Size of aperture as it affects sharpness

Trask · Aug 31, 2017

I couldn't come up with a subject line that captured my line of inquiry -- but here's the question.

We've all seen lens test charts that shows that as, for example, that as an f/1.4 lens is stopped down, its performance improves. So one might prefer to shoot at f/5.6 or f/8 for improved sharpness. Fine and good. But what if the maximum aperture of the lens in question is f/5.6, would we expect that lens to be at or near optimum performance because it is at f/5.6 even though it is wide open? Or would we expect that an f/5.6 lens would offer improved performance at f/16 (where perhaps diffraction might kick in)? An example: I'm considering two different medium format lenses, both 65mm, one f/5.6 and the other f/8. The f/5.6 lens provides a one-stop speed advantage, but at a significant cost. IF it is the case that the f/5.6 lens would be a bit sharper at f/8, maybe I'd wind up using it at f/8 all the time, in which case I might as a well buy the f/8 lens.

Perhaps it's the case that it's immaterial whether the lens is f/5.6 or f/8, as either is "middle range aperture" in terms of a medium format lens. Thoughts?

Prof_Pixel · Aug 31, 2017

It seems to me the general rule of thumb is that the best sharpness is two or three stops down from the maximum aperture.

Kodachromeguy · Aug 31, 2017

In the old days (meaning the traditional film days), a general rule was that most lenses performed best at about 2 stops closed down from maximum aperture and then approximately equally well for another 2 or 3 stops. So, for example, a Rolleiflex 3.5 Xenotar was excellent at f/5.6 to f/8.0 to f/11.0. Diffraction was generally considered to become a problem at about 1/4 of the focal length, so the 75mm Xenotar would start to deteriorate at about f/16. Whether you really saw it at f/22 is questionable, but that was the general rule. In your situation, do you need the f/5.6 lens to get a brighter viewfinder screen image? If so, go with that one and just stop down a bit. Cheers,

MattKing · Aug 31, 2017

I have a Kodak Tourist with a 105mm f/9 lens (IIRC). I doubt that stopping it down makes much of a difference.

piu58 · Sep 1, 2017

It depends on the construction of the lens. It is possible to have lenses with the best sharpness wide open (= least diffraction effects, "diffraction limited). Astronomical lenses are constructed in this way, but only for a tiny field. For most camera lenses a wide field combined with a large aperture should be obtained, for which sharpness is sacrificed.
Lenses for reproduction are usually sharpest wide open, many don't even have a stop (beyond the aperture which is a stop too).

TheRook · Sep 1, 2017

I think piu58 is quite correct; it depends on the construction of the lens at hand.

However, I strongly believe one shouldn't compromise the preferred depth of field or shutter speed for the sake of an extra bit of sharpness (or at least what one assumes will yield a sharper image). There are other things that matter, and oftentimes matter a great deal more than image sharpness.

Dan Fromm · Sep 1, 2017

All lenses suffer to some extent from off-axis aberrations that are reduced by stopping down. See, e.g., https://web.archive.org/web/20170628132157/http://toothwalker.org/optics.html This is why stopping down improves image quality off-axis.

Diffraction depends on the relative aperture. At some point, stopping down loses more image quality to diffraction than is gained from reducing off-axis aberrations. Where this happens is very lens-specific.

OP, if you want to find out what stopping your lenses down will accomplish, take some test shots.

Pioneer · Sep 1, 2017

I can't remember ever having shot a large format lens wide open before. I guess I'll have to try it out. I think depth of field would be so narrow as to be almost impossible to work with, no matter how sharp the lens was.

I consider the maximum aperture to be there for focusing. F/5.6 will be easier to focus with than f/8. Particularly when the light gets low.

I don't think that either maximum aperture by itself is the defining factor in lens sharpness. You may have to stop an f/8 lens down less than you stop down an f/5.6 lens to get to the sharpest apertures but that will be more dependent on lens construction and glass type then aperture.

Sirius Glass · Sep 1, 2017

Prof_Pixel said:
It seems to me the general rule of thumb is that the best sharpness is two or three stops down from the maximum aperture.

The sweet spot for sharpness is usually

f/8 for 35mm
f/11to f/16 for MF
f/16 to f/22 for 4"x5"

Dali · Sep 1, 2017

piu58 said:
It depends on the construction of the lens. It is possible to have lenses with the best sharpness wide open (= least diffraction effects, "diffraction limited). Astronomical lenses are constructed in this way, but only for a tiny field. For most camera lenses a wide field combined with a large aperture should be obtained, for which sharpness is sacrificed.
Lenses for reproduction are usually sharpest wide open, many don't even have a stop (beyond the aperture which is a stop too).

Correct. to be convinced, use 2 telescope with different mirror diameters. High chance the bigger might give you the best image. If it were not true, professional observatories would all be equipped with 4in. refractors...

WilmarcoImaging · Sep 1, 2017

The Nikkor W 150/5.6 on 4x5 wide open has great bokeh. Good for still life, portraits, flowers, etc. Thin DOF but you know that, and take it into account, when previsualizing the image.

Sirius Glass · Sep 1, 2017

Dali said:
Correct. to be convinced, use 2 telescope with different mirror diameters. High chance the bigger might give you the best image. If it were not true, professional observatories would all be equipped with 4in. refractors...

And engineer at Kodak had a telescope with a 40" mirror and was grinding a 60" mirror when I worked there.

