Depth of field (DoF) is not an absolute. Mathematical analyses that start with circles of confusion are pretty much a waste of time, rather less relevant to photography than knowledge of the precise chemical composition of the various pigments would be to painting. It does no harm to study either, and indeed, if you want to understand image permanence you would do well to study the latter; but both illustrate very well the limits of the interaction of art and science.There is more about focus and depth of field, along with zone focusing, on our old site, especially here and here.

Chapel, Pyrenees. This picture was taken with a 135/2.8 Tele-Elmar on full-frame digital (Leica M Typ 240). The trees in the background could be sharper; as far as I recall, I shot at about f/11. They could be softer, too: probably too soft if I'd shot at f/2.8. But across quite a wide range, it doesn't matter very much. Did you notice the trees first of all? I wrestled with the colour balance on this one: it was a strange, yellow storm light.

What is DoF, after all? It's the difference between "sharp" and "not sharp". In fact, it's not even the difference. It's the transition. It depends on all the following, and more: image size, viewing distance, the quality of the out of focus image (bokeh), visual acuity, and the very definition of "sharp". 

Soap, Arles. Part of the appeal of this picture is the texture of the soap and the legibility of the moulded writing. The most distant bars are however nothing like as sharp as the closer ones. My own view is that it doesn't matter: we have already "read" the picture before we get to the far end. But it would look very strange indeed to me if both the nearest and most distant elements of the picture were out of focus. It's also worth noting that contrasty pictures often look sharper than low-contrast ones. Leica M9, 35/1.4 Summilux at f8 or f/11.

Instead of just "sharp", many people use the phrase "acceptably sharp". This does not actually mean anything. Acceptable to whom? For what purpose? With what subject matter? Under what conditions? The variables are infinite in both kind and extent. The way we react to them can also be very personal. It may well be that I am unusually intolerant of what I see as needlessly shallow DoF because I am old enough to remember the days of relatively limited fast shutter speeds. We just couldn't shoot at 1/8000 second and f/1.4, because our shutters stopped at 1/1000 or 1/500 second, limiting us to f/4 or f/5.6 in the same light. The use of ultra-wide apertures in good light, as has been made possible by ultra-high shutter speeds, just doesn't look right to me. 

I'm also old enough to remember manual focus lenses with marked DoF scales, as illustrated in the piece on zone focusing on the other site. These were never any more than a quick'n'dirty approximation, and different manufacturers calculated them differently, but even if they were only a rough guide, they were quick and easy to use. Comparing actual pictures with what you got from using DoF scales helped instill a pretty good idea of what might be more or less in focus. Compare this with my 50/1.8 AFS-G Nikkor, where the only DoF scales are f/16 and an unspecified pair of dots inboard of the f/16 markings. Even if you look at the focused distance, you can't deduce much. Trying to use an app or any other form of calculation takes forever and provides a false sense of precision. Experience becomes the only teacher.

Philosophically drunk. We had been drinking all evening at a festival in Euskal Herria, the Basque country, and when the bar closed someone proposed that we move on to his cousin's bar which would still be open. At least I think it was his cousin, but details are understandably hazy. "Losing" the messy background was obviously desirable (and all but inevitable) but the shallow depth of field here also concentrates attention on Frances's interlocutor. Leica M9, probably 50/1.5 Zeiss C-Sonnar.

The first point to consider is that very often, depth of field is a minor consideration. It will not matter very much whether the background is sharp (deep field), totally out of focus (shallow DoF) or somewhere in between. With many subjects it won't even be a meaningful question. If the subject itself is fairly flat, without much depth, and not too close, you may well be unable to see the difference between f/2 and f/16.

Vintage thread. The makers' names on the two central boxes are what matters, along with the thread in them. Only afterwards do you notice that the hand-written orange labels are a bit out of focus. If they were too sharp, I think they would detract from the impact of the central part of the picture; but this is of course a matter of personal taste. Certainly, with more or less DoF it would be a somewhat different picture. Taken at a vide-grenier with a Leica M9 and (I think) Zeiss 50/1.5 C-Sonnar.

In any case, if you notice a technical aspect of a picture before you notice the content and composition, then the picture is likely (though not certain) to be a failure. In other words, if you notice the DoF before anything else, then the odds are that the technical tail is wagging the artistic dog. This is a common fault among photographers who do things because they can, instead of because they have any particular artistic vision. 

