What is the diameter of the human eye?
Human eye
Dimensions typically differ among adults by only one or two millimetres, remarkably consistent across different ethnicities. The vertical measure, generally less than the horizontal, is about 24 mm. The transverse size of a human adult eye is approximately 24.2 mm and the sagittal size is  23.7 mm with no significant difference between sexes and age groups. Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88). The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres (0.4 cu. in.), and a mass of 7.5 grams (weight of 0.25 oz.)..
Optography
An issue that Kühne encountered when attempting to produce an image from a human eye is that the size of the fovea centralis, the actual focal point of the image on the retina, is very small (about 1.5 millimetres). Kühne had considerably more success producing optograms from animals such as rabbits and frogs, and the Reif image ended up being the only known "human optogram". The original image from Reif's eye no longer exists, apart from a simple line drawing of the shape in Kühne's 1881 paper "Observations for Anatomy and Physiology of the Retina".
Ocular tremor
Ocular microtremor (OMT) is a constant, physiological, high frequency (peak 80Hz), low amplitude (estimated circa 150-2500nm (1)) eye tremor.
Eyepiece
There are six standard barrel diameters for telescopes. The barrel sizes (usually expressed in inches) are:Eyepieces for microscopes have barrel diameters measured in millimeters such as 23.2 mm and 30 mm.
Visual acuity
Vernier acuity measures the ability to align two line segments. Humans can do this with remarkable accuracy. This success is sometimes regarded as" hyperacuity". Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes (6/4) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone. Because the limit of vernier acuity is well below that imposed on regular visual acuity by the "retinal grain" or size of the foveal cones, it is thought to be a process of the visual cortex rather than the retina. Supporting this idea, vernier acuity seems to correspond very closely (and may have the same underlying mechanism) enabling one to discern very slight differences in the orientations of two lines, where orientation is known to be processed in the visual cortex.
Pupil
In humans the pupil is round, but other species, such as some cats, have vertical slit pupils, goats have horizontally oriented pupils, and some catfish have annular types. In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop. The image of the pupil as seen from outside the eye is the entrance pupil, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea. On the inner edge lies a prominent structure, the collarette, marking the junction of the embryonic pupillary membrane covering the embryonic pupil.
Mammalian eye
Dimensions vary only 1–2 mm among humans. The vertical axis is 24 mm; the transverse being larger. At birth it is generally 16–17 mm, enlarging to 22.5–23 mm by three years of age. Between then and age 13 the eye attains its mature size. It weighs 7.5 grams and its volume is roughly 6.5 ml. Along a line through the nodal (central) point of the eye is the optic axis, which is slightly five degrees toward the nose from the visual axis (i.e., that going towards the focused point to the fovea.
Luminous intensity
Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. This curve, denoted "V"(λ) or formula_1, is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten .
1958 Chandra
According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, the asteroid measures 36.2 kilometers in diameter and its surface has an albedo of 0.07, while the "Collaborative Asteroid Lightcurve Link" derives an albedo of 0.05 and a diameter of 33.8 kilometers with an absolute magnitude of 11.2.
Human eye
The human eye can detect a luminance range of 10, or one hundred trillion (100,000,000,000,000) (about 46.5 f-stops), from 10 cd/m, or one millionth (0.000001) of a candela per square meter to 10 cd/m or one hundred million (100,000,000) candelas per square meter. This range does not include looking at the midday sun (10 cd/m) or lightning discharge.
Non-line-of-sight propagation
By definition, line of sight is the "visual" line of sight, that is determined by the ability of the average human eye to resolve a distant object. Our eyes are sensitive to light but optical wavelengths are very short compared to radio wavelengths. Optical wavelengths range from about 400 nanometer (nm) to 700 nm but radio wavelengths range from approximately 1 millimetre (mm) at 300 GHz to 30 kilometres (km) at 10 kHz. Even the shortest radio wavelength is therefore about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz the difference is more like 60,000 times so it is not always reliable to compare visual obstructions, such as might suggest a NLOS path, with the same obstructions as they might affect a radio propagation path.
Feret diameter
The Feret diameter or Feret's diameter is a measure of an object size along a specified direction. In general, it can be defined as the distance between the two parallel planes restricting the object perpendicular to that direction. It is therefore also called the caliper diameter, referring to the measurement of the object size with a caliper. This measure is used in the analysis of particle sizes, for example in microscopy, where it is applied to projections of a three-dimensional (3D) object on a 2D plane. In such cases, the Feret diameter is defined as the distance between two parallel tangential "lines" rather than "planes".
Newton's reflector
"The diameter of the sphere to which the Metal was ground concave was about 25 English Inches, and by consequence the length of the Instrument about six Inches and a quarter. The Eye-glass was Plano-convex, and the diameter of the Sphere to which the convex side was ground was about 1/5 of an Inch, or a little less, and by consequence it magnified between 30 and 40 times. By another way of measuring I found it magnified 35 times. The concave Metal bore an Aperture of an Inch and a third part, but the Aperture was limited not by an Opake Circle, covering the limb of the Metal round about, but be an opake Circle, placed between the Eyeglass and the Eye, and perforated in the middle with a little round hole for the Rays to pass through to the Eye. For this Circle being placed here, stopp'd much of the erroneous Light, which other wise would have disturbed the Vision. By comparing it with a pretty good Perspective of four Feet in length, made with a concave Eye-glass, I could read at a greater distance with my own Instrument than with the Glass. Yet Objects appeared much darker in it than in the Glass, and that partly because more Light was lost by Reflexion in the Metal, than by Refraction in the Glass, and partly because my Instrument was overcharged. Had it magnified but 30 or 25 times, it would have made the Object appear more brisk and pleasant" ... "The object-metal was two inches broad, and about one-third part of an inch thick, to keep it from bending. I had two of these metals, and when I had polished them both I tried which was best; and ground the other again, to see if I could make it better, than that which I kept."
