critical aperture

Definition: The aperture setting which gives the best overall performance. * The relative aperture at which an optical system delivers the best balance between aberration correction and contrast rendition.
Stop stopping down two
The commonest advice for using critical aperture was to stop down two stops from maximum: this continues to be useful for standard lenses of modest aperture and enlarging lenses, but not for other types. High-speed telephoto lenses (e.g. 300mm, f/2.8) are often already at their best at full aperture or just half a stop down. Ultra wide-angle lenses often need to be stopped down three or four stops to be at the best. And zoom lenses, particularly those with large zoom range of inexpensive construction, may need to be stopped almost to minimum aperture to be at their best (and still none too good).

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