I have been experimenting with using very long daylight exposures with a 10 stop B+W ND Filter. When I use the filter, I get some pretty interesting shots, but I have found that I get a brown cast in the images. I can improve it slightly, but I want to figure out how to best correct the issue.
I have a graycard, and I was considering using it to set the middle gray, but I am not sure that will totally address the brown color cast in the highlights. Do I need to get a color checker and do a test calibration shot? Can I shoot a test shot once and get a general sense of the correction in daylight and then apply the same adjustments programatically to other scenes? I really don't want to carry a color checker or graycard around everywhere.
Another interesting note. 10 stops is really dark. I am going to get a 6 stop filter, because working with 10 stops basically means shooting totally blind. I think it claims 0.1% light transmittance. It results in some cool effects though.