Lens diffraction - truth or urban legend?

[tlitody]

Diffraction is always present. It is caused by the aperture edges which cause light to bend as it passes by them. Aperture diffraction increases as lens is stopped down. There is a point where it becomes more significant than other lens apertures.
But there are always other aberations in a lens too. So optimum aperture is a compromise between difffraction caused by aperture and other abberations. In 35mm format it is usally around f5.6 to f8. There are lenses for 35mm which perform best at wider apertures and some that perform best at smaller apertures. Each lens has a sweet spot where the minimum total abberationstake place(not the same as minimum diffraction which is always wide open). It is worth testing for this which is quite easy to do. Just make the same image at different apertures. You can actually see the difference. If using wet printing then do the same in the darkroom with your enlarger lens. i.e. print each image at the same aperture and pick the one which is sharpest (not DOF but sharpest in center). Then you know your camera lens sharpest aperture. Then take negative and print it at each enlarging lens aperture and pick the sharpest result. You will then know enlarging lens sharpest aperture.
When you combine your camera lens sharpest aperture with your enlarger lens sharpest aperture the difference is very obvious. Most people never bother to do this and wonder why their images are never as sharp as they think they should be. But you are then limited to dof obtained by camera lens sharpest aperture but you can always use enalrger lens sharpest aperture so its worth doing the test and can choose when to use camera lens sharpest aperture over using max or min dof.

Note: Large format lenses tend to have more of other abberations because of bigger elements. And LF lenses cannot reach the resolution of smaller lenses for 35mm cameras but then you don't the need the same enlargement so you can obtain a higher print resolution for the same size print with LF because of reduced enlargement factor. Again a LF lens will have a sweet spot you should test for sharpness so you know where it is and can choose when tou use it over dof.
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[ampguy]

If you have a really good lens, line prs per mm is theoretically best wide open:

you can measure it, and you can see it. If you are seeing more line prs per mm stopped down, and are accurately measuring, consider upgrading lenses.


f/ line pairs per mm

1.4 1,100
2 800
2.8 565
4 400
5.6 283
8 200
11 141
16 100
22 71
32 50

 

[semilog]

Roland, you might want to clarify that you shoot FF (and b/w mostly?)

Reds become diffraction limited soonest. Most everyone who has posted a u4/3 image with reds, I can see the diffraction.

Yeah, that 17% difference in image height between Canon APS-C and m4/3 is huge. 🙄

 

[semilog]

If you have a really good lens, line prs per mm is theoretically best wide open:

you can measure it, and you can see it. If you are seeing more line prs per mm stopped down, and are accurately measuring, consider upgrading lenses.

Very, very few *real* lenses (even expensive ones) are diffraction-limited wide open. Instead, real lenses are generally limited by optical aberrations at their widest apertures. This is especially true of fast lenses, which are particularly difficult to correct. For this reason, even the very best lenses are usually at their optimum when closed one or two stops.

See my post above, and the link to the review of the 150mm Zuiko for an example, with one of the very best lenses currently available.

 

[Sparrow]

So my 12mm CV, with a maximum aperture of f5.6 is compromised all across it's f-stop range?

 

[bigeye]

Gen. Barnard and his staff stopped at a potential crossing point on the Chickahominy River in pursuit of Confederate cavalry. Barnard muttered, "I wish I knew how deep it is." Custer heard this and spurred his horse out to the middle of the river, turned and shouted, "This is how deep it is, Sir!"

It might take a minute or 2 to test actual defraction with lens in a digital camera?

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[Sparrow]

To go back to the original question ... a good photo will still be a good photo even if it exhibits diffraction, and a poor photo will remain so however well it is technically crafted

Joe-public cares not for such things ...

 

[Juan Valdenebro]

Gen. Barnard and his staff stopped at a potential crossing point on the Chickahominy River in pursuit of Confederate cavalry. Barnard muttered, "I wish I knew how deep it is." Custer heard this and spurred his horse out to the middle of the river, turned and shouted, "This is how deep it is, Sir!"

It might take a minute or 2 to test actual defraction with lens in a digital camera?

.

And film and sensors' capture abilities are different too... On 35mm film I avoid f/32, f/22 and f/16, but f/11 and wider aperture values, are fine...

Cheers,

Juan

 

[ampguy]

That's what

That's what

she said :angel:

Yeah, that 17% difference in image height between Canon APS-C and m4/3 is huge. 🙄

 

[semilog]

To go back to the original question ... a good photo will still be a good photo even if it exhibits diffraction, and a poor photo will remain so however well it is technically crafted

Joe-public cares not for such things ...

It depends on what you plan to do with the image. For some applications it matters, for others it does not.

Remember that diffraction attacks overall image contrast, not just some abstract lp/mm value.

If you shoot landscapes using cameras with small sensors (FF DSLRs and 35 mm film are small, and APS-C/micro43 are smaller), then detail actually does matter, and you often need to balance DOF with degradation due to diffraction. This is even more true if you're using a truly tiny sensor like that in the G11. Same goes for still life photography.

I would suggest that to spend the sums that some here spend on their cameras and lenses (presumably, to achieve higher image quality), and then to pretend that diffraction does not matter, and to use those cameras and lenses under conditions that reduce their performance to that of a Canon Rebel with the kit zoom, might be pathological.

