Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Jonny Boyd Regular Member • Posts: 105
Re: Duck, duck, goose.

Ulric wrote:

Jonny Boyd wrote:

I get the impression that some people see the words 'diffraction' and 'limit(ed)' in close proximity and freak out without reading what I've written.

Johnny, for the benefit of everyone: just what is your definition of diffraction limited? A single, short sentence that anyone can understand. Mine is "Resolving power decreases when the lens is stopped down."

My definition comes at the end of this post. I wanted to put in some brief explanation first.

Diffraction always limits the resolving power of a lens, even wide open, and the effect gets stronger as you stop down. However lens aberations also limit resolution at all apertures but they're effect gets stronger as you open up the aperture. Hence all those curves we see of lens resolution at different apertures.

We could say that a meaningful definition of diffraction limited would be, not simply where diffraction limits resolution, because it always soes that, rather the point at which changing the aperture causes a drop in resolution due to diffraction effects. So if you take the peak aperture, then all smaller apaertures are diffraction limited while larger apertures are aberation limited.

What I was discussing though was a slightly modified definition which concerns when a lens is visibly diffraction limited. This would be when stopping down causes a visible decrease in resolution due to diffraction effects.

N.B. When a camera becomes dffraction limited at a larger aperture than another one and is therefore diffraction limited at a wider range of apertures,  that doesn't mean it has less overall resolution.

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