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Gamma is an intensity transfer function that maps between the ideal color space in PSP and the color space of a device like a monitor, scanner or printer.
If you aren't any wiser than you were a minute ago, we aren't surprised. The reason most people don't know about gamma is that they usually don't really need to know about gamma.
You know those knobs on your TV that are under a little panel that you never fool with? Well, gamma isn't like them, gamma is like those little knobs inside your TV the service guy adjusts with a screwdriver.
If you don't have an absolute requirement to make an image look a particular way on a device that isn't your monitor (this means if you aren't doing a commercial printing or advertising job) and if your monitor isn't broken, (it doesn't make everything look really dark or really green) then leave that monitor gamma alone.
If you don't know what it is or why you're doing it ... don't mess with it. If you've already messed with it, go set all three colors back to 1.0. When somebody asks you to do a print advertising or movie job, ask them how to adjust the gamma.
If you set the gamma to anything but 1.0, you'll end up with your images looking good on your monitor and terrible on everybody else's monitor.
OK, here's the exception. If you know that the image will never leave your computer, and if you can't adjust your color printer so that the image on the page looks like the image on your screen, you might be able to adjust your monitor gamma so that the two will look closer. And you might not. You'll just have to try it and see.
Then after you're done with that job,
set the PSP monitor gamma back to 1.0
or you could be sorry.