讨论:GIMP vs Photoshop
Alternative to 调整图层=
Regarding adjustments, the procedure that I normally adopt is to work from a TIFF file (but it will only be 8-bit). Load your image and immediately duplicate the layer. Switch to that duplicated layer and do not touch the original. Thus, you still have what you started with for comparison. (You should of course have a backup of your file anyway.) With adjustment layers you have the advantage of setting up several and adjusting each to see what happens to the image without committing to that change. Thus, instead, you have to carry out adjustments in a useful order. You can try creating more and more levels, but it's a waste of time. I suggest you adjust for exposure first (Levels and/or Curves), saturation tweaks next, followed by perspective corrections and cropping, and finally sharpening.
The Levels dialog and the perspective correction tool are very useful on the the GIMP. Download and try the latest range of "wavelet" tools from the Plug-In Registry - these are very useful and smart, especially the wavelet sharpen.
You mention layer masks. The GIMP has features associated with layer masks not available in other software, including as far as I know Photoshop. Ron Bigelow's tutorials on sharpening exploit layer masks and this turns out to be a dawdle on the GIMP once you've twigged what to do. However, if you've not yet fathomed layer masks in general the following comments may not make much sense (Even if you have the following may not make sense.) Look to your Layers palette, duplicate your base layer, open the Channels palette; now drag the copy layer from the Layers palette into the lower box in the Channels palette. Hey presto, you have just created a new channel based on your image. You can manipulate that B/w channel in any allowable way, then go back to your layers palette, choose a layer, open the options dialog with a right click, then add layer mask from the new channel. You can also drag the manipulated new channel back to the layers palette and it becomes a new layer. As I say I'm pretty sure that Photoshop does not have that facility. Nor is this facility explained anywhere. I just found it. Maybe in GIMP world you just know these things.