What is Lightroom *really* doing when I change a Camera Calibration?

Question

What does LR / ACR really do when a user changes the Camera Calibration setting?

Here's Adobe's statement (for LR4, but I didn't find a newer one and there's no reason for this to change):

Lightroom uses two camera profiles for every camera model it supports to process raw images. The profiles are produced by photographing a color target under different white-balanced lighting conditions. When you set a white balance, Lightroom uses the profiles for your camera to extrapolate color information. These camera profiles are the same ones developed for Adobe Camera Raw. They are not ICC color profiles.

(from http://help.adobe.com/en_US/lightroom/using/WS939594D8-4279-41b4-B8E9-B06BC919EC7C.html )

As far as I can read into this, the Calibration contains the map that transforms RAW data into what we see on our screens. IF my assertion is right, as long as a Calibration "makes sense" (that is, it's generating a result that "kind of" translates into what once was our perceived image, AKA "reality"), it shouldn't matter which one we use for further development.

EXCEPT it's altering the histogram and I can see - vaguely, as Adobe's histogram is more like a "guesstogram" - changes in color balance, shades, "exposure" or gamma AND I don't like when a program messes up with my images without leaving some kind of trace.

Any other changes made in LR can be read afterwards by looking at the settings. Even in what might be a complex preset by VSCO, for example, one can see the R/G/B color curves changing and the HSL setting changing as well, so one gets the (numeric) feeling of what was done.

Understanding settings enables a user to change what is needed.

So, why do I care? Because (a) Calibration does change color tones in a time consuming way to alter afterwards and (b) "Adobe Standard" is not always the most pleasing option.

Again, if it's indeed a complex mapping, or a color mapping, I believe that choice should be the starting point for further processing, except very few people ever mention it as a starting point. (Adobe does. In this obscure Help page I mentioned. And in a YouTube video that's not "by" Adobe itself. No Julieanne Kost sanctified video on that.)

Do note that it's possible to finish processing an image and just go to Calibration and try different settings "to see how it goes". It's just weird for me.

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I'd like to know if there are specific views here as to where in the workflow this should (or should not!) fit, learn if anyone has advice on how Calibration might impact results and, of course, try to learn a bit more about all the complex layers involved in Adobe's ACR / LR / PS.

Tks!

Answer 1

If you wait until late in the process to change camera profiles, what Adobe is really doing is going back, converting the RAW image using the newly selected profile, and then applying all of the adjustments to the newly created image (using the new profile) that you had made to the older image created from the same RAW file using the previous camera profile.

Answer 2

A camera profile may consist of several operators, including:

1. A linear transform (a matrix) from the camera RAW color space to CIE XYZ color space (used to compute the conversion transform to ProPhoto RGB, which is the working color space of Lightroom).

2. An HSV lookup table - Similar to the HSL function in Lightroom, but provides much more control on the transform.

3. Exposure baseline - can offset the baseline exposure (effectively it moves the 'zero' point of the exposure slider).

4. A "Look" table - Another HSV lookup table, to apply a custom look, on top of the "correct" colors.

5. A baseline tone curve - An RGB tone curve, usually brighten the image significantly.

Steps 1-3 should bring the image to the "correct" colors, while steps 4 and 5 aim to produce pleasing colors/tones (this enables to mimic the "as shot", a film-like, or other look that people may consider "better" for their purpose).

Adobe hides these baseline functions in the camera calibration, because they represent a starting point for the user edits. Also, most of them are too technical for the average user, or may even impractical to fine-tune by hand (e.g. large HSV lookup tables).

Answer 3

The profiles are necessary in the conversion process in order to make the colors look "correct". There are multiple profiles to choose from (and user can make additional ones) in Lightroom, because sometimes more accurate rendering is needed and sometimes more pleasant rendering is preferred. And some profiles are designed to match picture styles of the camera maker so that people can get what they see on the back of their camera and vs. using proprietary raw converters...

Some (camera default) profile is always applied before any other adjustments take place. As far as changing profile after editing, it seems to have the same effect as changing it before, so I don't have any strong preference.

Any other changes made in LR can be read afterwards by looking at the settings. Even in what might be a complex preset by VSCO, for example, one can see the R/G/B color curves changing and the HSL setting changing as well, so one gets the (numeric) feeling of what was done.

The profiles are part of the raw conversion process, not a color correction process - the values are not meant to be individually tweaked during image editing. If you want to see the changes the profiles make, make your own in Adobe DNG Profile Editor.