Workshop, Girona. The light was poor; I wanted as much depth of field as possible, so I did not want to shoot wide open on my 35/1.4 Summilux; and there is a bit of camera shake because of the long shutter speed. Look at the wood in his hands and you can see that there is some subject movement as well. This makes something of a nonsense of circles of confusion. Arguably, a slight overall softness is better than having some parts sharp and others not. Leica M9.

What is more, DoF can be (and often is) less important than camera shake. If nothing is really sharp, because the camera has moved during the exposure, then DoF really means very little, and circles of confusion mean even less. The old rule of thumb that the slowest "safe" shutter speed is 1/focal length is only of any use at all with the 24x36mm format, and even then, it's often too generous with long lenses and overly restrictive with wide-angles. With a 21mm lens, I find I can often get away with 1/15 second, rather longer than the theoretical 1/21 second, but with a 200mm lens I often get sharper pictures with 1/500 than with 1/250, which is itself faster than the theoretical 1/200 second. With smaller formats, where the image will be enlarged more, you need faster shutter speeds than the rule of thumb indicates, and with bigger formats, you can get away with (sometimes much) longer ones: with a 135mm lens on a 4x5 inch camera, you certainly don't need 1/135 second and indeed 1/15 may be okay if the camera is well braced. 

Arles. If you are going for a vintage look, it is worth remembering that in the days of slow films and what is now called "medium format" (roll film) it was often quite difficult to get much depth of field, even when you wanted it. Leica M9, 50/1.5 Zeiss C-Sonnar.

Of course there is always the option of a tripod, and of course it's true that if you don't at least try to take a picture, you'll get no picture at all. Subject movement can be as important as camera shake, too. But this brings us back to the question of "acceptability". You'll notice that I've used such phrases as "can get away with" and "may be okay". This is because it's art, not science. 

Spa, Igal, Hungary. The ripples in the water render "circles of confusion" nugatory, though to be sure, the drainage grille on the upper left could be (and arguably should be) sharper. Leica M-series digital; lens forgotten. 

What about visual acuity?This is often taken as 1 second of arc, enough to see a black hair on a white tile at 10 feet. It's a fair average, but we all know that some people are sharper-eyed than others. For an easy round number we can take 5 line pairs per millimetre (lp/mm) at the closest comfortable viewing distance. For most people, with eyesight correction if necessary, this is taken as 25cm/10 inches. Few people will be able to distinguish between a photograph with a resolution of 5 lp/mm and one with a resolution of 10 lp/mm: both will look very sharp. But even then, there's a difference between a set of parallel lines, and a single line with a tiny discontinuity, the so called Vernier resolution. This can be significantly more acute than the equivalent of 5 lp/mm. 

This is psychophysics, not mathematics, and the concept of the Just Noticeable Difference or JND is fundamental. The JND may also vary according to how hard we are looking for it. Sometimes we will notice it regardless; at other times (such as analyzing bokeh) we may have to look quite hard to notice it at all.  

Bicycle shadow, Cologne. Some pictures are carefully constructed; others, like this one, are "see it and shoot it". Most "see it and shoot it" pictures rely far more on content and composition than on technical quality, which need only be adequate. Given the sodium vapour street lighting, getting even an adequate colour balance took some effort. Leica M9, 35/1.4 Summilux.

Moving on to image size and viewing distance, it's a matter of common experience that if we can get close to a big poster we can quite easily see the blobs of colour that make up the image, and that from normal viewing distances we can't. This is an extreme case, but it also holds true as a general principle. Most of us, early in our photographic careers, have taken a picture that looks really sharp at postcard size; blown it up (or ordered an enlargement); and been disappointed by the lack of sharpness. There are various reasons why this happens, including poor quality lenses, poor quality enlargements and camera shake, but insufficient depth of field can be another factor. 

Bicycle race, Vienne. The plane of focus is plainly on the three cyclists leading the peloton or pack: the front-runner is both out of focus and blurred. But note "and blurred". This adds to the sensation of speed, and the feeling of being close to the action. It would be possible, with a high enough ISO speed (which I have on my Nikon Df but not on my Leica M9) to freeze everyone and to make a picture that is sharp from front to back; but that would, I think, be a much harder picture to take successfully.

