Stopping down

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Stopping down refers to a photographic technique that increases the depth of field by reducing the aperture of a camera. However, this comes at the expense of reducing the available light and results in dimmer images. Photographers can compensate for this by increasing the exposure time, using a photographic film with a higher ISO rating, or, in digital cameras, increasing the light sensitivity of the sensor.

A false impression abounds that stopping down increases image sharpness. This is true for out-of-focus objects, and for some cameras that have very cheap or poorly made lenses, because spherical aberration and certain other optical aberrations (e.g. coma and astigmatism) are less problematic at smaller apertures. High quality lenses, however, are corrected for these aberrations to a high degree, and usually give the sharpest images for in-focus objects at about one or two f-stops below full aperture. For very small apertures, diffraction makes the image fuzzier. The reason that one generally obtains a sharper image by stopping down one or two f-stops (consult manufacturer's recommendations) is that lens design is optimized for a combination of admitting the most light (to shorten the exposure time) and producing a sharp image. If the lens designer optimized for sharpness only, the lens would be delivered with a smaller aperture and would be sharpest at full aperture, but would produce too dim an image for some poorly lighted scenes.