Dave,
Here are two shots of ColorChecker, on under CFL and the other under daylight. The18% grey patch is equalized on both shots to 51-52%. All other parameters except for WB and Exposure are identical for the two shots.
The black patch is 11-12% on the CFL shot, whereas it is 13-14% on the daylight shot...a slight difference but not a signficant one. However the white patch is only 84% on the CFL shot, and 92% on the daylight shot!
Wilt,
This is probably not the best place to start a discussion about color management using the Color Checker. What color space are you using? If you are doing the readings in Lightroom then its ProPhoto by default. The grey square you are referring to has a 40% plus minus value in Prophoto RGB not 51%. Next did you do a white balance on the second square from the white square? That's the square that Adobe uses for its white balance reference. Then, what tone curve are you using? Unfortunately, Eric Chan, the principle engineer at Adobe, decided when doing the work on ACR and Lightroom raw conversion not to apply any tonal range default. It is his view that tonal range is too subject dependent and he feels that imposing a tone curve would limit creative potential.
That's why, as I noted earlier, in my experience using the white square to set exposure and then applying a strong tone curve in Lightroom brings the black square quite close to its reference value. I then let the 4 other square fall where they will and adjust the tone curve based on the image.
For critical documentation I have built a tone curve that has each square at its reference. The curve meanders from white to black with some significant hills and valleys.
If you think about it for a minute, I think you'll agree an uniform light source does not have inherent contrast. If a flat white card is placed in the sun so that the light is striking it evenly it will read the same value with a light meter at any point. Placing a Color Checker in the middle of the card and photographing it results in a reference for that light source.
Move the card into a room, illuminate it with one or more incandescent lights, metering to confirm consistent illumination across the card, place the color checker in the center and photograph it. Now you have a reference for the incandescent light source. Import the two images into Lightroom, adjust the white balance using the second square and we now have a reference for the spectral response of the light source based on a standard reference.
Lights with a red bias are different than lights with a blue bias, etc. That's what you are measuring not a contrast in the light.
Remember how we used to select the B&W film we used, or the color film, based on how it would "interpret" the light?
DXO, and others, have developed software plug-ins for Lightroom to emulate a wide variety of films, B&W as well as color.
If one takes a Color Checker image and applies those emulators, the grey scale and color reference squares change, in some cases dramatically. Again, its not because the light suddenly has more contrast or is bluer or more yellow. Its the color bias and tonal response of the film being emulated. These characteristics can be seen when opening the tone curve panel and HSL panel in lightroom. The sliders have moved to recreate in Lightroom the tonal and color response of the film.
At least in one instance, based on what the plug-in creator related on his website, these plug-ins are created by photographing a Color Checker in various light sources with the film, then reading the squares with a spectrometer and replicating those readings in Lightroom using the sliders in the tone curve and HSL modules.
