Posted By:
AnthonyOk, but it might get a bit geeky.
All parts of an image have a numerical value of 1-255. 1 being absolute black, 255 being absolute white. Generally if you want something to print with detail you need to be around 45 for black point and 243 for white, but it depends on how the image is being reproduced.
If you take an image into photoshop and pass the color picker (eyedropper) over it your info pallette will give you readings in the R, G, and B channels of 1-255.
Whenever I scan images (actually in my case I shoot copy shots with a digital back, but it's the same for scanning) I make sure the white of the flip is at a reading of about 241. When that is done the card is of the correct exposure, assuming the lighting is even (which it always is on a scan, or on a properly lit copy shot).
If you have photoshop the easy way to do this is open the scan, bring up curves (command-m on a mac, probably a similar way of those other computers) and then double click on the white eye dropper. It will bring up a menu with R, G, and B values, and you'd type in 241 for each. This makes them your new target colors. Now click on the flip with the white eyedropper, and it corrects both exposure and color shift (if any) in one move. Click ok to accept the new curve, click OK when it asks you if you want this to be the new target colors (you do) and you're good to go.
This works for both PSA and SGC flips. Global you have to eyeball, due to the silver flip. Haven't seen the new GAI flips yet but should get a submission back this week. I think they are white now as well.