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<blockquote data-quote="dslater" data-source="post: 363292" data-attributes="member: 32537">This is because most lenses are not diffraction limited. They have aberrations and usually it's the outer region of the lens that contribute the most to the aberrations. Stopping down masks out the outer regions of the lens removing these aberrations and yielding a diffraction limited result. ( Since stopping down also increases diffraction, it is easier for the stopped down lens to be diffraction limited. ). There are examples of diffraction limited lenses that do not improve when stopping down. The newest Nikon 600mm f/4 AF-S is an example - this lens actually performs best wide open. Other examples are the old Ultra-Micro Nikkor industrial lenses - these lenses all perform best wide open - stopping down reduces resolution in these lenses.However, for this discussion regarding DOF, I believe we're making an implicit assumption that we have an ideal diffraction limited lens.</blockquote>

[QUOTE="dslater, post: 363292, member: 32537"] This is because most lenses are not diffraction limited. They have aberrations and usually it's the outer region of the lens that contribute the most to the aberrations. Stopping down masks out the outer regions of the lens removing these aberrations and yielding a diffraction limited result. ( Since stopping down also increases diffraction, it is easier for the stopped down lens to be diffraction limited. ). There are examples of diffraction limited lenses that do not improve when stopping down. The newest Nikon 600mm f/4 AF-S is an example - this lens actually performs best wide open. Other examples are the old Ultra-Micro Nikkor industrial lenses - these lenses all perform best wide open - stopping down reduces resolution in these lenses. However, for this discussion regarding DOF, I believe we're making an implicit assumption that we have an ideal diffraction limited lens. [/QUOTE]

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