Re: The irony of camera lenses

Dave Martindale wrote:

On the other hand, when the lens is almost fully stopped down,
diffraction is the main source of blur, and if you stop down further you
won't decrease aberrations significantly but you will increase
diffraction blur, reducing resolution.

So any lens has an optimum aperture for resolution, and larger *or
smaller* apertures both give lower resolution. Of course, sometimes
there's still a good reason to work at those other apertures.

Dave

But the spot profile of diffraction is very different from the profile of many common geometric aberrations. The effect of diffraction is a softening, not a blur. Thus its effect on high contrast patterns is usually not as severe as defocus or most geometric aberrations.

In fact, it is often desireable in portrait work (thus the diffusion screens sometimes used in that work).
.

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