User ID wrote:
Color correcting black and white ?
Care to explain how that works ?
As an ex-lab guy, I'll take a stab.
Once upon a time, there was black-and-white film. (There still is, just a small fraction of what there once was.) It was printed on black-and-white paper. There was no need to worry about color, unless you ordered toning from a custom lab. But you did have to worry about contrast and gradation and tonality.
At some point, Kodak and others started making black-and-white CHROMOGENIC papers. These were papers that could be processed in color print chemistry, allowing labs to run everything through the same process. The images on these did not have the same depth and character as real silver-based emulsions, but they were cheap, convenient, and allowed big labs to streamline things. And they still produced "black-and-white" (grayscale) images. With that scenario, STAIN was an occasional issue, caused by a dirty processor and chemical neglect (inaccurate replenishment or replacement... Some labs get slack about it during busy seasons).
Then, along came the age of DIGITAL mini-labs. Unfortunately, these did not work very well with chromogenic black-and-white papers. So labs decided to print black-and-white files on COLOR paper. That's when the fertilizer hit the rotary distribution mechanism.
A digital mini-lab printer that is even slightly out-of-calibration (linearity of paper exposure in all three channels) or out-of-control (a chemical process imbalance) may produce a color cast in every image... even black-and-white ones. It is especially noticeable in black-and-white images. The lab I worked for struggled with this. When I ran the printing area, we always ran exposure calibration and process control tests before a batch of black-and-white, making corrections to calibration and replenishment as needed.
Color correcting black-and-white is required *partly* because files are submitted in RGB JPEGs. Exporting a grayscale bitmap or TIFF image as a JPEG converts it to an RGB bitmap with identical values in all three color channels. So when it's printed, it drives three color channel exposures. If the printer isn't calibrated to reproduce perfect gray at every level from 0,0,0 to 255, 255, 255, there's a color cast. If the process is out of control (no process is EVER perfect), there's a color cast. And you never know how much is calibration and how much is control, because the printer and processor drift independently.
The other issue with black-and-white is the same as it is with color... MONITOR CALIBRATION and MONITOR PROFILING. If the person who created the black-and-white image adjusted it on a computer monitor that is "visually dishonest" (i.e.; out of calibration, improperly profiled, or
off standard), then the tonal appearance of the print will not be good. It might be washed out, or murky and muddy, or otherwise not match the photographer's "rendering intention."
The solution to this latter problem is to "color correct" the photographer's *monitor* — i.e.; calibrate and profile it to achieve ICC standard linearity. But if the photographer won't or can't do that, paying a lab for file adjustments may produce a better result than leaving the file alone. Or not...