The conversion from negative to positive is different for each make/model of color negative film. If your scanning software doesn't support the kind of film you use, the results can be very strange. If you turn on any sort of automatic density adjustments in the scanning software, then you have to correct a moving target. The automatic adjustments will move the color problem around, making it necessary to do different tweaks to each image.
My usual process for color correcting negative scans with Adobe Photoshop is as follows:Auto-Levels
Probably the simplest solution is to waste a frame or two shooting a color target that has a reference white and black sample on it. Shoot this under the lighting that the rest of the shots will be taken under. Then use this to color balance the shots that follow. It's extra work and extra frames, but the difference is worth it. I use this technique with my digital camera and I always get really great results. Where this fails is when you are shooting under mixed lighting. If some of the light is sky light coming in through a window (blue) and some of the light is incandescent light in the room (yellow), then you can end up with a big mess no matter what you do. Flash is a good idea in these situations.
If you have a lot of time to kill, you can tweak each image to perfection. Personally, I think there has got to be a better way. Kodak Photo CD can handle hundreds of different kinds of color negative film, so when used properly, the Photo CD system does a pretty decent job with negatives. The results are very close every time, requiring only small color tweaks to get something useful. Unfortunately, Kodak hasn't been keeping their list of supported films up to date, so many films are currently not supported.
I think what is really needed is a way to give the user control over the negative to positive conversion. Pull in a raw scan of the image (or something similar to avoid any density tweaking by the software), and use a user-defined conversion to get a positive that is right every time. This would require that the user have control over orange mask removal, and color crossover (anything else I've missed?).
A simpler and more familiar model might simulate a darkroom. Allow the user to select color correction filters for the simulated enlarger, and provide models of various printing papers that are currently manufactured.
Any of these approaches would give us a tool that would take a lot of unnecessary burden off the scanner operator and give much more consistent results.
Converting Negative Film to Digital Pictures by Phil Williams 6/26/2003. This article shows how to convert a negative scan into a decent positive. Essentially, you use the orange mask on the film as the white point, then a good black part of the film as the black point, adjust the levels to neutralize these, then invert the image. I've not tried it, but it should work assuming you can find a good negative black point. The white point is easy since any portion of the film that is not exposed (like the leader) will be completely "negative white".
My Kodak Photo CD Homepage - I used Photo CD for three years, and although I certainly noticed variations in color with different color negative films, the corrections needed were always pretty small. Lately, my service provider has stopped specifying the proper Film Term, so I've discovered that Auto-Levels can get you very close. If not then you have to rely on experience to help you color correct.
My Polaroid SprintScan 4000 Review - I didn't realize how big a problem Color Negative film really was until I tried scanning it with my own film scanner. The results were so bad I returned the scanner.
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