This week I'm going to explain why RAW files aren't quite as raw as you think they are. This has been one of the most difficult of my columns to write. It all makes complete sense and is pretty simple...once you understand it. Getting to that understanding isn't easy: Mike, Oren Grad, and I had to go back and forth for several days before we finally grokked this stuff, and it was only due to the able guidance of the brilliant Eric Chan of Adobe that we ever got there.
Fig. 1. Here are plots of exposure vs. converted values for my two Olympus cameras. Not what you'd expect, if RAW conversions really were straight interpretations of sensor behavior. You'd expect the OMD curve to have the same overall shape, like the dashed gray line.
When I got my Olympus OMD, one of the first things I did was run a test to compare its exposure range to that of my Olympus Pen E-P1. Figure 1 shows a plot of my results (running both sets of RAW photographs through ACR version 7 using my default settings). I wasn't surprised that the OMD had a substantially longer exposure range; pretty much everyone had reported that. What surprised me was that the "characteristic curves" had very different shapes for the two cameras. Up through the middle-light grays, both cameras produced almost the same output values for the same exposures, but the E-P1 values shot straight up through 255, while the OMD showed a pronounced rolloff in the highlight contrast.Well, that explains why so many people like the "look" of the OMD photographs; they have a more film-like characteristic curve, and the highlights are less likely to blow out unpleasantly. Even if a photograph did have blank areas in the highlights, the very low contrast in that part of the curve means that there wouldn't be these glaring holes in the middle of the composition; there would just be a gradual fade-out of detail.
But, why the different curve shape? I thought RAW data was pretty much just what came out of the sensor (scaled to fit the number of bits in a RAW datum, of course). The sensors in the two cameras don't have differently shaped characteristic curves—they are pretty linear devices. So, why do the two RGB conversions appear so different? Shouldn't the OMD curve look more like the gray dashed line in the graph—just a stretched version of the E-P1 curve?
As they say in polite circles, WTF?
An absolute prerequisite for understanding what's going on is the column I wrote two weeks ago, "Why ISO Isn't ISO." Readers must take it as a given that camera manufacturers set camera ISOs to make what they consider the best-looking photograph. ISOs are not fixed by the sensor characteristics.
I've marked one point on the RAW characteristic curves of figure 1 as the "exposure point." This is the value you get out of the camera if you make an exposure of a uniformly lit surface; it's the camera's version of "average gray." In the case of both of my Olympus cameras, it's around 115 (it may be modestly different for different cameras). Whatever the camera maker decides is the proper ISO for their camera, that exposure still has to render as average grey or the picture will look too light or too dark.
Fig. 2. A set of hypothetical gray scales, covering the range of a sensor from no exposure on the left (black) to full saturation on the right (white). Changing the ISO set-point changes the distribution of tones.
So, on to figure 2. The top gray scale represents the output for the entire exposure range of some sensor (that I made up), starting from no exposure on the left to an exposure that will blow out the highlights on the right. The second gray scale from the top shows that same range with a vertical black bar marking that nominal, average gray exposure point (value=115). If the camera maker wanted an output that looked just like the top gray scale in this figure, this is where they would set the ISO point.
Now, suppose the manufacturer wanted to favor highlight range, adding more headroom there to avoid blown out highlights. You'd get something like the third gray scale from the top, with the exposure/ISO point moved to the left (less exposure) . The thing is, if we don't change the rendering of those values, that photograph is going to look underexposed because that point won't be converted to average gray. To get the exposure to look correct, the tones in the gray scale need to be shifted, which I've done in this illustration. The shadow values open up more quickly and the intermediate values become lighter to ensure that the ISO exposure is rendered with a value of 115 . The total range is still from black to white, but the distribution of tones within that range has changed; the curve shape is different, with more contrast in the shadows and less in the highlights.
Conversely, if the manufacturer wanted to favor the shadows and sacrifice some highlight range, they might move the exposure higher (more exposure), as in the bottom gray scale in the figure. Again, an adjustment of tones is necessary to ensure that that ISO exposure point reproduces as a value of 115. Now, the midtones and highlights are darker than nominal instead of lighter, so the photograph won't look overexposed.