Dynamic range
The human senses of sight and hearing have a very high dynamic range. A human cannot perform these feats of perception at both extremes of the scale at the same time. The eyes take time to adjust to different light levels, and the dynamic range of the human eye in a given scene is actually quite limited due to optical glare. The instantaneous dynamic range of human audio perception is similarly subject to masking so that, for example, a whisper cannot be heard in loud surroundings.
Optical microscope
The eyepiece, or ocular lens, is a cylinder containing two or more lenses; its function is to bring the image into focus for the eye. The eyepiece is inserted into the top end of the body tube. Eyepieces are interchangeable and many different eyepieces can be inserted with different degrees of magnification. Typical magnification values for eyepieces include 5×, 10× (the most common), 15× and 20×. In some high performance microscopes, the optical configuration of the objective lens and eyepiece are matched to give the best possible optical performance. This occurs most commonly with apochromatic objectives.
Cosmic Eye
Cosmic Eye is a short film and iOS app, developed by astrophysicist Danail Obreschkow. It shows the largest and smallest known scales of the universe by gradually zooming out from and then back into the face of a young lady called "Louise". According to the developer, the film and app were inspired by the essay "Cosmic View" (1957) and the short films "Cosmic Zoom" (1968) and "Powers of Ten" (1977), but use state-of-the-art know-how and new scientific imaging and computer simulations. "Cosmic Eye" was developed in 2012 and suddenly become famous in April 2016, when it scored 40 million views in ten days on the Facebook group page of "The Science Scoop". The video has since been viewed more than 200 Million times on Facebook (top-ten counts only) and was featured in major media, such as "BBC World News".
Monocular
Much of the basic design considerations and related parameters are the same as for binoculars and are covered in that entry, but some expanded comments have been added where appropriate:
Exit pupil is defined as the diameter of the objective lens divided by the magnification and expressed in mm. (e.g. a 8x40 will give an exit pupil diameter of 5mm). For a given situation, the greater the exit pupil, the better the light transmission into the eye. Hence a large objective lens with a low magnification will give good light admission, especially important in deteriorating light conditions. The classic 7x50 marine binocular or monocular is ideally suited to low light conditions with its relatively large exit pupil diameter of 7.1mm and a realistic magnification which is practical on a moving boat. However, the exit pupil should be considered in relationship with the human eye pupil diameter. If the exit pupil of the chosen instrument is greater than the human eye pupil then there will be no benefit, as the eye will be the limiting factor in light admission. In effect, the extra light gathering potential is wasted. This is a consideration as one ages, because human eye pupil dilation range diminishes with age, as shown as an approximate guide in the table below.
Glued IOL
The concept of performing "Vertical glued IOL" was first suggested by Dr. Jeevan Ladi from Pune, India.The vertical corneal diameter is always less than the horizontal diameter. In cases of big eyes, the least corneal diameter (i.e. at 6’o clock & 9’ o clock) along the vertical axis can be chosen for making the scleral flaps. The IOL’s currently available in the market are 13 mm in length. Choosing the least corneal diameter allows extra length of the haptic available for tucking and so gives extra stability.
Size
Humans most frequently perceive the size of objects through visual cues. One common means of perceiving size is to compare the size of a newly observed object with the size of a familiar object whose size is already known. Binocular vision gives humans the capacity for depth perception, which can be used to judge which of several objects is closer, and by how much, which allows for some estimation of the size of the more distant object relative to the closer object. The perception of size can be distorted by manipulating these cues, for example through the creation of forced perspective.
Human eye
As the eye ages, certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of diseases of the aging eye. While there are many changes of significance in the non-diseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between the eye and area of infection occurs, the STD can be transmitted to the eye.
Eagle eye
The eagle eye is among the strongest in the animal kingdom, with an eyesight estimated at 4 to 8 times stronger than that of the average human. An eagle is said to be able to spot a rabbit 3.2 km away. Although an eagle may only weigh , its eyes are roughly the same size as those of a human. As the eagle descends from the sky to attack its prey, the muscles in the eyes continuously adjust the curvature of the eyeballs to maintain sharp focus and accurate perception throughout the approach and attack.
Gradient-index optics
The lens of the eye is the most obvious example of gradient-index optics in nature. In the human eye, the refractive index of the lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This allows the eye to image with good resolution and low aberration at both short and long distances.
Monocular
As with binoculars and telescopes, monoculars are primarily defined by two parameters: magnification and objective lens diameter, for example, 8x30 where 8 is the magnification and 30 is the objective lens diameter in mm (this is the lens furthest from the eye). An 8x magnification makes the distant object appear to be 8 times larger at the eye. Contemporary monoculars are typically compact and most normally within a range of 4x magnification to 10x, although specialized units outside these limits are available. Variable magnification or zoom is sometimes provided, but has drawbacks and is not normally found on the top quality monoculars. Objective lens diameter is typically in the range 20mm to 42mm. Care is needed in interpreting some monocular specifications where numerical values are applied loosely and inaccurately - e.g. "39x95", which on a small cheap monocular is more likely to refer to the physical dimensions than the optical parameters. (This is covered in more detail in the section "Interpreting product specifications" below).
