It seems like there's a lot of confusion out there about exactly what "dynamic range" is. It's just a not-very-good term for how much subject luminance range that a given film or sensor can capture at one go.
It gets back to looking at the world. For any given framing of the world in a camera viewfinder—call it a "view"—the subject within the frame will have a certain range of luminances. Let's say your view contains both a tree trunk in shadow as its darkest object and a patch of sunlit grass as its brightest object. Photographers tend to measure what Phil Davis called this subject brightness range (SBR) in stops (as you know, the term refers to a halving or doubling of the amount of light or the exposure). If the subject—your chosen view of the world—has a measured seven stops of difference (two to the seventh, or 1:128) between these darkest and lightest subject areas, then the SBR for that view is 7 stops.
An imager (film or sensor) typically has an engineered-in maximum capacity to record a range of brightnesses. That's what's known as its "dynamic range." (I'm going to go ahead and abbreviate dynamic range as "DR" here, but note that in sensitometry, DR means density range.) The important part of D(ynamic)R is not its absolute specification—although more is generally better and less is generally worse—but how well it matches up to the subject you're trying to record.
Different scenes or subjects can have different brightness ranges. If you put a gray cat in front of a gray concrete wall in open shade, you might have an SBR of only two or three stops. Any currently available film or digital sensor will have no trouble recording that entire range of brightnesses. And note that this doesn't depend on the actual tone of the subject: white cat on white background, black cat on black background, doesn't really matter: each might have an SBR of two or three stops. What we're looking at is the range of relative brightnesses within the view. If you have a high-contrast subject—say, bright sunlit snow and an open barn door showing its dark, unlit interior in the same view—the SBR might be 10, 12, or 14 stops or even more.
The popular way of graphing SBR and DR these days is with the histogram. (Traditionally, it was with curves, originally called "H&D" curves, for Hurter and Driffield, who first devised the method of plotting density against exposure.) In a histogram, an SBR that fits within the DR will show all of the tabulated frequencies contained entirely within the display. As you know, this provides you a bit of leeway in exposure—you can move the "lump" of the histogram a little more to the right or a little more to the left and still have all the information there, enabling easy adjustment in post processing.
Here's a very low-contrast view (i.e., a scene with a low ratio of subject luminances) and its histogram. You can see that all the values of the scene "fit" within the DR of the camera sensor. In fact, I have room on both the left and the right, so I could have given more exposure or less exposure and still captured all the information in the scene. (That's called "exposure latitude.")
Now, typically, as long as the subject brightness range is equal to or less than the dynamic range of the sensor, you don't have problems. It's when the SBR exceeds the DR that you run into problems. That's when you've got a histogram that jams up against both the left and the right sides.
Here's an extremely high contrast view (a scene with a high ratio of subject luminances). This greatly exceeds the capture device's dynamic range, meaning that you can't get adequate detail in the shadows and the highlights simultaneously.
Those are both very extreme cases, of course.
In this pedestrian but more typical situation, the camera is confronting a scene with an SBR that it simply doesn't have enough dynamic range to cope with. So the auto-exposure has done what good AE is supposed to do: it splits the difference and hopes for the best! In this case, it's given a visually realistic value to the shadowed area of the building exterior because that's what takes up most of the central part of the frame. But the highlights are much too light and you can't see much inside the open doorway at all.
You have several ways of dealing with the issue when you're on the scene making the photograph: you can expose more, to get more shadow detail, like so:
This gives you a decent amount of shadow detail through the open doorway, but at a rather heavy cost—you lose half the picture to overexposure! Even the shadowed half of the exterior is much too light.
Or you can expose less to get more highlight detail—that is, expose the brightly-lit parts of the picture properly—and let the shadows fall where they may, like so:
In this case, not only do you lose any detail inside the open doorway, but the shadowed area of the building is much too dark—far darker than the subjective impression the eye might have of open shade.
So in this case, with this scene and this camera in this particular mode (ISO 200 JPEG), you really don't have enough dynamic range to record even just the shadowed and sunlit portions of the outside of the building—never mind the interior of the garage.
Don't get me wrong—dynamic range isn't just a problem with digital. Photographers have been struggling with these issues since Talbot pointed his camera at the sunlit wall of his manor house. It's just that digital is more frustrating because it has less DR than many commonly available films did—and many films had too little, too.
Any technical property can be exploited...
I should mention here, too, that like any other technical property of photography, low DR can be accommodated aesthetically and used to good artistic effect.
Magnum photographer Alex Webb, for one, often photographs in bright tropical locales with transparency (a.k.a. slide or chrome) film that probably has no more than five stops of range. But his pictures don't typically have dynamic range problems per se. Or, I should say, if they do have such problems as a disinterested technical matter, they don't as an artistic matter. Many chrome shooters traditionally had no choice but to let all the low values go to solid black—so they learned to accommodate to this and use the black shapes as graphic elements. An example is the shot below by David Alan Harvey, from the book Divided Soul.
