Материал из ФотоЭнциклопедия.рф
In photography, stopping down is to the act of increasing the f-stop number, thus decreasing the size (aperture) of the iris of a lens, for example, stopping down from f2 to f4. This increases the depth of field of the image and allows less light to reach the film plane. To compensate for the latter, it is necessary to increase the exposure time, use a photographic film with a higher ISO rating, using or increasing the strength of artificial lighting, such as electronic flash or, in digital cameras, increasing the light sensitivity of the sensor.
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 focussed image. If the lens designer optimised for sharpness only, the lens would have a small minimum aperture and would be sharpest at full aperture, but would produce too dim an image for some poorly lit scenes.