Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 41,736
Re: Thing is...

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.

If the entire scene is within the DOF and the diffraction softening outweighs the lens aberrations from wide open, the lens is "diffraction limited". An example of a "diffraction limited" lens would be the Panasonic 7-14 / 4:


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.

For example, the Panasonic 20 / 1.7 is "diffraction limited" in the center at f/2.2 (DOF permitting), and "diffraction limited" in the corners (or overall) at f/3.3:


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