Photographic optics

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 the 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.

Seidel aberrations

Spherical aberration occurs when the lens margins focus the light closer to -or further from- the sensor than the lens center.
Coma is associated with off-axis image points being smeared out in the characteristic comet shape.
Astigmatism implies that a lens can be sharper in one direction than another. The archetypal example is that of a wheel where the spokes and the rim are not rendered sharp simultaneously.
Field curvature leads to a curved image surface. Although the image can be sharp on this surface, it is incompatible with a flat sensor.
Distortion is not a sharpness issue but leads to straight lines in object space being rendered as curved lines on the film.

Color errors

Chromatic aberrations are due to dispersion and lead to color fringing.

Other phenomena and effects

Vignetting is the gradual or abrupt darkening of an image towards the corners. A related page discusses lens hoods.
Flare is the collective name for colored spots, ghost images or veils that impair the image. A separate page further exemplifies filter flare.
Diffraction is an unavoidable law of nature that says that a beam of light with a finite diameter spreads out as it propagates. It sets the ultimate limit to the resolution of a lens.
In focus: an article on depth of field, the theory and a DOF calculator.
Out of focus: the arcane bokeh.
Spurious resolution, a pitfall in lens resolution measurements.
On the center of perspective of a compound lens.

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.




Spherical aberration	D³	−
Coma	D²	Y
Astigmatism	D	Y²
Field curvature	D	Y²
Distortion (%)	−	Y²
Axial color	D	−
Lateral color	−	Y

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.

Credits

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.