Here is a collection of illustrated articles on the chief causes of image degradation in photography. Currently available pages can be accessed via hyperlinks. The reader should keep in mind that most articles are about principles of photographic optics and not about tests of specific lenses. Also, most of the phenomena have been captured on film. Since a film acts as no more than a light-sensitive medium, the examples equally apply to a digital recorder array. In the few cases where the nature of the recording medium is important, this will be emphasized.
Surely there are many misconceptions in the
field of photographic optics.
The below figure and table illustrate the dependence of third-order lens aberrations on the aperture diameter D and distance Y from the image center. It is an approximation that does not include higher-order terms, but it provides useful insight.
The aberrations are measured by the deviation from the ideal gaussian image point, either by the size of the associated blur disk on the sensor, or, in the case of distortion, by the displacement of the otherwise sharp image. For instance, spherical aberration leads to a blur disk for a point in object space. The diameter of the blur patch increases with the third power of the aperture diameter, but for a given aperture it is constant across the image. Distortion on the other hand is independent of the aperture, but increases quadratically towards the image corner. Photographers who notice color fringing in their retrofocus wideangle pictures may decide to stop the lens down further, but to no effect: lateral color is not cured by a small aperture size.
Alan Naylor is acknowledged for stimulating discussions, helpful remarks, and the preparation of experiments. Mikhail Konovalov is responsible for many appealing illustrations. Many thanks are also due to Carl Zeiss Oberkochen for comments and the ample supply of material.