 

[semilog]

So my 12mm CV, with a maximum aperture of f5.6 is compromised all across it's f-stop range?

On a sufficiently high-resolution sensor or film, yes. For practical photography, even with a good ISO 100 film and a tripod, no.

 

[ferider]

I would suggest that to spend the sums that some here spend on their cameras and lenses (presumably, to achieve higher image quality), and then to pretend that diffraction does not matter, and to use those cameras and lenses under conditions that reduce their performance to that of a Canon Rebel with the kit zoom, might be pathological.

So what degrades an image more: diffraction, use of UV protection filters, or proudly hand-holding down to 1/8 of a sec ? :angel: 🙂

 

[Sparrow]

On a sufficiently high-resolution sensor or film, yes. For practical photography, even with a good ISO 100 film and a tripod, no.

Ah ... so the answer to the OP's question is? urban myth?

this one would also be f16 on a 12mm so 0.75mm aperture well into the range

 

[ampguy]

Hi Stewart

Hi Stewart

Maybe a nice photo, but a lot of resolution reduction noise like stuff going on in the yellows here.

The photons are hitting the edges of the aperture blades and splattering across the film or sensor instead of going straight onto the medium.

It's like peeing, but missing the center of the bowl, it splatters.

Ah ... so the answer to the OP's question is? urban myth?

this one would also be f16 on a 12mm so 0.75mm aperture well into the range

 

[Sparrow]

Maybe a nice photo, but a lot of resolution reduction noise like stuff going on in the yellows here.

The photos are hitting the edges of the aperture blades and splattering across the film or sensor instead of going straight onto the medium.

It's like peeing, but missing the center of the bowl, it splatters.

Well that's fine, I'll concentrate on taking nice photos then, and leave you to keep an eye on the facilities ... btw I see no noise in a 12x18 print of that neg

 

[PKR]

Diffraction is physics. It depends on the aperture's actual dimension (i.e. diameter as measured in millimeters). A tiny lens in a P&S will be aperture limited to say f8 where diffraction is already a problem. A large format lens is much bigger because of the large film size, and f8 on that lens is a sizeable opening, so no discernible diffraction. They can go down to something like f64. Medium format is somewhere in between.

Some go to f 90 and smaller.

All math aside, the only way I will characterise (characterize) a lens is through testing. Lenses, regardless of design and manufacture, will vary from lens to lens. Test your lenses in real world situations. Get to know them. If you drop a lens, even on a fairly soft surface, where no visual damage is apparent, re-test the lens. I have seen vast differences in identical lenses. Nikon lenses with "floating elements" are subject to loss of sharpness, from mild shock. If you have one of these great, wide angle lenses, pack it with some good padding.

 

[Nikkor AIS]

Nikkor 8mm 2.8 AIS @F22 on D3

There is no doudt that diffraction is a reality. The thing is, sometimes the loss of sharpness at F22 (on 35mm) is a worth while trade off for the depth of field and ghost images of the aperture ring in the image. These dandelions where touching the front element.

 

[Ronald M]

Light is wave motion and acts like a stone in water.

It actually bends around corners like water does. When the aperture is small, all the image forming rays are bent. When the aperture is large, only a small percentage is bent thus having less effect.

 

[Doug]

From what I've read, the Heliar 12mm is at its best wide open at f/5.6... but that would be something to test on one's own lens.

The amount of light affected by diffraction varies with the circumference of the aperture opening, since as said the effect comes from light bending around the edges. So you'd think a wide aperture would have more diffraction, right? True that more light rays are bending, due to the large circumference, but they're overwhelmed by the non-diffracted light coming in the rest of the opening. As the aperture opening gets smaller, the circumference gets smaller in a linear relationship to the diameter, while the area available to non-diffracted light gets smaller faster, relating to the square of the aperture diameter. So at small apertures a larger proportion of the total light coming through is affected by diffraction than at large openings.

This is essentially independent of focal length or relative aperture (f/stop), and is just a matter of the physical diameter of the opening. A one-inch diameter opening has a certain amount of diffraction, while a 1/4-inch opening has more. Long lenses have larger openings for a given relative aperture, so diffraction effects are seen at higher-numbered f/stops than with short lenses.

 

[tlitody]

Diffraction is physics. It depends on the aperture's actual dimension (i.e. diameter as measured in millimeters). A tiny lens in a P&S will be aperture limited to say f8 where diffraction is already a problem. A large format lens is much bigger because of the large film size, and f8 on that lens is a sizeable opening, so no discernible diffraction. They can go down to something like f64. Medium format is somewhere in between.

I think you'll find that the term "diffraction limited" is b******t. All lenses for cameras that you or I use have diffraction. The need to close down to get your sharpest images is not because of diffraction but rather because closing down removes a large percentage of other aberations until the point where diffraction becomes more significant than the lens abberations. That is not a limitation of diffraction but rather the limit of the lens design which has nothing to do with the aperture. The aperture is just a hole. The lens design is what introduces lens abberations. If the lens design was good enough then you could always shoot wide open and get the fantastic sharpness and resolution. But lenses like that are very very very expensive to make. Or to put it another way, smaller apertures hide lens design faults to a point.

note: If you want max dof or min dof then you have to compromise on max sharpness in the focus plane. There is no way around that.