There's another way. Use Calculations. Select Image then Calculations. Try first channel as Red, Green, Blue, or Grey. Same goes for 2nd channel. Then select the blending mode and opacity. Try it around until you find one contrast level that you like. When you are done, go right to the bottom where there is a Result section and select "New Document". A new file will be presented to you which you can later convert to Grey Scale. This method gives me more control in the contrast and detail reproduction.