This method relates to an electronic correction method of distortion and lateral color aberrations introduced by lenses.
Distortion is an optical aberration introduced by a lens. Because of this aberration bowed lines instead of straight lines are created in a picture. Most camera lenses are designed to have less than 2% distortion. This amount of distortion is approximately what is just noticeable in a picture to a typical picture taker. Lateral color aberration is another aberration that often limits the lens performance. Lenses that suffer from this aberration create multiple different color images that overlap only partially, producing color fringing in the images. This color fringing reduces image quality. Lateral color aberration can be corrected with additional lens elements and judicial choice of lens materials, but this increases the complexity of lenses and their cost.
The need to minimize distortion and lateral color aberration is often balanced against the need to correct other aberrations and, as a consequence, other aberrations are either allowed to increase in order to have a lens with a reasonable amount of distortion and lateral color or, alternatively, additional lens elements are used to control these other aberrations. Furthermore, distortion and lateral color aberrations increase with increasing field of view. Because of the need to keep distortion and lateral color aberrations small, the field of view of many conventional taking lenses does not exceed 100 degrees. For example, a typical wide angle lens for 35 mm cameras has a full field of view of about 84 degrees. Even in a specially designed very wide angle lenses which contain a large number of lens elements the field of view is limited to approximately 100 degrees by distortion and lateral color aberrations.
Some wide angle lenses have much more than 2% distortion. For example, fisheye lenses have very large fields of view and because of this have a very large amount of distortion. Thus, a person who needs to take pictures with a very wide angle lens, (i.e. a lens that has a field of view of above 100 degrees) often has to settle for very distorted pictures. A distortion of 6% or more is very noticeable. A good example of a fisheye lens is disclosed in U.S. Pat. No 4,647,161. Except for distortion, this fisheye lens has an excellent image quality which is achieved with no vignetting over a total angular field of greater than 150 degrees. However, this lens suffers from 70% distortion. Thus, such lens would produce a very distorted an image.
In addition, lateral color aberration is very difficult to correct in wide angle lenses. Lateral color aberration also increases with the field angle because the image forming bundles of light pass through the periphery of some lens elements. Thus, images formed by such wide angle lenses often exhibit color fringing near the edges.
It is well known that single lens reflex cameras (also known as SLR cameras) employ a flipable mirror positioned between the taking lens and the image plane. It is difficult to control distortion in lenses designed for such cameras because these cameras require a minimum back focus distance in order to allow the mirror to flip out of the way during exposure. When the field angle is in the wide angle region (above about 45 degrees), this requirement forces the back focal length (i.e., the distance along the optical axis from the rear lens surface to the image surface) of the lens to be greater than the focal length of the lens. In order to achieve a back focal length that is greater that the focal length, the lens has to be of a retro-focus type. Such lenses have a front negative power lens group followed by a rear lens group of positive power. The asymmetry of this design form makes coma, lateral color and distortion aberrations more difficult to correct, limiting the maximum field of view of the lens. If the total field angle of the lens is required to be very wide, such as above 100 degrees, aberration control becomes especially difficult, and the lens becomes complex, bulky and very expensive.
It is desirable to have compact zoom lenses with a small number of lens elements that provide a large zoom range and an excellent image quality. However, many zoom lenses suffer from distortion and/or lateral color aberrations in one or more of their zoom positions. Of course, additional lens elements may be employed to correct for distortion and lateral color, but this raises the complexity and the cost of these zoom lenses. Furthermore, many zoom lenses have limited zoom ranges because distortion and lateral color aberrations have to be controlled in each of the zoom positions. Thus, distortion and lateral color are often limiting factors that determine the total zoom range of a zoom lens. For example, good five element zoom lenses, such as those disclosed in U.S. Pat. Nos. 5,825, 556 and 5,726, 810 have fields of view in their wide angle mode of about 30 degrees and provide a zoom ratio of about 2X. The field of view and the zoom range of these lenses are limited by the need to control distortion in each zoom position to about 2% or less and the need to control the lateral color aberration.