Millimetre
The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a human hair. A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately a tenth of a millimetre thick.
Draize test
Kirk Wilhelmus, professor in the Department of Ophthalmology at Baylor College of Medicine, conducted a comprehensive review of the Draize eye test in 2001. He also reported that differences in anatomy and biochemistry between the rabbit and human eye indicate that testing substances on rabbits might not predict the effects on humans. However, he noted "that eyes of rabbits are generally more susceptible to irritating substances than the eyes of humans" making them a conservative model of the human eye. Wilhelmus concluded "The Draize eye test ... has assuredly prevented harm" to humans, but predicts it will be "supplanted as "in vitro" and clinical alternatives emerge for assessing irritancy of the ocular surface"
Visual acuity
If the optics of the eye were otherwise perfect, theoretically, acuity would be limited by pupil diffraction, which would be a diffraction-limited acuity of 0.4 minutes of arc (minarc) or 6/2.6 acuity. The smallest cone cells in the fovea have sizes corresponding to 0.4 minarc of the visual field, which also places a lower limit on acuity. The optimal acuity of 0.4 minarc or 6/2.6 can be demonstrated using a laser interferometer that bypasses any defects in the eye's optics and projects a pattern of dark and light bands directly on the retina. Laser interferometers are now used routinely in patients with optical problems, such as cataracts, to assess the health of the retina before subjecting them to surgery.
Color
The ability of the [[human eye]] to distinguish colors is based upon the varying sensitivity of different cells in the [[retina]] to light of different [[wavelengths]]. Humans are [[Trichromacy|trichromatic]]—the retina contains three types of color receptor cells, or [[cone cell|cones]]. One type, relatively distinct from the other two, is most responsive to light that is perceived as blue or blue-violet, with wavelengths around 450 [[nanometre|nm]]; cones of this type are sometimes called "short-wavelength cones", "S cones", or "blue cones". The other two types are closely related genetically and chemically: "middle-wavelength cones", "M cones", or "green cones" are most sensitive to light perceived as green, with wavelengths around 540 nm, while the "long-wavelength cones", "L cones", or "red cones", are most sensitive to light is perceived as greenish yellow, with wavelengths around 570  nm.
Human eye
The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves rapidly to acquire a target (saccades), it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal rod photoreceptors is 80% complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by retinal disease, poor vascular circulation and high altitude exposure.
Angular diameter
The angular diameter of the Sun is about the same as that of the Moon. (The Sun's diameter is 400 times as large and its distance also; the Sun is 200,000 to 500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450 to 700, so a celestial body with a diameter of 2.5–4″ and the same brightness per unit solid angle would have the same brightness as the full Moon.)
Mammalian eye
The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina. Because of limited depth of field of the mammalian eye, an object at one distance from the eye might project a clear image, while an object either closer to or further from the eye will not. To make images clear for objects at different distances from the eye, its optical power needs to be changed. This is accomplished mainly by changing the curvature of the lens. For distant objects, the lens needs to be made flatter; for near objects the lens needs to be made thicker and more rounded.
Adaptation (eye)
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart. However, in any given moment of time, the eye can only sense a contrast ratio of 1,000. What enables the wider reach is that the eye adapts its definition of what is black.
Aperture
A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye – it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
Cone cell
Cone cells, or cones, are one of 3 types of photoreceptor cells in the retina of mammalian eyes (e.g. the human eye). They are responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. There are about six to seven million cones in a human eye and are most concentrated towards the macula.
The commonly cited figure of six million cone cells in the human eye was found by Osterberg in 1935. Oyster's textbook (1999) cites work by Curcio et al. (1990) indicating an average close to 4.5 million cone cells and 90 million rod cells in the human retina.
Optical telescope
Some telescopes cannot achieve the theoretical surface brightness of 100%, while some telescopes can achieve it using a very small-diameter eyepiece. To find what eyepiece is required to get minimum magnification one can rearrange the magnification formula, where it is now the division of the telescope's focal length over the minimum magnification: formula_64. An eyepiece of 35 mm is a non-standard size and would not be purchasable; in this scenario 
to achieve 100% one would require a standard manufactured eyepiece size of 40 mm. As the eyepiece has a larger focal length than the minimum magnification, an abundance of wasted light is not received through the eyes.
Vision span
Vision span or perceptual span is a controversial concept referring to the angular span (vertically and horizontally), within which the human eye has sharp enough vision to perform an action accurately (reading or face recognition). The visual field of the human eye spans approximately 120 degrees of arc. However, most of that arc is peripheral vision. The human eye has much greater resolution in the macula, where there is a higher density of cone cells. The field of view that is observed with sufficient resolution to read text typically spans about 6 degrees of arc, which is wide enough to allow a clear view of about five words in a row when printed text at ordinary size is held about 50 centimeters from the eyes. Regarding face processing, the field of view with a sufficient amount of information in order to recognise faces accurately spans about 7° which represents about 45% of a face. The brain creates the illusion of having a greater visual span by automatically and unconsciously moving the center of vision into any area of interest in the field of view.
Approximations of π
The magnitude of such precision (152 decimal places) can be put into context by the fact that the circumference of the largest known object, the observable universe, can be calculated from its diameter (93billion light-years) to a precision of less than one Planck length (at , the shortest unit of length that has real meaning) using expressed to just 62 decimal places.
Fisheye lens
However, there are new developments by the Japanese manufacturer Entaniya for the Micro Four Thirds standard, which offer an angle of view of 250 degrees with lenses that have a focal length of to , an aperture of f/2.8 to f/4.0, a weight of , a diameter of and a length below . In 2018 Venus Optics introduced a 210° fisheye lens for the Micro Four Thirds system.
