Diffraction

Platge, Evening #1. Barcelona, 2014 (A long exposure at f/22)

With my old film camera, and with my first digital cameras, the Nikon D70 and D90, if I wanted everything in focus in a landscape image, I’d just set f/22 (aperture priority), focus on my foreground object (or no more than 1/3 into the scene), and let ‘er rip. It seemed to work just as fine as a frog’s hair.

Enter the newer generation of digital cameras, such as my current Nikon D800. With these much more capable machines, you’ll hear the term “diffraction” sprinkled throughout conversations that focus on (sic!) image quality and sharpness.

Diffraction: A Poor Man’s Definition

So what is diffraction? I’ll explain it the way I think of it even though it might not be particularly scientific: When light passes through a hole, if the hole is big, the light passes through no problem. But, if the hole is tiny, the light tries to cram its way through and some of the beams get bent and scatter in the process. The effect in your image is that, with a small aperture (say, f/22) there might be some softness to the photograph. At a larger aperture (say, f/5.6) you might notice more sharpness to the scene. This is all very subtle and is more noticeable in larger print sizes (say, 16×20 and up).

If you want to read a great explanation–much more scientific than mine, but still understandable–of diffraction , airy discs, and how it might affect your photography, see the Cambridge in Colour website. Of particular note on that website, are two little interactive sections (scroll down a bit on that link). In both, you can enter your camera model and the desired f-stop for some interesting visual feedback on what the image quality impact might be.

Five of my personal and very unscientific observations about diffraction:

1) Other factors tend to be much more important. For example, are you using…A stable tripod? Mirror lock-up? A remote trigger (or delay)? Good focus technique? A good quality lens? Proper sharpening in post?

2) For my 12×18 prints on 13×19 paper, it seems like f/22 has little noticeable negative effect–as long as I also pay attention to the laundry list of items in #1.

3) If I don’t need f/22 I don’t automatically set it, as I used to. Instead, I try to see if f/8, f/11 or f/16 will give me the depth o’ field I need. (A max of f/8 will often work, especially with very wide-angle lenses, depending on the scene.)

4) If I ever need to print larger than 12×18, I might consider the idea of focus stacking several captures all shot at, say, f/5.6.

5) If you honestly analyze your photography, how many of your images really rely on sharpness to be effective anyway? (The below image, for example, could have been shot with just about any camera at most any f-stop–the effects of diffraction on sharpness didn’t really enter into the equation.)

As a side note, I purchased the D800 as opposed to the D800e model partly based on what I read about diffraction. The “e” model apparently is slightly sharper than the “plain” model, but generally at the wider apertures (f/5.6 and wider). At smaller apertures (f/8 and smaller), this difference tends to disappear, and these are the apertures at which I find myself shooting at quite often.

The Cave Man. Barcelona, 2014