Diffraction Limit Discussion Continuation

Started 10 months ago | Discussions thread
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 Re: Duck, duck, goose. In reply to Ulric, 10 months ago

Ulric wrote:

Jonny Boyd wrote:

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.

I realize that you understand that, but so do everyone else and yet there are different opinions on what the implications are.

Yes. i just wanted to be careful with the definition to make at absolutely clear that there is no disagreement over facts like this.

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.

So your single-sentence definition is something like "Resolving power decreases visibly when the lens is stopped down one stop"?

Close.

'The diffraction limit for a system occurs at the aperture at which any further reduction in settable aperture size will cause a visible decrease in image resolution.'

'for a system' clarifies that the sensor resolution and viewing method are factors.

'settable aperture size' allows for the fact that different lenses have different limitations on how precisely the aperture is controlled.

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