Display size
The resolution of the human eye (with 20/20 vision) is about one minute of arc. For full HDTV resolution, this one minute of arc implies that the TV watcher should sit 4 times the height of the screen away. At this distance the individual pixels can not be resolved while simultaneously maximising the viewing area. So the ideal set size can be determined from the chart below by measuring the distance from where the watcher would sit to the screen in centimeters (or inches), dividing that by 4, and comparing with the screen heights below. At this distance, viewers with better than 20/20 vision will still be able to see the individual pixels. If the user is replacing a standard definition TV with an HDTV this implies that the best visual experience will be with a set that is twice as tall as the standard definition set. As the average size LCD TV being sold is now 38", which is only about 15% taller than a 27" standard definition TV, this means that most consumers buy HDTV sets that are smaller than what they could utilize. Cost and budget also limit screen size.
Nudibranch
The eyes in nudibranchs are simple and able to discern little more than light and dark. The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.
Hipparchus
Hipparchus measured the apparent diameters of the Sun and Moon with his "diopter". Like others before and after him, he found that the Moon's size varies as it moves on its (eccentric) orbit, but he found no perceptible variation in the apparent diameter of the Sun. He found that at the "mean" distance of the Moon, the Sun and Moon had the same apparent diameter; at that distance, the Moon's diameter fits 650 times into the circle, i.e., the mean apparent diameters are = 0°33′14″.
Human eye
The visual system in the human brain is too slow to process information if images are slipping across the retina at more than a few degrees per second. Thus, to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Frontal-eyed animals have a small area of the retina with very high visual acuity, the fovea centralis. It covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Any failure to make eye movements correctly can lead to serious visual degradation.
Globe (human eye)
The globe of the eye, or bulbus oculi, is the eyeball apart from its appendages. A hollow structure, the bulbus oculi is composed of a wall enclosing a cavity filled with fluid with three coats: the sclera, choroid, and the retina. Normally, the bulbus oculi is bulb-like structure. However, the bulbus oculi is not completely spherical. Its anterior surface, transparent and more curved, is known as the cornea of the bulbus oculi.
Airy disk
The fastest f-number for the human eye is about 2.1, corresponding to a diffraction-limited point spread function with approximately 1 μm diameter. However, at this f-number, spherical aberration limits visual acuity, while a 3 mm pupil diameter (f/5.7) approximates the resolution achieved by the human eye. The maximum density of cones in the human fovea is approximately 170,000 per square millimeter, which implies that the cone spacing in the human eye is about 2.5 μm, approximately the diameter of the point spread function at f/5.
Fresnel lens
A first-order lens has a focal length of 920 mm (36 in) and a maximum diameter 2590 mm (8.5 ft) high. The complete assembly is about 3.7 m (12 ft) tall and 1.8 m (6 ft) wide. The smallest (sixth-order) has a focal length of 150 mm (5.9 in) and an optical diameter 433 mm (17 in) high.
Zygoballus nervosus
Adult females are 3 to 4 mm in body length, while males are 2.5 to 4.5 mm. The sides of the cephalothorax are nearly vertical in front, and more rounded in the back. The ocular area occupies nearly three-fifths of the length of the cephalothorax and is widest at the posterior lateral eyes (PLE). The anterior median eyes (AME) are twice as large as the anterior lateral eyes (ALE), while the PLE are roughly the same size as the ALE.
Eye
Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eyes". They are small, comprising up to about 100 cells covering about 100 µm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.
Lens (anatomy)
The lens is part of the anterior segment of the human eye. In front of the lens is the iris, which regulates the amount of light entering into the eye. The lens is suspended in place by the suspensory ligament of the lens, a ring of fibrous tissue that attaches to the lens at its equator and connects it to the ciliary body. Posterior to the lens is the vitreous body, which, along with the aqueous humor on the anterior surface, bathes the lens. The lens has an ellipsoid, biconvex shape. The anterior surface is less curved than the posterior. In the adult, the lens is typically circa 10 mm in diameter and has an axial length of about 4 mm, though it is important to note that the size and shape can change due to accommodation and because the lens continues to grow throughout a person's lifetime.
Angular diameter
Defect of illumination is the maximum angular width of the unilluminated part of a celestial body seen by a given observer. For example, if an object is 40″ of arc across and is 75% illuminated, the defect of illumination is 10″.
Visual acuity
A reference value above which visual acuity is considered normal is called 6/6 vision, the USC equivalent of which is 20/20 vision: At 6 meters or 20 feet, a human eye with that performance is able to separate contours that are approximately 1.75 mm apart. Vision of 6/12 corresponds to lower, vision of 6/3 to better performance. Normal individuals have an acuity of 6/4 or better (depending on age and other factors).
Central retinal artery
The central retinal artery is approximately 160 micrometres in diameter.
Eagle eye
Eagles blink up as their top eyelid is larger than the bottom one; the additional inner eyelid is known as a nictitating membrane, which “grows in the inner corner of the eye, next to the tear duct”. Eagle tears moisten the eyes and contain the chemical lysozyme which protects against salt water and also destroys bacteria, thus preventing eye infections. The nictitating membrane acts a sweeping wiper across the eye. The eagle iris is a pale yellow color, much lighter than human eyes. Both eagles and humans have a white area called the sclera, but in the case of eagles, it is hidden below the eyelid. Eyelid openings are oval-shaped in humans, while they are round in the case of birds' eyes.
