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In photography, stopping down is the act of increasing the f-stop number, thus decreasing the size (aperture) of the iris of a lens, for example, stopping down from f/2 to f/4. Reducing the aperture increases the depth of field of the image and allows less light to reach the film plane. To achieve the same exposure after stopping down, it is necessary to compensate for the reduced light by increasing the exposure time, using a photographic film with a higher speed, or adding more light by employing or increasing the strength of electronic flash. In digital cameras, increasing the light sensitivity of the sensor can also compensate for the reduced light
In general, stopping down increases image sharpness. This is true for out-of-focus objects, and for cameras with inexpensive or simple lenses, because spherical aberration, coma and astigmatism are less apparent at smaller apertures. High quality lenses are corrected for these aberrations, and usually give the sharpest images for in-focus objects at about one or two f-stops below maximum aperture. When using very small apertures, diffraction reduces sharpness. One generally obtains a sharper image by stopping down one or two f-stops because lens designers optimise their designs for a compromise between admitting the most light to shorten the exposure time and producing a sharply focused image. If the lens designer optimised for sharpness only, the lens would have a smaller maximum aperture which would be too dim for some poorly lit scenes.
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