Diffraction limited performance

warden · Apr 11, 2019

Hi all,

Maybe there is already a thread that shows this, and if so I'll be grateful for a link that I have failed to find so far. I'm wondering if someone has done a test to illustrate where a lens is diffraction limited and what the performance hit actually looks like. For instance two identical images, one at f8 and the other at f22, compared for quality. I'm expecting some "fuzziness" or other issues when diffraction appears on the f22 image.

My specific project involves up close work with a Hasselblad, 60mm lens and 10mm extension. Very little is in focus when the objects are 8" from the lens so I'm trying to maximize depth of field with smaller apertures. Examples look good so far even at f22 (the max for this lens) and the prints won't be all that large anyway, but the subject has me curious about where I should be seeing problems: in the focused area, in the out of focus areas, everywhere?

I'm doing my own comparisons but to my eye I'm not seeing problems with this lens maxed out. So if you have experience, links, examples or comments please fire away.

abruzzi · Apr 11, 2019

These sort of tests are pretty common with lens reviews on the web. Even a fair number of amateur reviewers I seen show enough detail to see the effect. For example this review of a Nikon Series E 100/2.8:

https://matthewdurrphotography.com/2012/07/01/lens-review-nikon-100mm-f2-8-series-e/

The first set of 100% crops at apertures 2.8-22. You can see that the sharpness falloff from 16 to 22 is noticeable in the testing circumstances. If they're noticeable in your photos if obviously a different question.

I haven't looked for similar tests on Hasselblad lenses, and they may not be out there. These sort of tests are popular in digital photography because its simple and cheap to setup.

M. Axel Wikstrom · Apr 11, 2019

I think diffraction is where digital photography will soon hit a wall no matter how perfect the lenses become - at least in the smaller formats.

Eric Rose · Apr 11, 2019

If it looks good to you then that's all that matters.

warden · Apr 11, 2019

abruzzi said:
These sort of tests are pretty common with lens reviews on the web. Even a fair number of amateur reviewers I seen show enough detail to see the effect. For example this review of a Nikon Series E 100/2.8:

https://matthewdurrphotography.com/2012/07/01/lens-review-nikon-100mm-f2-8-series-e/

The first set of 100% crops at apertures 2.8-22. You can see that the sharpness falloff from 16 to 22 is noticeable in the testing circumstances. If they're noticeable in your photos if obviously a different question.

I haven't looked for similar tests on Hasselblad lenses, and they may not be out there. These sort of tests are popular in digital photography because its simple and cheap to setup.

Excellent, thank you. That's exactly what I'm after. I don't follow lens reviews but already this looks interesting.

Edit: The one place that I'm seeing an issue on my negatives is in the out of focus areas with dappled sunlight, which somehow looked harsh compared to smaller apertures that I've used. I see from the article you linked that the lens this reviewer tested was also showing unwanted detail in the "bokeh" at f/22. Something for me to think about.

warden · Apr 11, 2019

Eric Rose said:
If it looks good to you then that's all that matters.

Well, yes. In my case I think I'd need to print larger than I do to see problems. And even if I did the tradeoff is worth it for the extra depth of field. I'm just curious to learn more, especially with analog/Hassy experiences.

abruzzi · Apr 11, 2019

Interesting thing about diffraction--that image just release of the black hole in galaxy M-87? It's blurry because it is at the diffraction limit of a virtual radio telescope the size of the earth. We could get sharper pictures if we had a much larger set of telescopes.

https://www.wired.com/story/the-perplexing-physics-of-imaging-a-black-hole/

Dan Daniel · Apr 11, 2019

There are some interesting effects possible with focus stacking. Yes, a digital process, but can't imagine that a series of film files wouldn't allow for the same results. Of course that could be a ton of film, and depends on if what you shoot is still.

MattKing · Apr 11, 2019

There are equations out there .
One thing to understand though is that diffraction effects are related to the relationship between the actual physical size of the aperture (not the f/stop!) and the wavelength of light.
For that reason, lenses for larger formats exhibit less diffraction at, for example, f/22 than lenses for 35mm.

