Hi everybody, I'd like to use a layer with a grayscale map to set the brightness for each pixel of an underlying image. E.g. Dark values lower the brightness and light values set the pixel to a brighter value. Is this possible with Photoshop and how? Maybe a stupid question, but I'm just a beginner...

In every adjustment layer you can paste a greyscale image as a mask. You have to alt-click (or similar, I am on Mac, option-clicking, so not sure) to access the mask, or call it 'jump in'. Also, I am not quite too bright on the theoretical side of things, but some of the layer blending modes do actually what you want when you apply them to a layer without color (not REALLY grey scale, because you would be in a RGB file I guess). Another way to this would be to use a dark version of your file (the not-bright-part), and stack a light version in a layer on top of that. And use the greyscale image as a mask in the top layer. As we say in Holland, there are a lot of roads that lead to Rome. Dunno why Rome, actually.

Thanks a lot, I'll try that

There are a couple other options... For a straight grayscale conversion without changing the color mode, try using a Hue/Saturation Adjustment Layer as your top layer with the Saturation set to -100. Takes all the color out, and can easily be turned on and off for quick previews. A slightly more complicated solution would be to instead of using Hue/Saturation, use a Gradient Map with the default Black-White Gradient as the basis. The gradient can then be managed in the Gradient Editor or a Levels or Curves Adjustment layer can be placed above it for smoother control.