Balancing Depth of Field with Acutance: Why you (should) care…

There is an inverse relationship between depth of field (DOF) and f-stop. As you reduce your lens’ f-stop, going from f/2.8 down to f/16, f/22 (and even f/32  and f/45 on some lenses), your DOF  increases. When shooting a macro photo this increased DOF is very tempting. It is easy to crank the aperture down as small as you can if you are using strobes, or if the subject is motionless and allows long exposure times with a tripod.

Then why not do it!? After all, you’ll have a better image because more if it will be in focus, right? Well, yes, you’ll have more DOF, but. . .  you won’t necessarily get a better photo. The smallest apertures on a lens rob you of that razor sharp edge detail that helps create a really stunning macro photograph. This sharpness is technically known as “acutance”.

Why does the sharpness decrease with smaller f-stops? Because all lenses have a  “sweet spot”, an aperture at which they are sharpest. A number of variables determine this sweet spot, including diffraction around the edge of the iris, the design of the lens elements, and the physical realities of how light behaves.

You don’t have to be an optical engineer to find the sweet spot of your lens. You can easily find your lens’ sweet spot with one quick test. The first lens you test may take 20 minutes to get set up, figure out how to use your software, and walk through the comparisons. But after that, each lens you test will only take five or ten minutes, and you will be rewarded with sharper images every time you shoot.

Here’s how. Put your camera on a tripod, set up a very clear target, and take a series of pictures, one at each f-stop. Compare them and find the sharpest ones. The difference will probably startle you.

Would you like detailed instructions? Click HERE.

To get a PDF containing a free testing target, click HERE.

If you don’t have time to run the tests, but you’d like a rule of thumb, here it is:

• Most modern lenses have good acutance between f/5.8 and f/11, with the best often found at f/8 – f/11. (But testing is the only way to tell which f-stop is the sharpest for your lens on your camera.)

Being aware of the impact f-stop settings have on sharpness can have big payoffs in your photography!

PLEASE NOTE: All Photos on this site are Copyrighted and may not be used without written permission.