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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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."
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.)"