Just to elaborate a bit, they’re linear multipliers meant to be applied to the raw data recorded by the camera. Take every red pixel, multiply it by 1.46399. Leave the green pixels as they are. They’ll be very specific to the camera model. The spectral sensitivity of a Canon 5DII is very similar to, but not identical to, that of a Canon 5DIII. And, of course, they correspond to the reference illuminant. You mention that this is for 6500K; the values would be different (but very close) for 6501K, for D65, for an actual illuminant with a measured CCT of 6500K, and for an actual illuminant with a measured CCT of 6500K in non-neutral surroundings. If you have a photo of a reference chart (a 24-patch ColorChecker is perfect), you can easily determine the exact figures for that scene. Use the raw developer to dump the linear raw data — no exposure, no gamma, no white balance, no nothing except demosaicing. Create an ICC profile from the chart and ask the chart what RGB values correspond with L*=100, a*=0, b*=0; this will be a reverse lookup. Basic high school algebra will tell you what multipliers you need to make that value be R=100%, G=100%, B=100%. This will, incidentally, also give you the perfect exposure adjustment. Note that most people who have little to no experience shooting raw inevitably wind up underexposing their images, often dramatically so. Digital cameras have an overabundance of dynamic range, so the raw converters have no trouble silently and automatically covering up the underexposure. But don’t be surprised if your image of the ColorChecker is so dark that you can’t even find where the chart is on the image. In that case, if at all possible, adjust your exposure accordingly and re-shoot the scene. Iliah’s RawDigger analysis software is an essential tool for anybody doing serious raw photography, and the Web site for RawDigger is possibly the best guide to raw exposure techniques (and related subjects) you’ll find anywhere. If you’re going to do anything like what I described above, you’ll want to use RawDigger to extract the values from the ColorChecker to create a GCATS file that you’d hand to your ICC profiler. There are other ways to do that sort of thing … but … Cheers, b&
On Dec 5, 2022, at 4:07 AM, Iliah Borg via colorsync-users <colorsync-users@lists.apple.com> wrote:
Hi Jeff,
Those are white balance multipliers, as the cameras write them. These multipliers equalize colour channel sensitivities with the aim that after multiplication raw values become equal on the gray.
The idea of expressing white balance as CCT + tint is a film paradigm, and it is not the best for digital (not directly measured data, difficult to calibrate, too many conversions back and forth).
On 12/5/22 01:22, Jeff Nova via colorsync-users wrote:
I’ve run into some data where the white balance is expressed in a manner befuddling to me: 1.46399. 1 1.52999 Is 6500K ?! There is a tint (4.2 for what that’s worth) applied to the white balance as well, which may confound this data. Anyone have an idea of what these numbers might be? XYZ and xyY both seem wrong…? Thanks! Jeff _______________________________________________ Do not post admin requests to the list. They will be ignored. colorsync-users mailing list (colorsync-users@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/colorsync-users/iliah.i.borg%40gmail... This email sent to iliah.i.borg@gmail.com
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