Image Diffraction in Camera Lens

Image Diffraction in Camera Lens

  • 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.

  • 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.

  • 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).

  • 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.

  • 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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