Angle of view
For comparison, the human visual system perceives an angle of view of about 140° by 80°.
Ocular prosthesis
The earliest known evidence of the use of ocular prosthesis is that of a woman found in Shahr-I Sokhta, Iran dating back to 2900–2800 BCE. It has a hemispherical form and a diameter of just over 2.5 cm (1 inch). It consists of very light material, probably bitumen paste. The surface of the artificial eye is covered with a thin layer of gold, engraved with a central circle (representing the iris) and gold lines patterned like sun rays. On both sides of the eye are drilled tiny holes, through which a golden thread could hold the eyeball in place. Since microscopic research has shown that the eye socket showed clear imprints of the golden thread, the eyeball must have been worn during her lifetime. In addition to this, an early Hebrew text references a woman who wore an artificial eye made of gold (Yer. Ned. 41c; comp. Yer. Sanh. 13c). Roman and Egyptian priests are known to have produced artificial eyes as early as the fifth century BCE constructed from painted clay attached to cloth and worn outside the socket.
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz.
X: The Man with the X-ray Eyes
Dr. Xavier develops eyedrops intended to increase the range of human vision, allowing one to see beyond the "visible" spectrum into the ultraviolet and x-ray wavelengths and beyond. Believing that testing on animals and volunteers will produce uselessly subjective observations, he tests the drops on himself.
Human eye
Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens; this process is called "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus.
Cephalopod size
The giant and colossal squids have the largest recorded eyes of any living animal, with a maximum diameter of at least and a pupil (Nilsson "et al.", 2012:683). This is three times the size of the largest fish eyes—up to in swordfish—and more than twice the diameter of the largest whale eyes—up to , , and in blue, humpback, and sperm whales, respectively—which are the largest among vertebrates (Nilsson "et al.", 2012:683). A large colossal squid caught in 2014 and dissected at the Museum of New Zealand Te Papa Tongarewa reportedly had eyes across (Farquhar, 2014). There are unconfirmed reports from the 19th century of giant squid eyes up to across (Land & Nilsson, 2012:86). Only the extinct ichthyosaurs are known to have approached these dimensions (Motani "et al.", 1999; Humphries & Ruxton, 2002), with some species having eyes up to in diameter (Nilsson "et al.", 2012:687).
Visual field
The normal (monocular) human visual field extends to approximately 60 degrees nasally (toward the nose, or inward) from the vertical meridian in each eye, to 107 degrees temporally (away from the nose, or outwards) from the vertical meridian, and approximately 70 degrees above and 80 below the horizontal meridian. 
The binocular visual field is the superimposition of the two monocular fields. In the binocular field, the area left of the vertical meridian is referred to as the left visual field (which is temporally for the left, and nasally for the right eye); a corresponding definition holds for the right visual field. The four areas delimited by the vertical and horizontal meridian are referred to as upper/lower left/right quadrants. In the United Kingdom, the minimum field requirement for driving is 60 degrees either side of the vertical meridian, and 20 degrees above and below horizontal. The macula corresponds to the central 17 degrees diameter of the visual field; the fovea to the central 5.2 degrees, and the foveola to 1–1.2 degrees diameter.
Architectural endoscopy
The opponents of controversial construction projects were never convinced, and they criticize the method. Their main critique was that the pictures were photographed from an ant's perspective. The eye level of a person is approximately . In a 1:100 scale, the eye level of a pedestrian in a model is at and this level can be precisely maintained with an endoscope, which preferably has a diameter of .
VISTA (telescope)
The objective to repeatedly image large areas of sky at seeing-limited resolution led to a unique optical design. The primary mirror is a concave hyperboloid with 4.1 m diameter and about f/1 focal ratio. The mirror has a meniscus shape of 17 cm thickness with a central 1.2 m hole to accommodate the camera at the Cassegrain focus. It was cast from Zerodur by Schott in Germany and subsequently polished and figured by LZOS, Moscow. It is the largest mirror of this shape and of such short focal ratio; polishing it took 2 years, which was longer than anticipated. The mirror is supported by a number of actuators (81 on the back and 24 around the edge), which allow its shape to be controlled by computers.
Retina
In adult humans, the entire retina is approximately 72% of a sphere about 22 mm in diameter. The entire retina contains about 7 million cones and 75 to 150 million rods. The optic disc, a part of the retina sometimes called "the blind spot" because it lacks photoreceptors, is located at the optic papilla, where the optic-nerve fibres leave the eye. It appears as an oval white area of 3 mm². Temporal (in the direction of the temples) to this disc is the macula, at whose centre is the fovea, a pit that is responsible for our sharp central vision but is actually less sensitive to light because of its lack of rods. Human and non-human primates possess one fovea, as opposed to certain bird species, such as hawks, who are bifoviate, and dogs and cats, who possess no fovea but a central band known as the visual streak. Around the fovea extends the central retina for about 6 mm and then the peripheral retina. The farthest edge of the retina is defined by the ora serrata. The distance from one ora to the other (or macula), the most sensitive area along the horizontal meridian is about 32 mm.
Aperture
The biological pupil of the eye is its aperture in optics nomenclature; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from 2 mm () in a brightly lit place to 8 mm () in the dark.
Angular diameter
The angular diameter of the Sun, from a distance of one light-year, is 0.03″, and that of Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of Earth.
