The Practical Side of Depth of Field

This week's column by Ctein

[A late addendum to last week's column—there are now links in the Featured Comments to the two photos by Kathy Li that I referred to, so you can see what I was talking about. —Ctein]

Some photographic subjects need to be periodically revisited, to inform newcomers, to stamp out persistent misconceptions, and simply to reach the folks who somehow managed not to read my deathless prose (ahem) the first time around.

Depth of field is one of those, as some recent questions I got from readers reminded me. Mike and I beat that topic thoroughly to death, or so you'd think, several years ago, in "Depth-of-Field Hell" and its Sequel. Of course, these are among the undead topics of the photographic world, and even shoving silver halide stakes through their hearts isn't enough to put them down permanently.

Now here's the thing. Depth of field isn't a theoretical concern; it's a supremely practical and pragmatic one. Theoretically, there is no such thing as depth of field—the lens image is sharp at one and only one distance from the subject. Well, okay, for real-world lenses, which can't take light down to an infinitesimal point of sharpness, there's actually a very, very thin range of distances. But you get the idea.

Depth of field assumes you can tolerate some blur in a photograph. Emphasis on you. The standard equations all make assumptions about what a standard viewer and viewing situation is: an 8x10-inch print viewed at normal close distances. For a viewer of normal sensibilities, the acceptable blur is in the range of four line-pairs per millimeter (4 lp/mm) in the print. Some of us print bigger, some smaller. Some of us press our noses against prints and others don't. Some of us are just plain fussier about fine detail. Failure to understand this variation is part of what drives some people crazy about depth of field.

The other practical concern is that it's all about how much total blur you can tolerate, not just how much blur comes from the lens being out of focus. There's blur caused by the lens being out of focus and that caused by the lens itself, and what you see is the combination of the two.

A mediocre lens, for example, one which is barely sharp enough to be acceptable to you, is going to have negligible depth of field. That's because you're already at the limit of the amount of blur you're willing to tolerate; adding in much more from the parts of the scene that are out of the plane of best focus is going to push beyond your pain tolerance. A sharper lens provides more depth of field.

The same thing can happen if you stop down too far, and macro photographers run into this all the time. The standard assumptions work out to around 30–35 lp/mm in 35mm film or a full frame sensor. Sparing you the math, once you've stopped down to a real aperture of ƒ/32 you've run out of depth of field: stopping down any further causes diffraction blur to increase so much that your total depth of field goes down. In fact, stopping down beyond ƒ/22 doesn't gain you much; diffraction is already important enough that you've got 90% of the maximum amount of depth of field you can get.

Now back to that matter of print sizes and tolerances. Some of the folks engaging in medium format digital photography are convinced that they're getting less depth of field than they did with film photography. Mostly, that's not true—it's a little bit true, because the digital sensors are modestly smaller than the film formats were, but this is not majorly visible. The larger problem is that the folks who are likely to indulge in that sort of equipment are also likely to be pixel peepers. Who can blame them! If they weren't that fussy about image quality and sharpness, they'd buy something cheaper. So, what happens when you pixel peep a medium format image?

Well, for a start, you're typically talking about 6 micron pixels, which works out to a real physical resolution of 50–60 lp/mm from the sensor. Now the thing is, for medium format film the standard assumptions about depth of field worked out to 15–20 lp/mm on film. If you're aiming for pixel-level sharpness from that medium format camera, you're talking about a tolerance for blur that is only one third of that assumed for film. That means much, much shallower depth of field, not because there's anything strange about digital but because you're working to much tighter standards.

Okay, you may already understand this, and you only view the file at 50% size on the screen, just to avoid dwelling too much on the minutia. Well, if you're talking about, oh, say, a 35-megapixel sensor and a normal resolution monitor, that's still a lot like looking at a 20x24" print. Which is a hell of a lot bigger than the canonical 8x10. Problem is, for previewing on screen, you can't really take it much smaller than that because screen resolutions aren't all that high. You really have to print out the photograph to get an idea of just how sharp it looks on paper.

When you do that, most of the imagined differences between digital and film depth of field disappear. As we often say about photography...It's all in how you look at things.

©2013 by Ctein, all rights reserved

Weekly columnist Ctein comes into focus on Wednesdays on TOP.

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(To see all the comments, click on the "Comments" link below.)
Featured Comments from:

Gordon Lewis: "Oh no! Not depth-of-field again! Lock the doors. Cover your children's eyes and ears. Pray that the violence and bloodshed will be brief."

Ed. replies:

Mark Johnson: "Ctein, that may have been the best article I've read of yours. Detailed, yet concise, with your normal sprinkling of humor. Well done. (BTW, thanks for turning me onto the wider world of tea. It has been a pleasure trying some of the teas you mentioned in articles past.)"

Comments

[Jeff]

I've been doing aperture bracketing and wonder when the mfg's will add it. On paper M4/3rds starts diffracting at f8. In practice f16 or f22 and it depends on light (or lack there of), distance between objects and to subject(closer being more noticeable).