Often, too, we look at big pictures from further away. If 5 lp/mm is adequately sharp at 25cm/10 inches, but we look at the picture from 50cm/20 inches (twice as far away), then the threshold for adequate sharpness falls to 2.5 lp/mm (half as much). It's simple geometry. It's an interesting question as to whether we could see much difference between 2 lp/mm and 3 lp/mm at 20 inches. My suspicion is that most people would find the image at 3 lp/mm sharper because it renders texture better, but it is only a suspicion.

This introduces yet another consideration. What we see is as much a function of our brains as of our eyes: Richard Gregory's books, especially Eye and Brain, make far more interesting reading than fruitless discussions of circles of confusion. Our eyes concentrate on what we find interesting; our brains fill in a lot of the detail. For example, our brains can equally well read film grain as the texture of a stone, or as sand on the beach, despite the fact that we are actually seeing neither. This is why we can quite easily "see" detail in someone's hair that isn't actually there. We know what hair looks like: our brains fill in the rest. 

Girona, near the university. Blow this picture up big enough, and the girls' hair isn't all that sharp. But it looks sharp, because we know what hair looks like, even though the exposure is partly "blown" from the powerful backlighting. Leica M9, Zeiss 50/1.5 C-Sonnar.

Hair also supplies an excellent example of how images that are partially out of focus can be more evocative than ones that are sharp from front to back. If we are physically very close to someone, such as a lover, parent or child, some of the hair on their heads may be visible but out of focus. A photograph can recreate this sense of intimacy via differential focus, though it is far from a simple question of focus: sometimes it works, and sometimes it doesn't. No mathematical formula will tell you when it will and when it won't.

The very words "differential focus" are central to DoF, too. There is a big difference between "losing" a distracting background that is some way behind the principal subject, the traditional use of differential focus, and taking a picture in which things slowly go out of focus on either side of the principal subject: imagine shooting the soap picture above in that way. Personally, I normally find the latter very distracting, but some people presumably like it. Either that or they do it because they can.

Tibetan woman, Dharamsala, 1980s. If the people behind her were in sharper focus, they would be distracting. Then again, with a long lens and a high shutter speed, a (fairly) wide aperture will inevitably result in shallow depth of field. Frances used a Nikon F or Nikkormat for this shot; lens forgotten.

Matters are confused still further by the option, in digital photography, of "sharpening". This has very different effects on in-focus and out-of-focus areas, and (like shallow depth of field) is easily overdone. Perhaps the worst examples are seen with skin, which if over-sharpened can look as if its owner is suffering from a remarkably nasty skin disease; but what can happen in out-of-focus areas is anyone's guess. Or sometimes, anyone's nightmare.

Window and shutters, Airvault. Everything in photography is an illusion, a two-dimensional reproduction of a three-dimensional reality. What prompted me to take this picture was the way the "old-fashioned" half timbering had at some point be covered over to make the building look more modern. Now, the textures and colours tell an historical tale. I sharpened this slightly more than I normally would, to emphasize the textures and shapes. Leica M9; probably Zeiss 50/1.5 C-Sonnar.

DoF can be reduced in at least three ways: bigger apertures, longer focal lengths, and moving in closer. It can also be modified heavily via camera movements on large format cameras. That's before you start printing bigger or smaller, and varying the viewing distance. Disregard anyone who tells you that all this is only "apparent" depth of field, and that "true" depth of field depends only on magnification. It's true, sort of, but it's worthless. Because we're dealing with psychophysics, both DoF and perspective are only ever "apparent". What you (think you) see is, in fact, what you see. 

This is the bottom line. As so often in photography (and indeed in life itself), the answer to the question "What works?" is a firm, unequivocal, "It depends..." You can't reduce it to formulae: all that matters is that the picture conveys what you want. Actually, it's not even that. It's whether you like the end result or not, even if it owes more at the taking stage to luck than to judgement.  The real judgement comes down to deciding whether it's any good once you see the picture. 

Shop window, Cologne. I hadn't the faintest idea if it would work, but as I said above, if you don't try, you'll never know. I focused on the image in the mirror and just took the picture. I rather like its  dreamlike quality: as in a dream, something can be several things at once. What does DoF mean here? Leica M9, almost certainly with a 35/1.4 Summilux, pressed against the shop window to stabilize it for the long exposure.

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Words and pictures copyright (c) Roger Hicks 2016