Telescopic sight
Telescopic sights are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. Those parameters are:
The main tube of telescopic sights vary in size, material, the applied production process and surface finish. The typical outer diameters vary between and . The internal diameter of the telescopic sight main tube influences the area light can pass through, lens elements and other parts can be mounted in and the amount the internal parts for elevation and windage adjustment can move. Telescopic sights intended for long range and/or low light usage generally feature larger main tube diameters. Besides optical, spatial and attainable range of elevation and windage adjustments considerations, larger diameter main tubes offer the possibility to increase the tube walls thickness without sacrificing a lot of internal diameter.
Eye
Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.
Hans Wallach
Since a human's two eyes are approximately 6.5 cm apart, they see the world from different viewpoints: the image projected on the left retina is slightly different from the image projected on the right. This difference (known as binocular disparity) is the fundamental cue underlying stereoscopic depth perception . The importance of stereoscopic perception is familiar to anyone who has ever attempted to thread a needle with one eye closed; and when two slightly disparate photographs are viewed through a stereoscope (a device that makes it easy to fuse the two images), the fused scene takes on a three-dimensional appearance.
Hair's breadth
This measurement is not a precise one. Human hair varies in diameter, ranging anywhere from 17 μm to 181 μm. One nominal value often chosen is 75 μm, but this – like other measures based upon such highly variant natural objects, including the barleycorn – is subject to a fair degree of imprecision.
Progradungula otwayensis
Odd-clawed spiders have eight eyes arranged in two rows of four. The front row curves slightly backward, while the back row curves slightly forward. The anterior and posterior lateral (left and right) eyes are clustered closely together, while the median eyes are spaced further from each other. In females, the anterior and posterior lateral eyes are the largest pairs, with a diameter of . The anterior median eyes are the smallest with a diameter of . The posterior median eyes are slightly smaller than the lateral eyes, at a diameter of . In males, the anterior lateral eyes are the largest pair, with a diameter of . Their posterior median eyes and posterior lateral eyes are about the same size, with a diameter of . As with the female, the anterior median eyes are the smallest pair, with a diameter of only . All of the eyes are white in color, except for the anterior median eyes which are black.
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK.
Atom
Atomic dimensions are thousands of times smaller than the wavelengths of light (400–700 nm) so they cannot be viewed using an optical microscope. However, individual atoms can be observed using a scanning tunneling microscope. To visualize the minuteness of the atom, consider that a typical human hair is about 1 million carbon atoms in width. A single drop of water contains about 2 sextillion () atoms of oxygen, and twice the number of hydrogen atoms. A single carat diamond with a mass of contains about 10 sextillion (10) atoms of carbon. If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.
Angular diameter distance
The angular diameter distance is a distance measure used in astronomy. It is defined in terms of an object's physical size, formula_1, and formula_2 the angular size of the object as viewed from earth. 
formula_3
Eyepiece
The field of view, often abbreviated FOV, describes the area of a target (measured as an angle from the location of viewing) that can be seen when looking through an eyepiece. The field of view seen through an eyepiece varies, depending on the magnification achieved when connected to a particular telescope or microscope, and also on properties of the eyepiece itself. Eyepieces are differentiated by their "field stop", which is the narrowest aperture that light entering the eyepiece must pass through to reach the field lens of the eyepiece.
Ciliary ganglion
The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It measures 1–2 millimeters in diameter and in humans contains approximately 2,500 neurons. The oculomotor nerve coming into the ganglion contains "preganglionic" axons from the Edinger-Westphal nucleus (a part of the brainstem) which form synapses with the ciliary neurons. The "postganglionic" axons run in the short ciliary nerves and innervate two eye muscles:
Binoculars
Binoculars concentrate the light gathered by the objective into a beam whose diameter, the exit pupil, is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image and to maximize the sharpness, the exit pupil should at least equal the diameter of the pupil of the human eye — about 7 mm at night and about 3 mm daytime, reducing with age. If the cone of light streaming out of the binoculars is "larger" than the pupil it is going into, any light larger than the pupil is wasted. In daytime use the human pupil is typically dilated about 3 mm, which is about the exit pupil of a 7×21 binocular. Much larger 7×50 binoculars will produce a cone of light bigger than the pupil it is entering, and this light will, in the daytime, be wasted. An exit pupil that is too "small" will also present an observer with a dimmer view since only a small portion of the light gathering surface of the retina is used. For applications where equipment has to be carried (birdwatching, hunting), users opt for much smaller (lighter) binoculars with an exit pupil that matches their expected iris diameter so they will have maximum resolution and are not carrying the weight of wasted aperture.
Egg cell
The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. The human ovum measures approximately 0.1 mm in diameter.
Optimum HDTV viewing distance
The human visual system has a fixed capacity to detect detail from a distance. Our understanding of limitations with regard to visual detail recognition and identification from a distance is primarily based on the work of Dr. Hermann Snellen. Dr. Snellen developed the eye examination chart that bears his name (Snellen chart). From his findings and the work of others over the last hundred years, one arcminute is seen as the threshold beyond which critical detail cannot be identified, by a person with normal vision. An arcminute is an angular measurement, which is equal to 1/60 of one degree of a circle. Normal vision is referenced as 20/20 or 6/6 vision in North America and Europe respectively. The visual acuity threshold has been identified as a constraint factor in the recommendations on the optimum viewing distance for HDTV, and also in formal research that comment on the subject of television and angular resolution. Assuming display is flat, with 1 arcminute as the constraint for seeing critical detail, in order not to miss any detail a viewer would need to be situated at a position where their view angle to a 1080p HDTV is approximately 31.2 degrees or greater (32 degrees for spherical display), for 2160p HDTV approximately 58.37 degrees or greater (64 degrees for spherical display) and for 4320p HDTV approximately 96.33 degrees or greater (128 degrees for spherical display). However, there is not always agreement that the Snellenian limit should be the constraining factor.
