Sometimes your exposure is so far off that you need more than a simple levels adjustment. Often, a really underexposed (dark) photo is better left alone and thrown away. However, there are times when a photo is important enough that it is worth saving. In this example photo we are going to save an otherwise too-dark photo using a SCREEN LAYER.
1. Open your photo (File > Open)
2. Make sure you layers palette is in view (Window > Layers) or click F7.
3. With your background layer highlighted, click Control J (Command J on a Mac). This will duplicate your layer. Alternatively, you can grab the layer and drag it to the new layer icon and it will duplicate.
4. You should now have two layers that are exactly the same.
5. With your BACKGROUND copy layer highlighted, you are going to click the blending mode box (that will by default say NORMAL). Blending modes tell the computer how the pixels in the top photo should interact with the pixels in the bottom photo. We are going to choose SCREEN MODE. Screen blending mode brightens the underlying layers depending on how bright the screened layer’s pixels are.
6. You should see your photo lighten up considerably. At this point, you can either stop and save or if it isn’t light enough for you liking, you can add another screen layer. With the BACKGROUND COPY layer highlighted, click Control J to duplicate. If this makes your photo TOO light, you can adjust the opacity of the layer.
For another example with further details, you can see THIS article