Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Ulric Senior Member • Posts: 4,242
Re: Thing is...

Great Bustard wrote:

Ulric wrote:

Great Bustard wrote:

Ulric wrote:

Great Bustard 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."

...that's true of all lenses. It's simply a matter of what aperture the increasing diffraction softening outweighs lessening lens aberrations or, according to Johnny Boyd, what aperture you notice that diffraction is resulting in softer photos (which, of course, also depends tremendously, even primarily, on how large we display the photos, how closely we view them, and our visual acuity).

It is certainly not true of all lenses that resolving power is at a maximum wide open and decreases as it is stopped down.

You didn't initially say "is at a maximum wide open". So, if you are defining a "diffraction limited lens" as a lens that resolves best wide open, then we are in agreement.

I was just noting that "Resolving power decreases when the lens is stopped down" is true of all lenses once past their peak aperture. But, for sure, if we add "right from wide open" to the end of that statement, as I said, I agree.

Then we don't agree. Or we are talking past each other. It seems that my request to Johnny was unreasonable, there is no way to get one person to explain to another what he means by 'diffraction limit'. I'm sure there will be a third thread to follow up on this one.

Let me have another go. The effects of diffraction exist right from wide open, and only get worse as you stop down. However, two other effects typically offset this increasing diffraction at the wide end of the aperture: lessening lens aberrations and increasing DOF.

Good so far.

If the entire scene is within the DOF and the diffraction softening outweighs the lens aberrations from wide open, the lens is "diffraction limited".

I assume you mean that the effect of increasing diffraction is greater than the effect of lens aberrations decreasing.

Interesting, I think we may have different definitions. Mine does not require that to happen from wide open, just from the peak.

On the other hand, if either lessening lens aberrations or increasing DOF outweigh the effects of diffraction softening, then there will be an aperture that results in "optimal resolution", and the lens will be "diffraction limited" past this optimal aperture.

That is my definition. The peak can be wide open (the 7-14, or the Olympus 12-50 at the long end) but doesn't have to.

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