Editing Aperture (section)

===Optimal aperture===
Optimal aperture depends both on optics (the depth of the scene versus diffraction), and on the performance of the lens.

Optically, as a lens is stopped down, the defocus blur at the Depth of Field (DOF) limits decreases but diffraction blur increases. The presence of these two opposing factors implies a point at which the combined blur spot is minimized ([[#CITEREFR Gibson1975|Gibson 1975]], 64); at that point, the <var>f</var>-number is optimal for image sharpness, for this given depth of field<ref>{{cite web|url=http://www.bobatkins.com/photography/technical/diffraction.html|title=Diffraction and Optimum Aperture – Format size and diffraction limitations on sharpness|website=www.bobatkins.com|access-date=15 April 2018}}</ref>&nbsp;– a wider aperture (lower ''f''-number) causes more defocus, while a narrower aperture (higher ''f''-number) causes more diffraction.

As a matter of performance, lenses often do not perform optimally when fully opened, and thus generally have better sharpness when stopped down some&nbsp;– this is sharpness in the plane of [[critical focus]], setting aside issues of depth of field. Beyond a certain point, there is no further sharpness benefit to stopping down, and the diffraction occurred at the edges of the aperture begins to become significant for imaging quality. There is accordingly a sweet spot, generally in the {{f/|4}} – {{f/|8}} range, depending on lens, where sharpness is optimal, though some lenses are designed to perform optimally when wide open. How significant this varies between lenses, and opinions differ on how much practical impact this has.

While optimal aperture can be determined mechanically, how much sharpness is ''required'' depends on how the image will be used&nbsp;– if the final image is viewed under normal conditions (e.g., an 8″×10″ image viewed at 10″), it may suffice to determine the <var>f</var>-number using criteria for minimum required sharpness, and there may be no practical benefit from further reducing the size of the blur spot. But this may not be true if the final image is viewed under more demanding conditions, e.g., a very large final image viewed at normal distance, or a portion of an image enlarged to normal size ([[#CITEREFR Hansma1996|Hansma 1996]]). Hansma also suggests that the final-image size may not be known when a photograph is taken, and obtaining the maximum practicable sharpness allows the decision to make a large final image to be made at a later time; see also [[critical sharpness]].