The Online Photographer

My apologies, but I've been sensitized. I was the editor of a technical photography magazine for six years. Depth-of-field (DoF) is a subject that is deeply beloved of wannabe photo-tech writers, and few things could create instantaneous dismay deep in my editor's heart like opening a submission and finding yet another beautifully prepared, extensive, arcane article on DoF. DoF is perhaps the ultimate hell of photo-tech—graspable enough so many people think they can understand it (or think they should!), complex enough so that very few really can. Whenever the subject really gets going, great reams of material are spewed forth "explaining" it, always—always—strewn with considerable content that happens not to be true. Assuming any given reader engages with said great mass of verbal matter, he comes out the other side usually no more enlightened, and possibly considerably more confused. Finally, add to those conditions the simple fact that a technical understanding of DoF is not required for the successful practice of photography, and you begin to appreciate the extent of my exasperation with the subject.

(By the way, the other day I finally figured out why "grammar police" types can seem to be so fussy and cross. It's because they keep correcting the World, and the World keeps making the same mistakes anyway. If you tell your child three times to hang up the wet towel in the bathroom and he doesn't, you're not that irritated. When you've told him 700 times and you still come into the bathroom and find the damp towel in a moldy pile on the counter, you might well be incensed. So do you see my point? No matter how many times I remind people that "loose" is the opposite of "tight" and "lose" is the opposite of "win," people are still going to misspell it—because it's different people all the time, and there are an inexhaustible supply of them. It's like having 40,000 children, each one of whom needs to be trained to hang up the towel. The first three people I correct, I'm perfectly calm. By the 700th, I'm about to "loose" my mind. Anyway, you see the problem: educate one photographer about DoF, or ten, or a hundred—at however great a cost of time and effort—and you still have numberless millions to go. It's Sisyphean.)

The above two paragraphs are from my article "Depth-of-Field Hell," from 2009. Ctein contacted me privately the other day and cracked, "It's DoF Hell Part III!" Ctein gave you his links the other day, but these are the links he listed in 2015:

What's a 'Fast Lens'?, by Mike

Depth-of-Field Hell, by Mike

Depth-of-Field Hell, the Sequel, by Ctein

The Practical Side of Depth of Field, by Ctein

It's useless, though. Trying to fight the falsehood of "equivalent aperture" (EA) is as hopeless as telling your kid for the 701st time to hang up the wet towel and expecting a different result from the first 700 times. I even had a photography instructor(!!) tell me once that he knew EA was wrong but he taught it anyway because his students found it easy to grasp. [*Smacks forehead*.] We ought to teach children that stars are much tinier than the moon. That would be a good thing to teach them because it's so easy for them to grasp.

Yes, it's true that people are describing real things when they talk about EA. But EA is not the reason for those things and it is not the correct explanation. The fact that it's the wrong theory to describe what people are trying to talk about should count for something.

This will probably annoy a few people, but I'm sorry, I think it's true, even if it's not true of you. I personally think the equivalent aperture fallacy mainly exists because it relates to status and prestige. It's usefulness for many (not all) of those who promulgate it is in asserting that one can achieve shallower DoF with a "full-frame" (FF) camera than others can using smaller-sensor cameras even if the latter have the same speed or slightly faster lenses. ("My FF ƒ/1.4 lens is better than your Micro 4/3 ƒ/1.2 lens.") At root it's a way of showing off a claimant's ownership of expensive equipment, and there's not really much more to it than that.

Three assertions in support of this contention: First, I've observed (over many years of observing) that EA is almost always asserted (not always, so don't take offense, please) as an argument against smaller-than-FF sensors, and of the superiority of FF sensors and fast lenses. Second, people seldom point out that you can get even shallower DoF with larger-than-FF formats. The reason for the latter is probably because shallow DoF isn't actually the point. Showing that one's camera is cooler and mo bettah and more he-man than gnarly liddle-sensor cameras and baby zooms is the point. Third are all those people who shoot wide open all the time even when they shouldn't, getting important areas of the image (like the dog's nose) out of focus even when more DoF would be better for the picture.

All of the EA adherents' assumptions also depend on the proposition that where depth-of-field is concerned, shallower is better. This is exactly contrary to how most photographers have felt over most of the history of photography. It's perfectly easy to turn that around and say something like: 4/3 sensors are more ideal because it's easier to get more DoF.

Of course there's no such thing as focal-length equivalency either; it's really angle-of-view equivalency we're talking about there. This might help EA proponents, though: consider that a maximum aperture value is a stable characteristic of a camera lens even when the lens is not attached to a camera. Even when it's not in front of any particular sensor. Even when pictures do not enter into it at all.

In every case, when someone asserts that a lens correctly calculated and correctly marked as an ƒ/1.7, for instance, is "really" an ƒ/2.8 lens or "really" an ƒ/4 lens or whatever, that is simply wrong. We shouldn't teach it and we shouldn't promote the idea. It's wrong; an ƒ/1.7 lens is ƒ/1.7 is ƒ/1.7. If you want to say that a photograph made with it on a particular sensor looks a lot like a picture made on a different sensor with a different lens, go ahead, but it's not because the lens magically acquired different properties it didn't acquire and doesn't have. Now then, for the 702nd time, please go hang up that wet towel.

