Critical aperture

Critical aperture
Setting at which a lens gives its best performance. The setting offers the best compromise between diffracting due to small aperture and lens aberrations apparent at wide apertures.
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Critical Aperture & Flash
"Who cares" what aperture I use when shooting a simple yet compellingly lit portrait against an old textured wall?

Critical Aperture
Critical Aperture is the f-stop setting where a lens provides the optimal image quality; critical aperture finds the best balance between diffraction from small apertures and lens aberration from large apertures.
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(You may prefer to stop down one or two f/stops to obtain the critical aperture of the lens for the sharpest image.) Make necessary exposure adjustments by varying the shutter speed.

See also: Lens, Photograph, Image, Aperture, Subject