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.
- 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
- 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
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
- 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.
- Thoughts on MTF measurements by target reproduction photography.
- Post-processing offers opportunities for correction of image faults in digital photography, but trees don't grow to the sky.
- 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 ||− |
| 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.
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.
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