Kodachromeguy · Apr 11, 2019

Back in the antediluvian era (the film era), Leica recommended that you not close lenses down to smaller than approx. ¼ the focal length to avoid visible degradation from diffraction. So a 50mm lens should not be closed down to smaller than f/12.5 or f/11. Recall that most 50mm lenses (German as well as Japanese) only closed to f/16. Macro lenses were different because the manufacturers assumed depth of field was a more important criteria, so they often closed to f/22 or even f/32. The relationship probably held to medium format lenses, but I am not sure about large format.

spijker · Apr 11, 2019

MattKing said:
For that reason, lenses for larger formats exhibit less diffraction at, for example, f/22 than lenses for 35mm.

But for the same Depth of Field you need to go to a larger F-stop number with a larger format and you'll end up with the same physical aperture. Therefore, for the same DoF, a larger format setup will give you the same amount of diffraction degradation as a 35mm setup.

MattKing · Apr 11, 2019

spijker said:
But for the same Depth of Field you need to go to a larger F-stop number with a larger format and you'll end up with the same physical aperture. Therefore, for the same DoF, a larger format setup will give you the same amount of diffraction degradation as a 35mm setup.

That is correct.
The point of my post was to avoid paying close attention to the f/stop. You need to pay attention to the actual size of the aperture itself.
Depth of field isn't the only reason that people choose small apertures - sometimes it has more to do with choice of shutter speed.
And of course, the relationship between diffraction and aperture size becomes very important when we consider pinhole photography.

Ian Grant · Apr 11, 2019

Eric Rose said:
If it looks good to you then that's all that matters.

I totally agree. There's discussions on this with LF lens stopped down past f22, and then someone looked at John Sexton's published images where he gave f stop data and he often used f32 or even f45 which were supposed to be diffraction limited but he had superb results.

It's more a matter knowing your own equipment and that has to be by trial and error, there's theory and then real world practice. I have a 150mm Schneider lens with an aperture scale to f90, it's the original scale so where does Diffraction limitation kick in ?

Ian

M. Axel Wikstrom · Apr 11, 2019

abruzzi said:
Interesting thing about diffraction--that image just release of the black hole in galaxy M-87? It's blurry because it is at the diffraction limit of a virtual radio telescope the size of the earth. We could get sharper pictures if we had a much larger set of telescopes.

https://www.wired.com/story/the-perplexing-physics-of-imaging-a-black-hole/

Speaking with astronomers recently I think the size of the telescope (aperture) is limited to the diameter of Earth's orbit around the sun, plus movement of our solar system relative to the target.

astroclimb · Apr 11, 2019

While I have not (yet) read the technical papers on the black hole image, only the popular press to date, I understand they were doing (virtual) radio interferometry across the earth with the 'scopes they used. I doubt they were able to perform such interferometry over the 1/2 year it takes the earth to get to the "other side" of the sun. So the aperture limit would be the Earth's diameter. However, also in the popular press, they went to much-shorter-than-usual wavelengths for these 'scopes, which improves the diffraction limit. Because the interferometry was "virtual", I suspect a fair bit of the fuzziness in the image is also due to noise, etc., in the data recorded at each site, not "only" diffraction limit.

abruzzi · Apr 11, 2019

Yeah, I haven't looked in dept, but this video:

explained that continuous observation from radio telescopes during the rotation of the earth created a simulated antenna the size of the earth. I'm actually curious how accurate that is given that the rotation of the accretion disk it about 2 days, and you care taking readings over on quarter of that time (12 hours for a telescope moving from one side of the earth to the other.).

Nonetheless, whichever is right, its pretty darn cool.

RalphLambrecht · Apr 11, 2019

jawarden said:
Hi all,

Maybe there is already a thread that shows this, and if so I'll be grateful for a link that I have failed to find so far. I'm wondering if someone has done a test to illustrate where a lens is diffraction limited and what the performance hit actually looks like. For instance two identical images, one at f8 and the other at f22, compared for quality. I'm expecting some "fuzziness" or other issues when diffraction appears on the f22 image.

