A Bit More on Aperture Selection (Morning Musing for 8/23/14)

(Thanks to a reader who posted a follow up comment on an earlier post — “Making Aperture Selection Easy (Morning Musing for 8/22/14)” — I wrote up a response with a quick explanation of why you might want to be careful about stopping down too far if you are trying to maximize image sharpness. I think it might be useful information for others, too, so I’m sharing it here as a new post.)

Aperture selection, among other things, allows us to control depth of field (DOF)— the range of distances in front of and behind (if not focused on infinity) the subject that is the center of the plane of focus. By choosing larger apertures (such as f/1.4) we narrow the DOF and can throw elements beyond or in front of the main subject out of focus, making them soft and diffused. Choosing smaller apertures (such as f/16) will increase the DOF, and subjects further behind or in front of the primary subject will be much sharper.

Some photographers make a logical leap from “increasing DOF with small apertures makes more things in the frame look sharp” to “smaller apertures are sharper.” It doesn’t actually work quite that way!

Most lenses have an aperture range in which the overall resolution of the image will be at its best. This point differs among various lenses (and due to other factors) but it will be somewhere in the middle between the largest and smallest apertures. In other words, things can be softer — in somewhat different ways — at both the smaller and larger ends of the aperture range.

At the small aperture end, the image will actually become less sharp as you stop down due to diffraction blur. In other words, if you shoot at an aperture like f/22 on most current cameras, you will increase DOF while you decrease maximum resolution. Like so many things in life, there is no free lunch, and compromises abound!

You may read all sorts of ideas about which apertures are sharpest, sharp enough, and too soft. In truth, there are a lot of subjective things that have to be balanced: are improvements in focus across the frame more important that maximum resolution in the center? are you willing to give up a bit of ideal resolution in order to obtain more DOF? etc…

A general rule of thumb that I use — based on looking closely at large prints of many photographs — is that if you are trying to get large DOF with good sharpness on a full frame DSLR or similar camera you can feel quite safe shooting at f/16. If you are willing to give up a bit of sharpness to get more DOF, f/22 is an option in some shots.

With smaller sensors the “ideal” point shifts in the direction of somewhat larger apertures. I generally don’t shoot a cropped sensor camera at apertures much smaller than f/8 without good reason, though I think you can do quite well at f/11 if necessary. I avoid f/16 and have a hard time thinking of many situations in which I would shoot a cropped sensor camera at f/22.

G Dan Mitchell is a California photographer and visual opportunist whose subjects include the Pacific coast, redwood forests, central California oak/grasslands, the Sierra Nevada, California deserts, urban landscapes, night photography, and more.
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Text, photographs, and other media are © Copyright G Dan Mitchell (or others when indicated) and are not in the public domain and may not be used on websites, blogs, or in other media without advance permission from G Dan Mitchell. do quite well at f/11 if need be. I avoid f/16 and have a hard time thinking of many situations in which I would shoot a cropper at f/22.

2 thoughts on “A Bit More on Aperture Selection (Morning Musing for 8/23/14)”

  1. Dan, I don’t suppose you’d be willing to post some 100% crops of shots suffering from diffraction versus shots with inadequate DOF? I think I’m starting to be able to spot diffraction issues in my own work, but it would be nice to see examples from you. When I used to shoot mostly wide angles, I never worried much about it, since wide aperatures generally got the job done. But now that I’m doing a lot more telephoto work, the DOF/diffraction tradeoffs are becoming much more of an issue.

    1. Jackson, at some point I should run off a series of images at whole apertures through about f/32 and share the examples. It may not be right away since I’ve got a few other things going right now.

      A few years ago I did a sequence of tests with some of my lenses in this way, though at that time I was only really trying to look at whether there might be a difference on a full frame camera between f/8 and f/16, so I didn’t push it too far. Essentially, I figured out that if I looked quite closely at 100% magnification crops of full frame images shot at f/16, I could barely make out some slight overall softening of the image. It would be essentially invisible even in quite large prints.

      I do occasionally shoot at f/22, but only with subjects where I want to squeeze out a tiny bit more DOF or in which I need to double the exposure from what I could get with f/16 and in which absolute sharpness is not the point of the photograph. For example, I’ve done some long exposures with motion blurred water at f/22… but nothing it the frame needs to be extremely sharp.

      By the way, you’ll get diffraction blur at these apertures from any lens — wide, normal, long, it doesn’t matter. And lens quality doesn’t really make a difference, since diffraction is actually a normal optical process affecting all lenses.

      Dan

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