Patrick Robert James · Sep 1, 2017

Sharpness is relative. Of course a lens will usually be the sharpest a couple of stops down, but you may make your subject appear sharper by making everything else less sharp.

Hatchetman · Sep 1, 2017

I'm not an lens engineer or scientist of any sort. my mathematical instinct (what little I have) hunch is that a spherical aperture would give you the optimum sharpness at the optimum width. like if you have a five blade aperture the aperture would have different radii at different points. only one radius aperture would be optimum, no?

Theo Sulphate · Sep 1, 2017

Dali said:
Correct. to be convinced, use 2 telescope with different mirror diameters. High chance the bigger might give you the best image. If it were not true, professional observatories would all be equipped with 4in. refractors...

Telescope requirements are different: it's not sharpness, but light-gathering ability in order to see faint stars, nebulae, &c. Regardless of mirror or main objective size, sharpness is probably hampered mostly by our atmosphere: equivalent to looking through 30 feet of water. Hence the Hubble platform.

Dali · Sep 1, 2017

Theo Sulphate said:
Telescope requirements are different: it's not sharpness, but light-gathering ability in order to see faint stars, nebulae, &c. Regardless of mirror or main objective size, sharpness is probably hampered mostly by our atmosphere: equivalent to looking through 30 feet of water. Hence the Hubble platform.

Well, for faint deep sky objects, you are right but you can easily verify the increase in image quality is correlated with the diameter with planets. I highly doubt you can see as many details on Jupiter with a 4" scope than you can with a 16" light bucket even under crappy atmospheric conditions (and small scope are more immune than big guns to less than perfect conditions).

Sirius Glass · Sep 1, 2017

Hatchetman said:
I'm not an lens engineer or scientist of any sort. my mathematical instinct (what little I have) hunch is that a spherical aperture would give you the optimum sharpness at the optimum width. like if you have a five blade aperture the aperture would have different radii at different points. only one radius aperture would be optimum, no?

Do you mean a circular aperture? As the aperture decreases the sharpness increases and the diffusion increases. At some point the diffusion starts to dominate. Hence the results sighted in post # 9.

cliveh · Sep 1, 2017

"Sharpness is a bourgeois concept" - Henri Cartier-Bresson

Hatchetman · Sep 1, 2017

Sirius Glass said:
Do you mean a circular aperture? As the aperture decreases the sharpness increases and the diffusion increases. At some point the diffusion starts to dominate. Hence the results sighted in post # 9.

yes, circular. so at f8 would a circular aperture be sharper than a square one with the same glass?

Sirius Glass · Sep 1, 2017

cliveh said:
"Sharpness is a bourgeois concept" - Henri Cartier-Bresson

Zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz ..............................................................

One really big yawn.

DREW WILEY · Sep 1, 2017

It varies both by the type of lens and the focal length relative to the format. The character or look of what is out of focus is a slightly different subject. Either way, just take a roll of film and shoot a fence or something like that at each aperture and look at the result under a loupe.

DREW WILEY · Sep 1, 2017

Apo Nikkor process lenses actually exist which give you a choice of multi-bladed circular aperture, a square aperture, or waterhouse stops. This is related to optimum dot shape reproduction in offset printing. Or maybe they just wanted a lens which they could market both to Damien Hirst and you Hasslebland squares!

M Carter · Sep 1, 2017

All of these "rules of thumb" don't really give an adequate answer. There are millions of lenses out there, and they're all different.

If you want to know, do a test. Get a test chart or pin up some printed pages and do a close test, then go outside and shoot a detailed building or something. Use a tripod, get every aperture, take notes.

It's an unanswerable question (my .02 anyway) unless you test the lens itself, and then you have the answer.

RalphLambrecht · Sep 1, 2017

Trask said:
I couldn't come up with a subject line that captured my line of inquiry -- but here's the question.

We've all seen lens test charts that shows that as, for example, that as an f/1.4 lens is stopped down, its performance improves. So one might prefer to shoot at f/5.6 or f/8 for improved sharpness. Fine and good. But what if the maximum aperture of the lens in question is f/5.6, would we expect that lens to be at or near optimum performance because it is at f/5.6 even though it is wide open? Or would we expect that an f/5.6 lens would offer improved performance at f/16 (where perhaps diffraction might kick in)? An example: I'm considering two different medium format lenses, both 65mm, one f/5.6 and the other f/8. The f/5.6 lens provides a one-stop speed advantage, but at a significant cost. IF it is the case that the f/5.6 lens would be a bit sharper at f/8, maybe I'd wind up using it at f/8 all the time, in which case I might as a well buy the f/8 lens.

Perhaps it's the case that it's immaterial whether the lens is f/5.6 or f/8, as either is "middle range aperture" in terms of a medium format lens. Thoughts?

it's a compromise in finding the 'sweet' spot.as you stop down lens aberrations are better controlled and the image becomes sharper but beyond certain point, the sweet spot, diffraction takes over and the image becomes more fuzzy again.every lens is different but the sweet spot is usually between f/8-11 for 35mm and MF lenses. You have to try it outs, buy with the full right to return only.

Alan Edward Klein · Sep 1, 2017

When shooting landscapes with MF, I put the DOF i want to achieve first and ignore diffraction and lens sharpness. In fact, I'll stop down one additional stop to make sure the DOF is covered. I'm using Mamiya RB67. Am I wrong?

Size of aperture as it affects sharpness

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