Image Diffraction in Camera Lens
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When the light forming an image passes
through a lens aperture, the wave front is distorted in much the same
way as an ocean wave is distorted as it passes through an opening in a
breakwater. The result is reduced image resolution or picture
sharpness. The smaller the aperture or the higher the f-number will
result even more distortion. This distortion effect is called image
diffraction.
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Diffraction, being produced by the
presence of the lens aperture (usually an iris diaphragm) rather than
by refractive qualities of the lens, is not itself a lens aberration.
However, since all camera lenses have an aperture, and since
diffraction impairs image quality, photographers must take it into
account.
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Closing the lens aperture to reduce the
effects of residual lens aberrations introduces image diffraction.
Diffraction itself can only be reduced by widening the aperture (that
is, using a lower f-number).
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Therefore, camera lenses have an optimum
aperture: a point beyond which further closing down will introduce
diffraction more than it will correct aberrations. This optimum
aperture differs from lens to lens, but usually occurs between f/4 and
f/8.
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Fortunately, because its onset is
relatively gradual, diffraction is not a serious problem in most
photography. It is more of a problem in technical photography, where
the optimum lens aperture must be determined for very critical
sharpness. The most drastic effects occur at very small apertures; a
sharp drop in lens performance occurs after about f/90 to f/128. Since
few, if any, lenses are calibrated to such small apertures, the worst
effects are limited to high-magnification photograph and close-up
photography.
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