Fig. 3. The same information that's shown as gray scales in figure 2 is shown here as curves. An ISO/exposure point that favors more highlight range lightens tones overall, while one that favors more range in the shadows darkens them, even though the average gray point remains the same.
I hope you're all still with me. Figure 3 conveys the same information as figure 2; it's just that here I'm showing it as a curves plot in Photoshop. If you're more comfortable reading curves than gray scales, pay attention to figure 3. If curves give you a serious headache, you can skip this paragraph. The faint gray diagonal line would be the curve for the nominal exposure point. The upper curve shows how the tones get adjusted to favor an extended highlight range. The lower curve shows how they get adjusted to favor more shadow range. The horizontal red bar marks the value (115) of the average gray.
Remember, the choice of ISO is arbitrary; it's a decision made by the camera manufacturer, in order to produce what they think is the best looking photograph, as I explained two weeks back. Any of these ISO points would be a legit choice; no one is more "correct" than another.
The exact shape of these curves is not fixed. In fact, it can be pretty much anything you want, so long as those ISO exposure points stay locked to an output value of 115. Messing with the curve shapes will change the look of the photograph, the same way choosing different films with different characteristic curves will change the look of those photographs, but the overall exposure will still be correct. Remember this; I'll get back to it in a bit.
That's what happens when different cameras (or camera makers) settle on different ISOs for the same sensor. Now, what happens when the sensor changes, like in my OMD vs. my E-P1? That's simulated in figure 4. The OMD has a longer total exposure range, which corresponds to the greater width of the OMD gray scale. Up to the exposure point, both gray scales look very similar. Above that, the OMD scale is stretched out and has lower contrast to accommodate the greater exposure range between the ISO point and pure white.
Now, here's the import of all of this. Your RAW converter has to deal with this. It not only has to have that RAW data on how much exposure each pixel received, it has to know what the camera considers the correct ISO. Sometimes that information is there in metadata that the RAW software author can read (though not usually in the EXIF data); in some cases the author will need to make camera measurements directly. Regardless, a description of the camera's handling of ISO must get built into the RAW converter. The reason the RAW converter needs to know about this is because it needs to automatically make the kind of curves adjustment that's shown in figure 3. If it doesn't, a "normal" exposure won't yield a "normally-exposed" photograph.
In other words, the RAW converter has to be able to say to itself, "Aha, I know this particular exposure level is supposed to correspond to the average gray setpoint for this camera at this ISO, and I will adjust the curve shapes to ensure that it gets rendered correctly."
Now, exactly how it adjusts the curve shapes depends on the design of the converter. Looking back at figure 1, you can see that ACR doesn't do quite what I did in my simulations. It keeps the values tracking pretty much the same for both cameras until the light grays and then the OMD curve goes very flat. That's an arbitrary design decision in ACR; a different converter or a future version of ACR can decide to use a different characteristic curve, so long as it intersects the exposure point the same way.
All of this can be changed by the photographer, too. You can change the settings for the exposure slider and for the curves tool in ACR to render your RAW files with an entirely different characteristic curve shape. You're not locked into one particular characteristic curve. But that doesn't change the underlying fact—you can't give RAW photos from both cameras the same curve shape and have them both produce correct-looking exposures.
Think of it as being analogous to this: T-Max 400 and Tri-X have inherently different tonal renditions. You can change how you process the film and get different characteristic curves out of the two films, but you cannot make them look the same.
To sum up: Different cameras' RAW files cannot always be interpreted the same way, regardless of the RAW converter one uses, even if the cameras have identical sensors. RAW rendering isn't just determined by the black point and the white point of the sensor, it's also determined by the exposure point, and that is different for different ISO decisions. When cameras have different ISO or exposure points, then it's impossible for their RAW files to be rendered with the same characteristic curve and exposure. Just as different films are predisposed to different "looks," so are different cameras' RAW files.