My specific project involves up close work with a Hasselblad, 60mm lens and 10mm extension. Very little is in focus when the objects are 8" from the lens so I'm trying to maximize depth of field with smaller apertures. Examples look good so far even at f22 (the max for this lens) and the prints won't be all that large anyway, but the subject has me curious about where I should be seeing problems: in the focused area, in the out of focus areas, everywhere?

I'm doing my own comparisons but to my eye I'm not seeing problems with this lens maxed out. So if you have experience, links, examples or comments please fire away.

I'm more than happy to offer the attached:

Maris · Apr 11, 2019

When balancing depth of field versus diffraction a rule of thumb with a subtle mathematical basis is:
The 3 millimetre rule.
This states that when the final picture size is an 8"x10" then the minimum lens aperture diameter can be as small as 3 millimetres before diffraction begins to take away visible detail. The rule applies to all lens focal lengths and film formats. A 50mm lens on 35mm can stop down to f16. My 600mm Apo-Nikkor working on an 8x10 camera can stop down to f200. I've done the comparative experiment and it looks like the rule works pretty well.
The final picture size is an critical factor. If I was making 16"x20" enlargements then the critical diameter becomes 6 millimetres. The more picture detail is enlarged the more diffraction effects are enlarged too. Can't repeal optical law, no free lunch, etc.

MattKing · Apr 11, 2019

Thanks Maris.
So for 11x14 prints, 4.5 millimetres should work
And f/22 should be fine for that 100mm Anaston lens on my Kodak Tourist.

RalphLambrecht · Apr 11, 2019

Maris said:
When balancing depth of field versus diffraction a rule of thumb with a subtle mathematical basis is:
The 3 millimetre rule.
This states that when the final picture size is an 8"x10" then the minimum lens aperture diameter can be as small as 3 millimetres before diffraction begins to take away visible detail. The rule applies to all lens focal lengths and film formats. A 50mm lens on 35mm can stop down to f16. My 600mm Apo-Nikkor working on an 8x10 camera can stop down to f200. I've done the comparative experiment and it looks like the rule works pretty well.
The final picture size is an critical factor. If I was making 16"x20" enlargements then the critical diameter becomes 6 millimetres. The more picture detail is enlarged the more diffraction effects are enlarged too. Can't repeal optical law, no free lunch, etc.

I wasn't aware of this ryle but, it seems to match the smallest available f/stops of many lenses.

jim10219 · Apr 11, 2019

The only time I worry about diffraction is when enlarging negatives or focus stacking digital macro photography. In reality, the areas out of focus will look worse than the same areas in focus yet hampered by diffraction. So to get the best photo, just set your aperture to the maximum opening that will still get the appropriate depth of field, and let the cards fall where they may. If the image is soft due to diffraction, then you weren't going to be able to capture that image sharply anyway.

Ian Grant · Apr 12, 2019

My 360mm f9 Apo Ronar has an F stop Aperture scale as well as the Aperture diameter in mm, the smallest marked setting is 4mm which is f90.

The aperture is easy to calculate in mm as it's simple maths - Focal length divided by the F stop = Aperture diameter in mm.

Ian

trendland · Apr 12, 2019

jawarden said:
Hi all,

Maybe there is already a thread that shows this, and if so I'll be grateful for a link that I have failed to find so far. I'm wondering if someone has done a test to illustrate where a lens is diffraction limited and what the performance hit actually looks like. For instance two identical images, one at f8 and the other at f22, compared for quality. I'm expecting some "fuzziness" or other issues when diffraction appears on the f22 image.