Mike

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Featured Comments from:

JOHN B GILLOOLY: "Yes, to each their own. But to be completely honest, this '80% guy' commercial photographer has never been able to understand the objective obsession with depth of field. (I recognize that I am primarily a 35mm full-frame photographer and if I were shooting larger formats one might have to be more intentional.)

"Depth of field is a gradual moving target depending on the three factors of f-stop, focal length and distance from subject. When approaching every shot, one of the first questions is how much depth do I want? I can pick an f-stop and focal length that will be close to what I need and tweak from there. And now we are looking at the results immediately.

"I am a photographer who is interested in photographs. I am not really interested in using the photography as part of a math and science proof method."

Mike replies: That about sums up my approach as well. Of course more succinctly than I ever could do!  :-)

Tahir Hashmi: "Regarding equivalence, Ctein is right in highlighting that from a point of view of photometry the aperture of a lens doesn't change with sensor size. E.g. an ƒ/1.7 lens needing 1/50 exposure at ISO 100 on FX sensor would need exactly the same parameters of exposure for Micro 4/3 as it would on a larger 645 sensor (assuming all other lens characteristics are the same).

"However, while there is no equivalence in exposure parameters, there is absolutely undeniably an equivalence in visual parameters, i.e. framing and depth of field. It's trivially verifiable and has been demonstrated thousands of time online.

"And it is useful.

"Since Mike talked about DoF equivalence being used by FX [i.e., full-frame —Ed.] owners to show off, let's take an example from Weegee, who shot with a Kodak 127mm Ektar on a massive 127mm x 101mm (5x4-inch) film 'sensor.' His favourite shooting apertures...ƒ/11 (longer distance) through ƒ/22 (close-ups).

"It's impractical for me to acquire a 5x4 camera today, so I must make do with my puny little 35mm sensor. Applying the equivalence math, I ought to be using a 35mm lens set to ƒ/3 through ƒ/6 or thereabouts. Seems plausible; looking at his published photos, I reckon those settings would fit.

"If I were to ignore DoF and FoV equivalence and shoot with 127mm at ƒ/22, I don't think I'd be doing close-ups anymore.

"And, oh, getting Weegee's ƒ/22 DoF on my humble iPhone...impossible! But somehow the iPhone gets everything in focus even at its ƒ/1.8 aperture.

"There is such a thing as DoF equivalence, and it shouldn't be denied. What's needed is clarity around where to apply aperture invariance (exposure calculations) and where to apply aperture equivalence (visual approximation)."

Mike replies: Okay, but let's call it DoF equivalence then, as you do here. It's not aperture equivalence. As John G. notes above, DoF is a combination of three things anyway. As an aside (not as a refutation of your points, that is), I think one thing that annoys me about EA is that the observation of effects is just so sloppy. If not missing. EA adherents seem to assume that out-of-DoF blur (bokeh) is always the same, that is it's just one effect universally and it just has to be applied, like it's a standard property that is interchangeable in every situation. An analogy from my point of view might be to assert that all singers sound the same as long as they are singing the same note. As someone who observes the effects closely, I believe that it's not only not a standard property optically, but (even) that it varies with the subject being rendered.

A good comment, though, and I take your points and respect your point of view even if we do not entirely agree.

Sean: "As a poor speller, I’ve had my collar felt by the 'grammar police' in every neighbourhood of the internet. They get quite upset when I tell them that, technically, spelling isn't grammar. A furious ‘no true Scotsman’ response often follows. Let he who is without nits...."

Mike replies: I'd never heard of "no true Scotsman" before, and thanks for that. 

Here's a snippet from Wikipedia about the expression:

No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect an a-posteriori claim from a falsifying counterexample by covertly modifying the initial claim. Rather than admitting error or providing evidence that would disqualify the falsifying counterexample, the claim is modified into an a-priori claim in order to definitionally exclude the undesirable counterexample. The modification is signaled by the use of non-substantive rhetoric such as 'true,' 'pure,' 'genuine,' 'authentic,' 'real,' etc.

Philosophy professor Bradley Dowden explains the fallacy as an 'ad hoc rescue' of a refuted generalization attempt. The following is a simplified rendition of the fallacy:

Person A: 'No Scotsman puts sugar on his porridge.'
Person B: 'But my uncle Angus is a Scotsman and he puts sugar on his porridge.'
Person A: 'But no true Scotsman puts sugar on his porridge.'

Wonderful! "Signaled by the use of non-substantive rhetoric" made me laugh.

I like and enjoy reading and learning about fallacies even though I've found I can't memorize them and have little aptitude for applying them. (Same goes for formal logic, and for the identification of trees. Although I've always been a very good speller. It's always interesting sorting out what we are good at and what we are not, although it was more involving when I and my brain were young.)

Bear.: "DoF is really very simple and may be calculated by reference to the circle of confusion; namely, for any given field of view for any sensor (or film) size, it is a depth equal to the diameter of any given number of people confused by the concept at that time standing in a circle holding hands multiplied by the inverse of the chosen f-stop."