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An optical effect which can soften photographs and make them less sharp. As long as light travels in straight lines, this phenomenon will not occur, but as soon as it starts to bend - disperse or "diffract" - when it has to travel through a hole so small that it has to squeeze through, it will begin to interfere with the quality of the final result. Although a negligible effect in most situations, it actually increases with smaller apertures. There is a break-even point at which the disadvantage of the diffraction of the light captured is still compensated by the advantage of extra sharpness due to greater depth of field. But beyond that point the softening effect of the diffracted light is only partly compensated by the sharpness due to the greater depth of field. Finding the break-even point can help prevent any negative effects of diffraction. And as a bonus it will limit the length of the exposure or the ISO needed to take a photo with a very small aperture. The difficulty is that the effect isn't the same for different cameras and lenses. The aperture isn't the only critical factor - the size of the film or sensor recording the photo counts as well, and so does the quality and the focal length of the lens. For those who don't want to get into complicated mathematical calculations in order to find the ideal aperture, it is good to remember that the sharpest results for most lenses are found around two or three stops below their maximum aperture. Especially cheaper lenses can give very bad results at full aperture.
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