U.S. Pat. No. 5,461,440 discloses a photographic image correction system for correcting degraded images for distortion and light irregularity. The distortion is corrected by knowing the location of the distorted data and, then, by solving a complicated equation to find the ideal pixel location for that data. This is done for each pixel in the image medium. The calculation of the ideal pixel location using this equation for each pixel of the image medium is excessively time consuming. Once the ideal pixel location for a given image data is determined, the image data for the ideal pixel is calculated by area weighted average of four adjacent pixels. This averaging operation acts as a smoothing filter. Therefore, due to this averaging operation some of the image information is lost. This is undesirable.
Furthermore, the disclosed technology is based is based on use of conventional camera lenses and photographic film. The original negative with a distorted image is scanned by a film scanner and a new (improved) image is produced by a printer. No adjustment is made for the difference in the amount of pixels in distorted versus undistorted image, and in case of the pin cushion distortion (unless used with a smaller field of view) some of the data near the edge of the film is lost.
U.S. Pat. No. 5,739,924 is directed to a photographic image printing apparatus capable of correcting image quality. This patent discloses image sharpening for a moving object, which results in focus error, using a subtraction of a Laplacian function of the image data from the image data itself. That is, the disclosed method emphasizes the outlines of the image. This patent is silent with respect to distortion or lateral color correction.
An object of the present invention is to provide a method and an apparatus for correcting distortion and lateral color introduced by lenses. Another object of the present invention is to provide a distortion and lateral color correction method that is simple and more efficient than that of the prior art. Another object of the present invention is to efficiently use sensor arrays capturing the distorted image.
According to one aspect of the present invention, an apparatus comprises: a lens with a predetermined full field angle of more than 50 degrees, the lens having at least 6% distortion; and a sensor array receiving a distorted image created by the lens. At this full field angle, the distorted image just fits within the sensor array and paraxial image corresponding to this field angle does not touch the sides of the sensor array.
According to another aspect of the present invention an apparatus comprises: a lens with a predetermined full field angle of more than 50 degrees, the lens having at least 6% distortion; a sensor array receiving a distorted image created by the lens, such that at this full field angle, the distorted image just fits within the sensor array and paraxial image corresponding to this field angle does not touch the sides of the sensor array; and an image processing unit processing the data representing the distorted image and transposing the data to represent an undistorted image.
According to an embodiment of the present invention a method of producing a distortion-free image comprises the following steps:(i) producing an image with at lest 6% distortion; (ii) for a predetermined image location of the distortion-free image finding a corresponding location in the distorted image; (iii) copying data located at the corresponding location of the distorted image into the predetermined image location and copying corresponding data from three other quadrants of the distorted image into their corresponding locations in the distortion-free image. The corresponding image locations are characterized by the same radial distance from the center of the distortion-free image.
According to an embodiment of the present invention, an apparatus for forming distortion free images comprises: (i) a rotationally symmetric lens system capable of forming a distorted image and of introducing lateral color aberration; (ii) a photosensitive image capture medium capable of capturing the 5 distorted image created by the lens system; (iii) an image storage medium storing data that represents the distorted image captured by the image capture medium; (iv) an image buffer for holding data representing the distortion free image; and (v) an image processing unit transposing the data representing the distorted image and creating a set of data that represents an undistorted image. The image processing unit has information describing distortion and lateral color aberration characteristics of the lens system. The image processing unit calculates, for at most one quadrant of the image, given any ideal radial position of a pixel, the actual radial position of the distorted data pixel from which to retrieve the data. Then, the image processing unit moves the data corresponding to the radial value of the pixel with the distorted data to its undistorted pixel location in each image quadrant, thereby building a distortion-free image. The data representing the distortion-free image is stored in the image buffer medium; and then moved from the buffer to the image storage medium.
One of the advantages of the present invention is that it alleviates the need to correct distortion and lateral color in a lens. Thus, very wide angle lens designs could be achieved if distortion need not be corrected in the lens design.
If the lenses are permitted to have large amounts of distortion and lateral color aberrations, other aberrations can be better controlled, the lenses can comprise fewer lens elements, have very wide field angles, and be less expensive. Thus, a lens can be designed to have a very large field of view and a large amount of distortion and lateral color, which will be eliminated digitally. A zoom lens that has excellent performance except for distortion and lateral color will also achieve a larger zoom range and have a wider field angle in its wide angle mode then a comparable zoom lens that is also corrected for distortion and lateral color. If a digital camera is used, the view seen in the viewfinder will be accurately captured in the image, resulting in higher resolution.