In general (guess you'll know ) a lens at "max open aperature" let's say at 2.0 will normally have a real profit (more resolution in lines per mm/or MTF) from "closing" the lens 2.8, 4 , 5,6 , a.s.o.!
EXEPTION : very good and max. corrected AND real expensive lenses from type Leica and others
(guess most modern Hassi lenses you also can count to that types)!
There you can reach max. resolution with max. open lens! But it is dependable!
In all cases (from my point) you can higher the performance a little bit example :
Zeiss lens at about 1.2 has extreme resolution but at 1.4 you can maximal it a bit ~ 9,5% and with 2.0 again 3,5 % and this is it (at 4.0 the profit in direction of better characteristics isn't real messurable (+ 0,967 for example) but it is enought to define then the sweetspot for this lens at 4.0 = best characteristcs you can get with this lens!
What will happen with this Zeiss (it is just an example to make clear the general mechanism - so be aware of to unterstand the mentioned datas precise) - what will happen with the Zeiss at 5.6?
In best case you will have identical characteristics (no messurable difference) in fact (in reality) you will louse a bit of resolution (lets say 0,856%) - because of what mechanism?
Because of DEFRACTION at 5.6 ! REALY ? Yes realy (because the definied sweetspot is INDEED at
4.0) What will happen at f8 ? AND NOW LISTEN IT IS NOT SO MUCH COMPLICATE TO UNDERSTAND BUT HERE IS THE CENTRAL KEY TO YOUR QUESTION :
THE DEFRACTION WILL PROCEED IN A FORM OF A LOG. FUNCTION!!!!
So at f8 you can louse - 3,9% resolution, f11 - 5,9%, f 16 - 12,2 % f22 - 31% f32 - 49% f 45 -68%!
If your lens has no good characteristics your sweetspot can be just at 5,6 a bad lens from old type may have it at f8! But a bad lens may have a higher log witthin DEFRACTION (from my point)!
With Zeiss for Hassi it will not be the last mentioned case = no bad lenses at all!
I noticed real unsharpness at f22 and wonder about (many years ago) and that was
DEFRACTION - NOTHING ELSE!

THE NEXT PARAMETER TO DISCUSS (IT WOULD HELP YOU NOT VERY MUCH FOR YOUR WORKFLOW IN CONCERN OF DEEP OF FIELD SHARPNESS) WOULD BE : HYPERFOCAL DISTANCE!

with regards

Dan Fromm · Apr 12, 2019

Maris said:
When balancing depth of field versus diffraction a rule of thumb with a subtle mathematical basis is:
The 3 millimetre rule.
This states that when the final picture size is an 8"x10" then the minimum lens aperture diameter can be as small as 3 millimetres before diffraction begins to take away visible detail. The rule applies to all lens focal lengths and film formats. A 50mm lens on 35mm can stop down to f16. My 600mm Apo-Nikkor working on an 8x10 camera can stop down to f200. I've done the comparative experiment and it looks like the rule works pretty well.
The final picture size is an critical factor. If I was making 16"x20" enlargements then the critical diameter becomes 6 millimetres. The more picture detail is enlarged the more diffraction effects are enlarged too. Can't repeal optical law, no free lunch, etc.

Hmm. So if I hang a 35 mm Nikon behind my 610/9 Apo-Nikkor and stop the lens all the way down to f/128 (as far as it goes) I'll be able to get a good 8x10 print?

This seems impossible. At f/128, the diffraction limit is approximately 1500/128 = 12. So I'll have 12 lp/mm at low contrast on my 24x36 piece of film. Enlarged 8x to make an 8x12 print that crops to 8x10, I'll have 1.2 lp/mm at low contrast in the print. Fuzzy, fuzzy.

Reconsider y'r rule of thumb. It seems absurd.

Might work for contact printing an 8x10 negative, except for the low contrast.

trendland · Apr 12, 2019

Kodachromeguy said:
Back in the antediluvian era (the film era), Leica recommended that you not close lenses down to smaller than approx. ¼ the focal length to avoid visible degradation from diffraction. So a 50mm lens should not be closed down to smaller than f/12.5 or f/11. Recall that most 50mm lenses (German as well as Japanese) only closed to f/16. Macro lenses were different because the manufacturers assumed depth of field was a more important criteria, so they often closed to f/22 or even f/32. The relationship probably held to medium format lenses, but I am not sure about large format.

I never noticed visibly DEFRACTION before I first identified it.... it is a real need to identify
your enemy in photographers workflow before you are able to eliminate !
In parts I agree what you stated here!

with regards

Diffraction limited performance

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