Old thread. Just to report on some further conclusions here,
after some more tests with bracketed exposures of the ColorChecker.
The question was about Exposure-dependency of the DNG profiles created with the Chart wizard of the DNG Profile Editor.
/> The Hue/Sat.-corrections from the Chart Wizard are indeed found to be dependent on +/- EV camera exposure.
/> The effect is less pronounced with the Hue shifts, but the Saturation shifts vary significantly.
/> Surprisingly, it was found to be a systematic deviation: the lower the camera exposure, the more are the Sat.-shifts pulled down to lower or negative numbers. With a dark target capture, it seems that the Chart wizard tries to brighten the patches by reducing saturation. Could be an issue with the underlying HSL separation, rather than sensor imperfections calling for Lightness-dependent Hue/Sat.-corrections (what I initially thought).
I think a most correct exposure for the purpose of running the chart wizard should yield something around L* 96 for the white patch corresponding to the synthetic target in Lab
. The value can be measured directly in the DNG Profile Editor while the readouts are computed based on scene-referred ProPhoto RGB linear coordinates.
L* 96 for the white patch may in practice require a +EV camera exposure setting (depending on camera of course). It is basically in line with Eric’s recent recommendation as quoted below.
The DNG PE needs to be somewhat conservative in its chart wizard with regards to clipping because color tables are always applied after white balance. Your raw capture may not have clipped data in its native coordinate system (native RGB, without WB applied), but that data can become clipped after WB is applied. That leads to problems with the color mapping. DNG PE will generally detect this case and prevent you from proceeding.
My recommendation is simply to bracket exposures and then pick the brightest one that DNG PE will accept without giving you an error.