When Shooting in RAW, What Color Space Should You Use?
Most cameras that allow you to shoot raw also have numerous other options including the choice of color space between sRGB and Adobe RGB. But if you use raw, does the color space really matter?
To answer this question, it's worth starting from a common understanding of what a raw file is. It's nothing like a regular image and is really just a bunch of data that can later be turned into an image. There aren't separate red, green and blue channels at all. A raw file has just a single channel of information with half of the pixels distributed throughout the image representing green information and the remainder split 50/50 between red and blue. But even given that, the values for each color have an entirely different range of values and interpretation from the closest RGB equivalent.
The truth is, a raw image doesn't really have a color space at all in the traditional sense. Yes, there are limits to how bright as well as how "green" or how "red" a camera can record, but if you considered these limits as part of a color space definition, they wouldn't describe either sRGB or Adobe RGB at all. The total gamut of a typical raw file actually exceeds even that of ProPhoto RGB in many respects, but a lot depends on just how you convert it to a usable image.
But if one accepts that the in-camera choice of sRGB versus Adobe RGB for raw capture doesn't make much sense, does it nonetheless make a difference?
At least on my camera, the most obvious difference is the file naming scheme that results. In sRGB image file names have an underscore in the middle whereas the underscore gets moved to the beginning of the name when the camera is set to Adobe RGB. As do most photographers though, I rename all my images once I copy them to my computer, so this difference is irrelevant. If I shot much in jpeg, and if I somehow lost the embedded profile originally placed there by the camera, I might be happy to have the file name to fall back on, at least until I renamed the file, but this seems like a marginal benefit.
If I shoot raw+jpeg, the resulting jpeg images definitely have been generated using the selected color space choice. Raw+jpeg mode though seems geared toward users that can't decide between the shooting raw and shooting jpeg and besides this article is about shooting raw.
The camera histogram for raw images is based on the embedded jpeg preview image which does honor the color space choice. You can see the difference by shooting two otherwise identical images, varying only the color space choice. The actual embedded jpegs don't contain profiles but the wider gamut possible in Adobe RGB is definitely evident in the resulting histograms. There are programs that can extract the embedded jpeg images on your computer where you can do further comparisons to support this observation.
When shooting raw, the camera doesn't actually embed a profile, but amongst all the embedded EXIF metadata, the MakerNotes area does contain an indication of the selected color space to be used when you convert each raw image. But this only influences raw conversion for those users who use a raw converter that honors this value. Speaking as a Nikon shooter, this translates to just Nikon's own Capture NX software (or Nikon View, etc). Adobe Lightroom and Camera Raw ignore this value. Even when using Nikon Capture NX, you can override the default in-camera color space as you see fit.
When you convert a raw image in Adobe Camera Raw, you can select between sRGB, Adobe RGB and ProPhoto RGB. The default is based on your Camera Raw preferences, not on your in-camera color space choice at all. I always convert to ProPhoto in ACR. Since Lightroom editing operates directly on your raw files, it doesn't really convert them until you export to some other format. Internally, Lightroom operates in ProPhoto RGB.
So in every meaningful sense for raw shooters, the in-camera color space choice makes no difference.
This leads me to two questions for further contemplation.
First, why does Nikon (and I believe Canon too) allow users to select the color space for raw capture? Obviously Nikon does use this choice as the default for raw conversion in Nikon Capture NX and they undoubtedly do want to support users of their software. Clearly they have judged that at least some users would prefer to determine color space when shooting rather than when converting. Myself, I'd rather concentrate on shooting when shooting, and converting when converting, but maybe that's just me.
My second follow up question to scratch your head over is, if Adobe Camera Raw allows the choice of ProPhoto RGB in addition to the basic sRGB versus Adobe RGB dichotomy, why doesn't Nikon (and I believe Canon too) allow users to select ProPhoto RGB in camera? On this one, I have no clue. Early versions of Nikon Capture didn't support ProPhoto RGB conversion, but the program has now for several years. As such, this seems to be an inconsistency of design I can't account for.