(Thanks to Dave Polaschek, David Dyer-Bennet, Oren Grad, Carl Weese, Jeffrey Goggin, Ken Tanaka, and especially Eric Chan for fact-checking this and for their great suggestions on how to make it more comprehensible.)
Ctein's regular weekly column on TOP appears on Wednesdays.
Original contents copyright 2012 by Michael C. Johnston and/or the bylined author. All Rights Reserved. Links in this post may be to our affiliates; sales through affiliate links may benefit this site.
A book of interest today:
(To see all the comments, click on the "Comments" link below.)
Featured Comments from:
Riley: "Shouldn't Wednesday come more than once a week?"
David Miller: "Bingo! I got it—and the first time through! Brilliantly written and a model of clarity: well done Ctein. Even better, it has practical application the next time I'm camera shopping, in helping me to understand the reviews and their implication for my particular style and preferences in photography. Thanks."
Mike Shimwell: "Good article Ctein, but I would take some issue with your title. :-) The issue you address is the interpretation of the RAW file (as you state throughout) not the 'rawness' of the data in the file.
"Adjusting the highlight curve to take advantage of the increased exposure range possible with newer sensors is a key way to obtaina more pleasing (film like) rendering. It's also an approach I've used with my digital cameras by deliberate 'under-exposure' to give more highlight headroom and then adjusting the midpoint and highlight curve in raw processing.
"One area that still interests me is how raw is the data in raw files. Accepting the processing chain from the voltage output from the photon counter through the amps, ADCs, etc., at what point does any noise reduction or other processing become 'intrusive'? A few years ago Canon RAW files were thought to be less tampered with than Nikon (re dark curent subtraction) and hence more useful for astrophotography. I wonder to what extent this remains the case and how the more modern senor/processing systems compare."
Ctein replies: Article titles are a funny thing. It's amazing how much discussion goes on behind the scenes about what they should be. Being exactly factually correct is, surprisingly, not at the top of the list. For example (though not in this case) Mike frequently rewrites my titles, which lean towards factual correctness, because either they won't draw a reader into reading the article or, more seriously, they don't include important keywords that will allow search engines to find them. That's a much bigger issue. Clever wordplay (well, what the author/editor imagines is clever—readers don't always agree) counts for a lot, too.
If folks look through all my old columns, they'll see that many of the titles are not literal. Sometimes they're not even close. The one about fitting a pint into a 12-ounce can had nothing to do with beverages. My column of two weeks ago had the title, "Why ISO Isn't ISO," which was not merely misleading but entirely false, because the article was about how ISO really is ISO. There are legitimately several different kinds of ISO. It was a catchy phrase that got people to read. This week's title is a riffoff of that one, and it's a lot more factual.
So, just lay back and enjoy. You can't take the titles too seriously.
As for me writing articles that dig even deeper than this one, describing what goes on between the moment a photon hits the sensor and data gets written into a RAW file. I'm probably not going to write those articles (though I always reserve the right to change my mind). The reason is that I don't think they'd be particularly relevant to the photographic experience. It's not that I'm not interested in that stuff, but I'm interested in it in a more abstract way. I might extract nuggets of practical information to include in articles, but, honestly, I don't think there's much there that improves one's photography.
Truth is, I wouldn't have even written this column if I hadn't butted up against this "different curve shape" thing, which does directly affect my photography.
I was pretty much that way on the film side of things, too. I mean, I can take the process all the way back to the fundamental quantum-mechanical level. Doesn't mean I'll write articles about that. Heck, I wasn't that interested in ordinary photochemistry. With no offense meant to David Jay and Tinsley Preston, who put out an excellent magazine, I never even read the original Darkroom Techniques (which was only half-jokingly referred to as "The Journal of Applied Sensitometry and Photochemistry"). I like knowing how things work, but I'm not a gearhead.
Don't get me wrong. I don't think there's anything wrong with having that deep knowledge; all knowledge is good. But I haven't come anywhere close to exhausting the knowledge I want to write about, and none of us have come anywhere close to learning everything useful we could about photography. What goes on behind the RAW file is, in my judgment, not very important knowledge.