Visual angle
That is just a bit larger than the retinal image size for the moon, which is about formula_23, because, with moon's mean diameter formula_24 formula_25 , and earth to moon mean distance formula_2 averaging formula_27 (formula_28), formula_29formula_30.
Cooperative eye hypothesis
Unlike other primates, human beings have eyes with a distinct colour contrast between the white sclera, the coloured iris, and the black pupil. This is due to a lack of pigment in the sclera. Other primates have pigmented sclerae that are brown or dark in colour. There is also a higher contrast between human skin, sclera, and irises. Human eyes are also larger in proportion to body size, and are longer horizontally. Among primates, humans are the only ones where the outline of the eye and the position of the iris can be clearly seen.
Normal lens
The eye has a nominal focal length of approximately 17mm, but it varies with accommodation. The nature of human binocular vision, which uses two lenses instead of a cyclopean one, and 'post-processing' by the cortex is very different to the process of making and rendering a photograph, video or film.
Pupil
The pupil gets wider in the dark and narrower in light. When narrow, the diameter is 2 to 4 millimeters. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8 mm. In any human age group there is however considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals. After 25 years of age the average pupil size decreases, though not at a steady rate. At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus. The constriction of the pupil and near vision are closely tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.
Cornea
The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance, the healthy cornea does not have or need blood vessels within it. Instead, oxygen dissolves in tears and then diffuses throughout the cornea to keep it healthy. Similarly, nutrients are transported via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface, and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue.
Angular diameter distance
In the currently favoured geometric model of our Universe, the "angular diameter distance" of an object is a good approximation to the "real distance", i.e. the proper distance when the light left the object. Note that beyond a certain redshift, the angular diameter distance gets smaller with increasing redshift. In other words, an object "behind" another of the same size, beyond a certain redshift (roughly z=1.5), appears larger on the sky, and would therefore have a "smaller" "angular diameter distance".
Reduced eye
The reduced eye is an idealized model of the optics of the human eye. Introduced by Franciscus Donders, the reduced eye model replaces the several refracting bodies of the eye (the cornea, lens, aqueous humor, and vitreous humor) are replaced by an ideal air/water interface surface that is located 20 mm from a model retina. The converts a system with six cardinal points (two focal points, two principal points and two nodal points) into one with three cardinal points (two focal points and one nodal point).
F-number
Computing the f-number of the human eye involves computing the physical aperture and focal length of the eye. The pupil can be as large as 6–7 mm wide open, which translates into the maximal physical aperture.
Hobby–Eberly Telescope
The Hobby–Eberly Telescope (HET) is a 10-meter (30-foot) aperture telescope located at the McDonald Observatory. It is one of the largest optical telescopes in the world and combines a number of features that differentiate it from most telescope designs, resulting in greatly lowered construction costs. For instance, the primary mirror is constructed from 91 hexagonal segments, which is less expensive than manufacturing a single large primary. Furthermore, the telescope's main mirror is fixed at a 55° angle and can rotate around its base. A target is tracked by moving the instruments at the focus of the telescope; this provides access to about 70–81% of the sky at its location and allows a single target to be tracked for up to two hours. The telescope is named for former Texas Lieutenant-Governor Bill Hobby and for Robert E. Eberly, a Penn State benefactor.
Ear canal
The ear canal (external acoustic meatus, external auditory meatus, EAM; ) is a tube running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter.
Mammalian eye
The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.
William Herbert Steavenson
Steavenson studied how the human eye operates, particularly at the low-light levels encountered in visual astronomy. He measured the diameter of the pupil of a dark-adapted eye to be 1/3 inch (8mm), which was larger than the figure that was believed at that time.
Star-nosed mole
The star nose is a highly specialized sensory-motor organ shaped by 22 fleshy finger-like appendages, or tendrils, that ring their nostrils and are in constant motion as the mole explores its environment. The star itself is a centimeter across and thus has a diameter slightly smaller than a typical human fingertip. Nevertheless, it is much larger than the nose of other mole species, covering per touch compared to covered by the noses of other mole species. This structure is divided into a high resolution central fovea region (the central 11th pair of rays) and less sensitive peripheral areas. In this way the star works as a "tactile eye" where the peripheral rays (1–10 on each side) study the surroundings with erratic saccade-like movements and direct the 11th ray to objects of interest, just like the primate’s foveating eye.
Bird vision
The eye is also cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian glands which coats the cornea and prevents dryness. The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull. The ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm (2 in), twice that of the human eye.
Human eye
Having two eyes allows the brain to determine the depth and distance of an object, called stereovision, and gives the sense of three-dimensionality to the vision. Both eyes must point accurately enough that the object of regard falls on corresponding points of the two retinas to stimulate stereovision; otherwise, double vision might occur. Some persons with congenitally crossed eyes tend to ignore one eye's vision, thus do not suffer double vision, and do not have stereovision. The movements of the eye are controlled by six muscles attached to each eye, and allow the eye to elevate, depress, converge, diverge and roll. These muscles are both controlled voluntarily and involuntarily to track objects and correct for simultaneous head movements.
