I've been using Photoshop CS2 for several years to enhance my photographs, but just now have encountered something simple I cannot find out how to do. I have on the background layer a picture of a room interior with the exposure set mostly for the open window, but the rest of the room is very dark. On the only other layer, the upper one, I have the same shot exposed correctly for the room, which overexposes the window aea. The upper and lower layers are perfectly superimposed, and the upper layer is at 100% opacity. I have selected the window on the upper layer and want to change the selected area's opacity to blend properly with the window on the bottom layer without changing opacity of the rest of the layer. No matter what I do, changing opacity seems to affect only the entire layer, and I haven't figured out how to isolate the selected area for the opacity change.
I can cut the selection to reveal the darker window on the bottom layer, but this doesn't give me the flexibility I need to balance the two exposures. An opacity change would be ideal.
Okay -- I figured out a workaround. I did the opacity change to the whole layer so that the window looked the way I wanted,and flattened the image. This left me with a great rendition of the window, but not of the room. Then I added a new layer at 100% opacity with the good room image but the overexposed window. I repeated the window selection on the new layer and did a cut, which revealed the good window in the good room image. Then I flattened the image and proceeded from there
I'm closing this question as solved.
You should read up on Layer Masks. Masking would have allowed you to work much faster and non-destructively, as you wouldn't have had to flatten your layers.