[Eduard de Kam]

If the lens is 'bad', and the sharpness is low, would that not give you a larger DOF, because it is harder to see where the marginal sharpness ends and it might be so bad that you cannot clearly see what the point of focus was.
And I did try such lenses! Early cheap zooms from 10 years ago, you could not tell where the focus was!

[KeithB]

This might be another can-o-worms, but when does printer resolution come into play? My post-it calculation says that at 300 dpi, I have a resolution of about 12 dots per mm. Is that enough to resolve 4lp/mm? It seems awful close.

[Manuel]

Depth of field, what an endless theme for debate... especially when it brings about a subject everybody loves nowadays: 'Bokeh'.
Ctein's article matches my own empyrical findings. My experience with 35mm film (which is considerably vaster than with 'full frame' digital) tells me shallow depth of field is something we have to deal with when shooting. It could come as a surprise to some, but with this kind of equipment it is actually harder to keep sharpness in every plane than to get the so-called 'bokeh'. With film it is usual for me to use very narrow apertures - say f/8 and f/11 - in order to increase depth of field. And even then it's hard to avoid that some portions of the image appear out of focus.
There's no science in taking pictures with lots of 'bokeh'. Making photographs with acceptable back-to-front sharpness is the real challenge when using larger formats. 110, APS and now 'crop sensors' made it easier to get sharpness across the picture and harder to get 'bokeh'. No wonder newcomers used to smaller formats believe 'bokeh' is a difficult technique.
So, despite the fact that many photographers in the film days used narrow depth of field for creative purposes, what we've got now with this 'bokeh' psychosis is a technical shortcoming being promoted to artistic status. Oh well.

[Mark Johnson]

Ctein, that may have been the best article i've read of yours. Detailed, yet concise, with your normal sprinkling of humor. Well done. (BTW, thanks for turning me onto the wider world of tea, it has been a pleasure trying some of the teas you mentioned in articles past.)

[Steve Jacob]

Dear Ctein, much as I respect your intellect and accumulated wisdom, I really don't think it's entirely reasonable to reduce everything to mere logic, experience and common sense. I mean where would the world BE if that was the answer to everything?

Say what you really think for heck's sake! This IS the internet you know. You can't argue with a flawless premise, where's the fun in that?

[Dennis]

Paul Krugman has confronted this phenomenom of the repeating subject and has identified it as the Zombie problem ie no matter jow many times you step on it it still come back to haunt you. Lol. And no I didn't read the comments.

[ctein]

Dear Keith,

I wrote a column on how to determine your printer's sharpness back in 2010: http://theonlinephotographer.typepad.com/the_online_photographer/2010/01/how-sharp-is-your-printer-how-sharp-are-your-eyes.html .

Getting a quantitative measurement in terms of line pairs per millimeter is extremely tricky. You can't just print a bar chart and get anything meaningful, because the dither algorithms frequently respond oddly to repeating patterns. I've tested the large handful of good printers over the years, nowhere near a majority of the ones on the market. But the worst results I got were 8 lp/mm and the best 15. So it's a pretty safe bet that printer resolution isn't likely to come into play when you're worrying about depth of field.

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
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[Jarrett]

There are no Great Photographs with a shallow enough depth of field to draw attention - think about it! Show me a clasic photograph where a shallow depth of field has enhanced it.

[ctein]

Dear Jarrett,

Who cares? Really!

pax / Ctein

[Ed]

Mostly, that's not true—it's a little bit true, because the digital sensors are modestly smaller than the film formats were, but this is not majorly visible.

Eh, Ctein this needs explaining optically the DOF would increase with a smaller sensor (since the smaller the sensor the bigger the DOF right), but this would be counteracted by the smaller sensor pixel versus film grain. So if you pixelpeep (which is photographycally/artistically nonsense but fun to do) you see less DOF but in print you'd see more. So it's all about pixelpitch/Mpixels versus final print size? And both live in different universes.

In that case a pixelpeeper would see less DOF in a D800 then in a D600 right, while a photographer would be amazed at the large format prints he can get from the former? Now I mentioned that bacause that was exactly what was happening when I went from GF1 (12 Mpixel) to an OM-D (16 Mpixels), the prints (at the same size limited by the R2400) looked significantly sharper and crisper (at that size comparable to a GSW690 even to the point of not bothering with it anymore) while the image pixelpeeped looked less sharp especially in the DOF. So I looked at both images at 50% resolution and the difference fell to the positive for the OM-D.

So this article is a explaination for that feeling or hunch or bewildering experience?

Thanks Ctein.

Greets, Ed.

[David Bennett]

So for the kind of photography that the Olympus 17mm on 4/3 is likely to be used for - where shooting with an open aperture is not a priority - is the f1.8 actually sharper than the f2.8?

[Geoff Wittig]

Ctein, it's common knowledge that Zombie lies and 'factoids' are not killed by stakes through the heart. No, they must be dispatched by shooting them in the head. It's probably most effective to use bullets made from silver recovered from Kodachrome processing, which is now in very short supply.