Third eye
Adherents of theosophist H.P. Blavatsky have suggested that the third eye is in fact the partially dormant pineal gland, which resides between the two hemispheres of the brain. Reptiles and amphibians sense light via a third parietal eye—a structure associated with the pineal gland—which serves to regulate their circadian rhythms, and for navigation, as it can sense the polarization of light. C.W. Leadbeater claimed that by extending an "etheric tube" from the third eye, it is possible to develop microscopic and telescopic vision. It has been asserted by Stephen Phillips that the third eye's microscopic vision is capable of observing objects as small as quarks. According to this belief, humans had in far ancient times an actual third eye in the back of the head with a physical and spiritual function. Over time, as humans evolved, this eye atrophied and sunk into what today is known as the pineal gland. Dr. Rick Strassman has hypothesized that the pineal gland, which maintains light sensitivity, is responsible for the production and release of DMT (dimethyltryptamine), an entheogen which he believes possibly could be excreted in large quantities at the moments of birth and death.
Human eye
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Fixational eye movements include drift, ocular tremor, and microsaccades. Some irregular drifts, movements smaller than a saccade and larger than a microsaccade, subtend up to one tenth of a degree. Microsaccades move the eye no more than a total of 2° in adult humans.
Reflector sight
Since the collimated image produced by the sight is only truly parallax free at infinity, the sight has an error circle equal to the diameter of the collimating optics for any target at a finite distance. Depending on the eye position behind the sight and the closeness of the target this induces some aiming error. For larger targets at a distance (given the non-magnifying, quick target acquisitions nature of the sight) this aiming error is considered trivial. On small arms aimed at close targets this is compensated for by keeping the reticle in the middle of the optical window (sighting down its optical axis). Some manufacturers of small arms sights also make models with the optical collimator set at a finite distance. This gives the sight parallax due to eye movement the size of the optical window at close range which diminishes to a minimal size at the set distance (somewhere around a desired target range of 25-50 yards).
Universe
The proper distance—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe times the speed of light, , but this does not represent the distance at any given time because the edge of the observable universe and the Earth have since moved further apart. For comparison, the diameter of a typical galaxy is 30,000 light-years (9,198 parsecs), and the typical distance between two neighboring galaxies is 3 million light-years (919.8 kiloparsecs). As an example, the Milky Way is roughly 100,000–180,000 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away.
Optimum HDTV viewing distance
A 1998 Sun Microsystems paper on the limits of human vision and video display systems uses a different constraint value of approximately ½ an arc minute (or 30 arc seconds), when estimating the saturation point for the human visual system. With 30 arc seconds as the constraint, the view angle necessary to see all the detail provided by an HDTV with a 1080p resolution drops to approximately 16.1 degrees. Furthermore, several academic articles have challenged the notion that 1 arcminute of resolution is the typical resolving power of the human eye, suggesting that on average, we can resolve detail smaller than that. Also, there is the issue of vernier acuity, which is the eye's ability to detect an offset between 2 lines and stereoacuity, which is the ability to discriminate depth by the use of both eyes. Vernier acuity and stereoacuity are cited as being detected with only a 2–4 arc second degree of separation. Ultimately all of the various types of acuity play a part in how we see things and more importantly, how we perceive what we are witnessing. The complexities of the human visual system and the relationship between different types of acuity are not yet fully understood. Thus, depending on which human visual system constraints are applied, viewing angles calculations will vary to some degree, especially when technological constraints are factored in.
Circle contact lens
The diameter of regular contact lenses that are sold in the United States are on average 14mm–16mm. Similar to the diameter of regular contact lenses, circle lenses have no more than 15mm diameter since larger sizes would be harmful to the eyes at daily wear. When the diameter of the circle lens is described as 16mm or 18mm, it is only the provided visual effect of the circle lens. The difference between the two types of lenses is that circle lenses are tinted not only in areas that cover the iris of the eye, but also prominently in the extra-wide outer rim of the lens. The result is the appearance of a bigger, wider iris with a limbal ring, creating an illusion of large eyes.
Eye relief
The exit pupil width of say, a binocular, can be calculated as the objective diameter divided by the magnification, and gives the width of the exit cone of light in the same dimensions as the objective. For example, a 10 × 42 binocular has a 4.2 mm wide exit cone, and fairly comfortable for general use, whereas doubling the magnification with a zoom feature to 20 × results in a much more critical 2.1 mm exit cone. 
Eye relief distance can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the eyepiece so needs a longer eye relief in order to still see the entire field of view. A simple practical test as to whether or not spectacles limit the field of view can be conducted by viewing first "without" spectacles and then again "with" them. Ideally there should be no difference in the field.
2060 Chiron
The assumed size of an object depends on its absolute magnitude (H) and the albedo (the amount of light it reflects). In 1984 Lebofsky estimated Chiron to be about 180 km in diameter. Estimates in the 1990s were closer to 150 km in diameter. Occultation data from 1993 suggests a diameter of about 180 km. Combined data from the Spitzer Space Telescope in 2007 and the Herschel Space Observatory in 2011 suggests that Chiron is in diameter. Therefore, Chiron may be as large as 10199 Chariklo. The diameter of Chiron is difficult to estimate in part because the true absolute magnitude of its nucleus is uncertain due to its highly variable cometary activity.
Eye
For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.
Corrective lens
The eye's Abbe number is independent of the importance of the corrective lens's Abbe, since the human eye:
In contrast, the eye moves to look through various parts of a corrective lens as it shifts its gaze, some of which can be as much as several centimeters off of the optical center. Thus, despite the eye's dispersive properties, the corrective lens's dispersion cannot be dismissed. People who are sensitive to the effects of chromatic aberrations, or who have stronger prescriptions, or who often look off the lens’s optical center, or who prefer larger corrective lens sizes may be impacted by chromatic aberration. To minimize chromatic aberration: