It has been a while since I posted an article in my “Photographic Myths And Platitudes” series — so here is a new one! It is a bit different than some of the previous posts in that it is based on something I wrote elsewhere in response to a lengthy (and long-winded!) discussion that suggested hyper-awareness of the so-called diffraction-limited aperture.
What is the diffraction-limited aperture, you ask? It is a real thing — not a myth — though it is sometimes over-emphasized by overzealous techno-photographers. In simple terms, as you stop down a lens its potential maximum resolution declines as a result of diffraction blur. (Keep in mind that other factors affect sharpness, too. Also, this happens to every lens, from the cheapest to the most expensive — it is a universal optical phenomenon.) At some aperture, the increase in blur becomes significant enough, in a technical sense, to be more of a “limitation” on image sharpness than the ability of the sensor to record detail — e.g. the “number of megapixels” of the sensor. A simplistic interpretation of the concept suggests that a photographer should avoid other apertures in order to “get the sharpest image.”
As with many things in photography, it isn’t that clear-cut.
Now on to my original post. It is a bit technical, though I think most photographers should be able to follow it — and I will conclude with some simple, straightforward practical advice. (And here I remind readers that sharpness is not the most important, much less the only important thing in photography. Far from it!)
Sharpness, or at least the perception of sharpness, is a more complex thing than choosing the aperture the provides (to the extent that this can be determined) the highest optical resolution at the sensor plane, measured at either a) the best performing point in the frame, or b) the average across the frame.
(Speaking of “the extent that this can be determined,” I wonder how folks would answer the following question: Which is “sharper,” the image with the best center resolution but slightly lower corner resolution or the image with slightly less center resolution but better overall resolution across the frame?)
While we might consider whether f/16 will be softer than f/8 on some lens/camera combination — it almost certainly will be softer — it isn’t irrelevant to ask: “How much softer, and will this affect my print?” In quite a few cases the difference in maximum resolution in the print will be essentially invisible. In other words, while you will get optimal resolution at some particular aperture, you will actually still get extremely good print resolution at a smaller (or larger) aperture, too.
If there is no particular photographic reason to choose a smaller (or larger!) aperture, you might as well use whatever aperture you think will produce the highest resolution. That best resolution aperture will vary based on the lens you are using, the camera format, and arguably the photo site density of the sensor. To generalize, if you are shooting full frame it will probably be somewhere in the f/5.6-f/8 range with many lenses. (Other things can affect that — for example, what the maximum aperture of the lens is.) On a cropped sensor camera you could, in many cases, use either the same aperture or guess at one stop larger or so — while realizing that there could be resolution downsides to going larger with some lenses. Trade-offs abound! (I’ll spare you the technical discussion of all of the variables. You can think me later.)
But, seriously, if you are calculating the “sharpest” aperture to the closest 1/3-stop for each lens and using that aperture in the field and avoiding others that are slightly different, you probably aren’t really gaining anything significant from your efforts, and you may be sacrificing things that could make your photographs better.
That said, if we know that some mid-range aperture can provide the highest resolution, why use other apertures? And if we do use other apertures, won’t we end up with a softer print?
It should be pretty obvious that there are photographic reasons to use smaller (or larger) apertures, right?
Consider depth of field, or DOF. In some photographs you can make your primary subject subjectively sharper (in other words, it looks sharper to a viewer) by choosing a very large aperture (which will probably produce an objectively softer image of the main subject!) in order to produce an out-of-focus background, in comparison to which the primary subject stands out… and seems sharper. Or, if you have a scene with subjects at varying distances and it is important to try to bring them all into sharper focus, you can expand the DOF by using that objectively softer small aperture and produce a print that is, again, subjectively sharper (and objectively softer!) since the overall level of apparent detail across the frame is higher.
It is useful to understand how sharpness is affected — including optimal sharpness — by aperture selection, but to focus (pun intended!) too much on using the supposedly perfect, highest resolution aperture, dependent on format and photo site density, is to miss the forest for the trees.
Real World Aperture Selection Simplified
In my experience, many photographers frequently take a much simpler and more practical approach in much of their work, roughly dividing aperture choice into three categories:
- When aperture selection isn’t critical to the nature of the photograph (little distance in front of and behind main subject, for example) we tend to use some middle-of-the-road aperture. It used to be “f/8 and be there,” though today it might be f/5.6 is you are on a cropped sensor camera.
- When maximizing depth of field is critical, we tend to go for the smallest aperture that we think will produce a fine image and avoid (except in outlier cases) using smaller apertures than that. You may have your own ideas of what this aperture is — depending on what you do with your photographs and on the format you shoot — but my general rule is to go to f/16 on full frame without concerns if I need to maximize DOF and to go to f/11 on crop in similar situations. (I’ll sometimes push things one more stop if I’m willing to give up a bit of maximum sharpness in exchange for DOF.)
- When minimizing DOF is critical (and/or in low light where we may have no choice) we simply open up as far as we need to in order to get what we’re looking for.
Simple. Effective.
This article is part of my Photographic Myths and Platitudes series.
- Photographic Myths and Platitudes — Diffraction Limited Aperture
- Photographic Myths and Platitudes — ‘Landscape Photography Lenses’ (Part I)
- Photographic Myths and Platitudes — ‘Landscape Photography Lenses’ (Part II)
- Photographic Myths and Platitudes — ‘Photographer’ versus “Photoshopper’
- Photographic Myths and Platitudes — Primes Make You a Better Photographer
- Photographic Myths and Platitudes — New DSLR? Why You Do NOT Need a 50mm Prime
- Photographic Myths and Platitudes — No Post-Processing!
- Photographic Myths and Platitudes — That Noise is Awful!
- Photographic Myths and Platitudes — The Very Best Camera! (Part 1)
G Dan Mitchell is a California photographer and visual opportunist. His book, “California’s Fall Color: A Photographer’s Guide to Autumn in the Sierra” is available from Heyday Books and Amazon.
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