[Archie Noble]

That may be the best explanation of a very complex subject that I have ever read. That's very hard to do. Richard Feynman would have been proud.

[Nicolas M.]

Agree 150 % with Mark Johnson. You were talking about your "perfect post" not so long ago. I loved the one you wrote on Jim Marshal (which lets me find his book with his contact sheets). But for technical discussions this one is just cristal clear. Well done !

[ctein]

Dear Ed,

I don't know the pixel pitches in the various cameras you've mentioned, so I'm not 100% certain, but it does sound like the bewildering experience you're describing exactly what I'm talking about: You are pixel-peeping photographs made from different cameras that have different physical pixels sizes, so the “circle of confusion” that you're looking at changes from camera to camera, instead of just looking at the results in a print.

~~~~~

Dear David,

I don't understand what this has to do with depth of field. Aren't you really asking about what aperture your lens is sharpest at? Well, that depends on the lens. It's not unheard of for lenses to be sharpest wide open, but it's far more common that stopping down 1-2 stops improves their definition.

I don't own the lens you're talking about. Easy enough for you to find out: put the camera and lens on a tripod, focus really, really carefully, and make a series of aperture-bracketed exposures. Then pixel-peep to your heart's content. You'll have your answer!

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
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[ctein]

Dear Richard,

It's so irritating when you ask me questions that make me think [grin]!

I believe Mike had covered a lot of the practical aspects you're interested in in previous articles of his, and with everything archived here I didn't really want to cover previously-mined territory.

But your comments about microns and theory got me to thinking (always a dangerous thing). What you're talking about is the qualitative vs. quantitative. That's kind of orthogonal to theory vs. practice. Sometimes qualitative explanations work better; sometimes quantitative examples do. And different ones work better for different people. I felt like this article needed some numbers to tie it down to reality, because unspecific statements about different size blur circles and circles of confusion wouldn't register as well.

Clearly that worked for many readers, but not for you. One of those courses/horses things (or is it the other way around), I imagine. if you look back at the “sequel” article I linked to at the beginning of my column, you'll see that I presented no less than five different framings of the argument, to really nail a stake through its heart. Different ones resonated for different people.

But back to that orthogonality. Most of the columns on theoretical science I've written here haven't had quantitative examples. Even though they're about theory. There are couple of reasons for that. One is that I feel that when I'm trying to explain a conceptually-difficult subject, throwing numbers at laypeople usually doesn't make it any easier. Except when it does. When I was writing about the breakdown of local reality, I tossed a couple of made-up numbers into my cereal box analogy just to make it feel more concrete.

The other reason my theoretical science columns don't include a lot of quantitative stuff is that I can't do most of the maths! A curiosity of modern physics is that most physicists run into a math wall at some point. Mine occurs at a point before I am proficient in either quantum mechanics or general relativity. But I can read the stuff, and I can explain it to other people… Just so long as I don't have to create numerical examples to go with my explanations.

If some readers are having trouble imagining how this works, think of it this way: it's a lot easier to read enough of a foreign language to be able to get the gist of some text than it is to be able to speak it. I can read a moderate amount of French and Spanish. Don't ask me to converse!

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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-- Ctein's Online Gallery http://ctein.com 
-- Digital Restorations http://photo-repair.com 
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[latent_image]

Jarrett, you're kidding, right? Excepting the f/64 School, most photographers working with large cameras (Sally Mann, Jock Sturges, Joel Meyerowitz, Alec Soth, etc.) can hardly avoid using shallow DOF from time to time, and they often use it to great effect artistically. And that's not to mention the legion of acclaimed photographers who have employed selective focus in medium and small format.

[ctein]

--begin inspirational rant, here.--

Dear Latent,

Oh, don't even encourage this sort of intellectually bankrupt trolling by arging with it. Folks who start going on about how "Great Photographs" are or aren't made with X, Y or Z are either just trying to score status points in a meaningless debate or have no clue why people make photographs.

Really, none.

'Cause, show of hands, now -- how many folks reading this are into photography primarily because they want to make a "Great Photograph" (or, worse yet, some random Netizen's notion of what constitutes a Great Photograph)? Show of hands? Chorus of voices? Anyone?

{insert sound of crickets, here}

Yeah, exactly. Except for an insignificant number of poseurs, we all make the photographs that make **US** happy, using whatever techniques, aesthetics, and tools please us the most. Nothing, not one whit, more than that.

Ever so occasionally, one of us will land on a Great Photograph. Yay for us, when it happens.

But nobody, and I mean nobody, goes out to photograph each morning saying to themselves, "Today I must make a Great Photograph or my time will have been wasted, and so I must hew to the rules* of Great-Photograph-Making."

(*[imaginary])

--end inspirational rant, here.--

pax / Ctein

[Ed]

Are you claiming that 15-20 lp/mm is the limit achievable by medium format film? It sounded like that in the text of the article.

[ctein]

Dear Ed,

Not in the least. That's a complete misread. I'm not discussing inherent film or lens resolutions at all.

pax / Ctein