Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, they connect through adjoining cells within the retina to send an electrical signal to the optic nerve fibres. The optic nerves send off impulses through these fibres to the brain. Pigmentation
The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related—although problems of convergence do exist.Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells. The eyes of vertebrates usually contain ciliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes. Likewise, c-opsins have been found to be expressed in the brain of some invertebrates.
They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form. C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins. Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins—but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment. Additional images
See also
Adaptation (eye) (night vision)
Emission theory (vision)
Eye color
Eye development
Eye disease
Eye injury
Eye movement
Eyelid
Nictitating membrane
Ophthalmology
Orbit (anatomy)
Simple eye in invertebrates
Tapetum lucidum
Tears

Notes
References
Citations
Bibliography
Ali, Mohamed Ather; Klyne, M.A. (1985). Vision in Vertebrates. New York: Plenum Press. ISBN 978-0-306-42065-8. Further reading
Yong, Ed (14 January 2016). "Inside the Eye: Natures Most Exquisite Creation". National Geographic. External links

Evolution of the eye
Anatomy of the eye – flash animated interactive. (Adobe Flash)
Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most towards graduate students. Eye strips images of all but bare essentials before sending visual information to the brain, UC Berkeley research shows