Effect of Aperture on Sharpness

Peeb · Sep 30, 2015

Was looking for a DOF app for iOS when I stumbled upon this bit of wisdom previously unknown to me: an increase in aperture value increases DOF but slightly DECREASES focal sharpness.

E.g.: going from wide open to f/16 will increase the width of your in-focus depth but decrease your max focus clarity.

Could this be so? Had never heard this.

KmH · Sep 30, 2015

Lenses have a 'sweet spot' of focus sharpness that is usually a couple to a few stops wide.
Each make/model of lens has it's 'sweet spot' range of mid-size lens apertures.

Few fast lenses are as sharp as they can be when used wide open. Stopping a fast lens down 2 or more stops usually shows an improvement in focus sharpness.
At the other end, as lens aperture gets smaller diffraction starts to degrade focus sharpness.
Again each make/model of lens will start loosing focus sharpness at somewhat different smaller apertures.

WayneF · Sep 30, 2015

Peeb said:
Was looking for a DOF app for iOS when I stumbled upon this bit of wisdom previously unknown to me: an increase in aperture value increases DOF but slightly DECREASES focal sharpness.

E.g.: going from wide open to f/16 will increase the width of your in-focus depth but decrease your max focus clarity.

Could this be so? Had never heard this.

Yes, it is correct. Diffraction increases as the lens is stopped down, reducing sharpness.

And yes, depth of field does increase then too.

We don't always need maximum depth field, so in most pictures, we can easily stay around the best compromise, often maybe f/5 to f/10....

But other times, the depth of field helps more than diffraction hurts.

Perhaps we do ordinarily need a good reason to stop down "excessively"

but sometimes Depth of Field is that exceptional reason when it is definitely needed.

Keep your bag of good tricks full.

Some samples of this at Diffraction limited images? Really?

See the animated images of it.

runnah · Sep 30, 2015

F/5.6-f/8 are normally the sweetest of spots for a majority of lenses.

Peeb · Sep 30, 2015

If you're focusing on far-away subjects (mountains, e.g.) so that the focus is set to infinity, wouldn't wide open be the sharpest then?

Ysarex · Sep 30, 2015

Peeb said:
If you're focusing on far-away subjects (mountains, e.g.) so that the focus is set to infinity, wouldn't wide open be the sharpest then?

No. Wide open the lens suffers from various problems like spherical aberration, astigmatism, coma, field curvature, etc. Most of these are dramatically cleared up by stopping down one or two stops. Lens aperture use and sharpness are the perfect example of moderation as the best choice -- avoid extremes.

Joe

KmH · Oct 1, 2015

Slow lenses, or lenses that have a maximum aperture of f/3.5 or f/4 often do not need to be stopped down much, if any.

Stopping down is for fast lenses, say in the the f/1.o (or less) to f/2.8 range.

Ysarex · Oct 1, 2015

KmH said:
Slow lenses, or lenses that have a maximum aperture of f/3.5 or f/4 often do not need to be stopped down much, if any.

Stopping down is for fast lenses, say in the the f/1.o (or less) to f/2.8 range.

They're pre-stopped down by the manufacturer who opted to cheap-out and very likely they're cheaped-out across the board.

Joe

Derrel · Oct 1, 2015

runnah said:
F/5.6-f/8 are normally the sweetest of spots for a majority of lenses.

The lens test sites and their charts and graphs tend to very strongly bear this out. Those two f/stop settings on many,many lenses are where the absolute peak optical performance is realized on a majority of lenses. I would say the vast majority of lenses are at their best in terms of sharpness and contrast and overall optical performance at those two specific settings. it is around f/5.6 or f/8 where many,many lenses see the far corners and the outer edge areas "catch up with the center" to the highest degree, and where contrast seems to be the highest. Still--there are a LOT of shooting situations where it's really much more critical to have the lens stopped down to get deep depth of field, which really overrides all the technical B.S. if deep DOF is what is required to make a good picture in a single click of the shutter.

Braineack · Oct 1, 2015

typical sharpness chart:

here's one for a m4/3 lens:

Solarflare · Oct 1, 2015

Peeb said:
Could this be so? Had never heard this.

Err yes.

You can make lenses sharper and sharper by introducing stricter and stricter tolerances and adding more and more elements into the optical formula. Until you have reached huge lenses that can focus so precisely they can be used in computer chip production: diglloyd: A Visit to Zeiss, and back (2nd photo from the top).

However, if you instead stop a lens down, the possible ways of light from the subject before they hit the target spot get smaller and smaller, i.e. lens errors are masked more and more. This battles with the effect of diffraction.

Fuji claims they have algorithms to reduce resolution loss from diffraction. I dont know how efficient those are, if at all. Theoretically its certainly possible, since the blurr introduced from diffration has a well defined structure. Its however pretty much the same problem as trying to correct for the blurr resulting from a misplaced focus, and theres no good algorithm for that one either.

WayneF · Oct 1, 2015

Derrel said:
runnah said:

F/5.6-f/8 are normally the sweetest of spots for a majority of lenses.

Click to expand...

The lens test sites and their charts and graphs tend to very strongly bear this out. Those two f/stop settings on many,many lenses are where the absolute peak optical performance is realized on a majority of lenses. I would say the vast majority of lenses are at their best in terms of sharpness and contrast and overall optical performance at those two specific settings. it is around f/5.6 or f/8 where many,many lenses see the far corners and the outer edge areas "catch up with the center" to the highest degree, and where contrast seems to be the highest. Still--there are a LOT of shooting situations where it's really much more critical to have the lens stopped down to get deep depth of field, which really overrides all the technical B.S. if deep DOF is what is required to make a good picture in a single click of the shutter.

Easily agreed, except I would also include f/4 with optimum sharpness for a good f/2.8 lens (but certainly not for a f/4 lens.

)

A good f/2.8 lens is a very valuable asset for bounce flash at f/4.

And yes, need for Depth of Field can override all else. f/22 is a fairly minor problem for sharpness, but f/22 can be extremely significant for Depth of Field, when needed (not always needed).