[Fat Albert]

Why do lenses get sharper when stopped down? Is it because stopping down increases DOF (which doesn't sound like the right answer to me), or are there other optical reasons for this phenomenon?

[goubejp]

No it is ony because by stopping down you use the center part of the optical elements and you reduce aberrations

[vonBaloney]

*REALLY* good lenses are just as sharp (or even sharpest) wide-open, reportedly. So say the folks with the best Leica lenses, anyway.

[Ole]

As goubejp said.

And when you stop down "too far", then image quality reduces again due to diffraction. Diffraction is due to light hitting and being deflected by the aperture blades. As apertures get smaller the relative amount of scattered light to the "good light" increases and diffraction becomes more visible.

[Canada_Rockies]

If you have a Pentax lens with an A setting on the aperture dial, you can set your program line to MTF and Mr. Pentax will tell you what he thinks is the sharpest aperture. A rule of thumb often used is that a lens is sharpest stopped down two stops. Use an f/1.4 lens at f/2.8, for example. I find that even my old M 400/5.6 follows this rule quite closely. I get much better aberration control at f/11 than wide open. Of course, I do have some problems with 8 second shutter speeds ....

[Just1MoreDave]

The aperture blades block light as they are closed. The light rays that get blocked tend to be at odd angles and the lens edges, so what gets through are rays that look more like the lens designer first drew, ones that go through the elements more predictably.

[Digitalis]

REALLY* good lenses are just as sharp (or even sharpest) wide-open, reportedly. So say the folks with the best Leica lenses, anyway.

Most Leica lenses still gain from being stopped down a stop, SLR lenses typically are at their best at two stops down from their widest aperture. There are some pentax lenses like the Pentax FA31mm f/1.8 that are impressively sharp at f/1.8.

But stopping down too much has its drawbacks, Diffraction:


100% crops, un-sharpened from the Sigma 8-16mm f/4.5-5.6 ASPH on a Pentax K5IIs

[tim60]

Most Leica lenses still gain from being stopped down a stop, SLR lenses typically are at their best at two stops down from their widest aperture. There are some pentax lenses like the Pentax FA31mm f/1.8 that are impressively sharp at f/1.8.

But stopping down too much has its drawbacks, Diffraction:


100% crops, un-sharpened from the Sigma 8-16mm f/4.5-5.6 ASPH on a Pentax K5IIs

Very interesting to see the impact of diffraction in a side-by-side. Sorry about the site though, used to be the best cricket gorund in the world, in the days of the members where the white tent thing is and the adjacent George Giffen stand. Used to work with a descendent of George. And George's brother's test cricket career was ended witht he loss of fingers in an industrial accident at the Brompton Gas Works in the 1880's. Were you half way up the hill from the cathedral (of cathedral end fame) towards Aquinas College?

[Digitalis]

Were you half way up the hill from the cathedral (of cathedral end fame) towards Aquinas College?

no, I was standing right on the top of that hill.

[bdery]

*REALLY* good lenses are just as sharp (or even sharpest) wide-open, reportedly. So say the folks with the best Leica lenses, anyway.

Coma, vignetting and chromatic aberration will always be worse on the sides of an optical element. Even when Leica is written on it.

[nightfly]

Good lenses can be sharp wide open, but focus is critical at longer focal lengths. I used the Pentax SMC 67 105mm f/2.4 on my K-01 to produce amazing wide-open aperture performance. Granted, I am using the lenses sweet spot as this lens was designed for 6x7 frame coverage. On my Pentax 6x7 the outer frame would have exhibited coma, vignetting, and other aberrations. A 105mm f/2.4 lens made for a cropped sensor, or even for 35mm full frame might not have produced such a result as below.

[fb_penpho]

The site Welcome to Photozone! has tests on many lens. In general in the centre of the lens MTF does not change a lot. At the edges of a lens MTF will change much more, especially if it is a fast (eg f1.4) lens.

[normhead]

The one thing you have to remember in all this, is that even though Diffraction may cost you sharpness in huh f-stop image, nothing causes more loss of sharpness, than the effect that creates narrow Depth of Field in a wide open lens. Sites like photo zone concentrate on absolute sharpness in a given plane. But many times even though the lens may be sharpest at ƒ5.6 the overall sharpness of a 3D image may be improved by stopping down, sometimes to as much a ƒ22, if you are trying to keep your whole subject in focus. Good lenses can be sharp wide open, but they can't provide the amount of print area in acceptable focus a stopped down lens can.

SO the issue is not just what is the sharpest focus, but also what is acceptable focus.

DoF so narrow you almost can't find it, but great bokeh (out of focus areas). The subject is mostly out to focus because of the narrow depth of field, and that can be really irritating.
ƒ2.8


ƒ5.6 the sharpest resolution for this lens, but still not a lot of depth of field.


ƒ11… much wider DoF and more of the subject in focus. The bokeh is starting to get messy... in a commercial image where there my be text placed in the OoF areas, that may become a problem.


ƒ22 even the little bits of dirt in the foreground are in focus… very wide Dof, out of focus areas have become pretty messy. Noticeable loss of sharpness compared to even ƒ11 comparing the sharpest areas, but is much sharper than ƒ11 in out of focus areas, with some portions of the image that are soft at ƒ11 being in acceptable focus @ ƒ22, even if they aren't razor sharp. Bokeh is becoming distracting.


So how you define sharpness in the 3D world is very differential from how you define sharpness on a test chart. IN the real world, ƒ22 is a real option. Not so much on a test chart. But it's still about sharpness.

I find what I end up thinking of as the sharpness, is often the best compromise between absolute sharpness and a DoF that matches the subject matter. And that really isn't predictable using any kind of formula I know.

IN terms of the original question, you need to see a lens diagram….
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The wider the area you collect light from, the bigger your circle of confusion will be in your out of focus areas. A pinhole camera has infinite D0F but it's image is greatly limited by diffraction, because the pin hole is very small, so light isn't being collected from a wide surface but diffraction is a problem. Using a lens theoretcial sharpness exists only at the "image plane". But in the areas in front of and in back of the image plane the image may be in acceptable focus, because the circles of confusion are so small, they are judged by the eye to be "sharp".Theoretically any lens below the point at which diffraction starts to degrade the image should be equally sharp if the images is in sharp focus from it's widest to it's narrowest aperture setting. (APS-c is Aperture limited after 5.6) However practically it's very difficult (and expensive) to do that. Most lenses have a sweet spot, somewhere around ƒ5.6 that is the best combination of sharpness based on a smaller aperture, and before diffraction becomes an issue. Although I find diffraction doesn't really become and issue until after ƒ11 if the bokeh is OK.

[6BQ5]

normhead,

Thanks for the great explanation and the very detailed post!!

[normhead]

No problem....