If you’re at all tuned in to the world of digital photography you’re probably already aware of RAW files and why you should use them. But just in case you’re not, the quick answer is that RAW files allow you to preserve as much data as possible from what the sensor captured. This is in contrast to what happens when you shoot JPEG, where your camera makes some fairly arbitrary decisions about what it thinks the photo should look like, often throwing away a lot of detail in the process. Specifically it will throw away details in the high end of the image – the brightest parts.
But what a lot of people don’t realize is that even if you are shooting RAW, any review you do when looking at the display on the back of the camera is also not showing you any of that highlight detail. In other words, even if you’re shooting RAW your camera will only show what you’d get as if you were shooting JPEG.
Why is this an issue? Because it means that you may be tempted to underexpose the image in order to prevent, for example, your sky from ‘blowing out’. Even though in reality your RAW photo may have already captured the full tonal range of the sky.
Here’s a quick example – first of all, a photo that I took directly off the display on the back of my camera. (Yes, this is somewhat meta :-). As you can see, the sky appears to be completely overexposed to white.
Here, I even made an animated GIF that shows you what the flashing overexposure warning on my camera looks like (you may have to click on the image to see the animation).
Yup, the camera is telling me that, without a doubt, I’ve really overexposed this shot and have lost all that detail in the brightest part of the sky. And if I had indeed been shooting jpeg this would be true. I’d get home and view the file on my computer and I’d get an image that looks pretty much the same as what the display showed me, i.e. this:
But I wasn’t shooting JPEG, I was shooting RAW. And if I take the RAW file that I actually shot and bring it into something like Aperture or Lightroom and work a little magic, you can see that there’s a whole bunch of beautiful blue sky in the allegedly overexposed area. Like this:
Tricky tricky. Note that even the histogram display is misleading you – it, too, is showing you data relative to the JPEG file, not the underlying RAW file.
See how the right side of the histogram slams up against the wall? This also indicates that you’ve clipped data. Only you haven’t.
So beware, gentle readers. Beware of the camera that lies. Unfortunately there’s no good solution to this other than to develop some instincts about how much you should (or shouldn’t) trust your camera. It sure would be nice if manufacturers offered the option to display the image (and histogram) relative to the complete data captured in the RAW file but I’ve yet to find a camera that allows for this. Anybody seen one?
In the meantime, just be aware of the situation and plan your exposures accordingly!
Digital Composting 
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red cameras allow for raw histogram… and a ton of other useful exposure tools.
Yeah, RED really really understands what raw workflow is all about.
There is a very easy way for a correct histogram. You just need to determine wich value your camera sees as “true grey”, and just use this value for whitebalance. This gives you green Camera-Previews, but correct histogram. To do this we create a photoshop document:
1. Creating a square testchart.
Create a square testchart (1024×1024) with the following values and name the layer “basechart”.
Red: black to white, horizontal gradient.
Green: Constant 0.25 (63)
Blue: black to white, vertical gradient
2. Shooting the chart and developing it without whitebalance.
Maximise this “basechart” layer on your monitor and shoot it with your camera as raw. Develop the raw with NO whitebalance. (Dcraw can do this, use this commandline: dcraw -v -r 1 1 1 1 -T). Bring it into Photoshop (crop and transform it to a square again if you had perspective errors). Blur it slightly to remove the monitor pixels and scale to 1024×1024 again. Place this layer above “basechart” and name it “NoWB”.
3. Determining the proper white balance color
To determine the true grey in the “NoWb” layer, crank the saturation all the way up to the maximum. This will show a “pole” in the image from which the colors originate, at a certain x and y coordinate. Looking at the “basechart” layer, these x and y coordinates from the “NoWb” layer will point you to an exact RGB color, typically some kind of dark magenta.
Create a new image, and fill it with this color. Shoot this magenta image with your camera and use it as whitebalance. Depening on how exact you determined the true grey, your histogram will now show the true raw rgb proportions.
Point I was making, though, is that the major camera MFR’s don’t generate their histogram from the RAW file but rather from the JPEG. Yes I can get an accurate histogram back at home in the studio, but I want it out in the field!
good point – I still shoot the camera like slide film, so if I’m not seeing detail in the whites I re-shoot.
And your point is of course correct. It’s a shame the histograms can’t be simply switched to Raw! But, well, except of the green colors, this workaround works of course everywhere, just put this WB into a custom slot and be done. Sorry if this sounds too basic, Im sure you knew all this. Best regards!
No you’re right, it’s an interesting strategy. Have you actually used it in practice?
Yes, I use it all the time since 2 years. I discovered that this WB setting (or any other WB setting for that matter) once loaded into camera, does not longer require the image on CF-card it was created from. (At least with Canon.) Wich is nice, it just sits in a custom wb slot ready if I need it. At first, I shot all the time with this setting to get a feel for it. Nowadays, I use it until the final shot, if required, and then turn Whitebalance back to something more appropriate. Just another little option in the toolbag.
just tried it – you can simply shoot something blown out white and set it as a custom white balance. (Canon DSLR). You can see the difference in the histogram straight away.
Yes you are right I left this out.
Ron
Clear, valuable, and practical piece. Congrats on a well-written, thoughtful explanation that every beginning photography student should read. Make it into a book and sell it.
Jim