This shows even less DR than the last (lowest) garage shot. But the photographer has anticipated this—he knows what his materials are going to do—and he's used it to good effect.
At the other extreme, virtuoso black-and-white large-format photographers such as Ray McSavaney can create gorgeously nuanced prints from subjects with SBRs of as much as 20 stops. Don't try that at home, grasshoppahs.
A big source of confusion is the range of the display media, whether it's printing paper or a monitor or anything else. You'll constantly come across people saying that since a certain range is all you can display, then that's all the DR you can have, or can use, or whatever. Not so. Any subject brightness range can potentially be represented accurately and proportionately within a given display range—as long as you captured the brightness levels of the subject correctly relative to each other in the first place.
Where the display's range of values comes into play is in its representation of relative values in smaller areas within the image, also called "local" contrast. On a piece of paper—let's just deal with that for the moment, for simplicity's sake—you can record a range of tones from the "Dmax" (maximum black) to "paper white"—and no more. Everything has to be represented within that scale. As a general rule, paper has a much more limited range of brightnesses than most scenes do. And if your scene had, say, four or five more stops of range than your film or sensor can record, what's your "capture system" going to do with that extra information? Basically, it "dumps" successively more information into nearly featureless black or nearly featureless white (as demonstrated in the garage shots above). But what that does to the information that's left—the information that it does record, typically at only one end or in the middle of the scale—is that it increases its contrast, so that it looks more vivid. Conversely, when you take a scene with a high SBR, record it with a device with a suitably high DR, and compress it into your Dmax-to-paper-white display range, you can show all the tones relative to each other with accuracy, but your local contrast will diminish.
In order to "fit" high-SBR scenes into the image file, you have to have a capture device with high dynamic range. But the greater the SBR you capture, the lower the local contrast will be in any given part of the image once you try to match it to the limited range of the display. Again, that's because you're only recording relative values. And it's why some people can look at an illustration like Fuji's F200 high-DR-mode simulation (shown below again for reference) and actually prefer the left-hand version; it's because it has higher local contrast, even though it has less information in both the highlights and the shadows. (Look at the lit side of the frontmost whole column facing us, for example. See how the contrast of the bricks or stones is higher in the picture on the left that has the lower DR?)
So why, then, if people like higher contrast in the midtones, do photographers want devices with greater dynamic range?
The answer comes down to two things. The first is options. Creative options. Having more information in the file to start with simply gives you more creative and interpretive options for the end result. In the Fuji example, if you start with a file that looks like the one on the right, you can create the picture on the left. But if you start with a file that looks like the one on the left, you can't create the picture on the right. (You can try, using things like Shadow/Highlight in Photoshop and the Recovery slider in ACR or Lightroom. But, as most of us know all too well, those are of limited value if your SBR has exceeded the DR of your camera; you can't recover information that isn't in the file to begin with.)
The second reason is that for those who love photography because of its power to show what the world looks like, adequate DR is a critical tool in the service of realism.
State of the art
Many DSLRs have good but not great dynamic range, and so far the improvements have tended to be slow and incremental. The Sony A900 has limited high-ISO ability, but the very best dynamic range of any digital camera I've used yet. But even the mighty A900 is not perfect. (Although I think it could easily handle the garage test shot scene above without convoluted post-processing antics. At least the exterior.)
The Fuji F200 EXR sensor is interesting simply because Fuji seems to be the only company actively addressing the DR problem with sensor architecture (or at least, it's the only one telling us about it). The company made an earlier try with the Super CCD SR sensor in the S3 and S5, with marked but mixed success. The EXR sensor is another avenue of approach to the same goal. The F200, as the first EXR-sensor camera, will be the first implementation of Fuji's latest ideas on the subject, and as such will be interesting to investigate.
Remember, though, with a camera that has high dynamic range, you don't necessarily have to accept the full-range result—just as you don't have to "overcook" an HDR image that merges two separate exposures until it looks completely unrealistic. When you don't have adequate dynamic range to begin with, though, you're out of luck. From a single file, you can always get less; you just can't always get more.
Oren Grad contributed to this article and Ctein and Carl Weese read early drafts and helped with useful suggestions. Thanks to all three.
Postscript: There are a fairly large number of issues related to this subject. For instance, how to choose a camera with good DR; how to get better DR from the camera; "fixing" pictures using software; ways of reducing the contrast in the scene to match the sensor you're using, for instance with lighting; the effects of using raw capture; cases in which you want and expect to lose highlight or shadow detail; techniques to restore mid-tone contrast to a high-DR image; selective area contrast enhancement; multi-exposure HDR; and so on. In this particular essay, which is pretty long for a TOP post already, I didn't want to stray too far from the basics—can't write a textbook in a single post. However, I get the feeling this is a general topic we might well revisit. —MJ.