Balancing Light and Dark Areas
This process can be achieved using slides and negatives (which must have been taken on a tripod to get exact registration), but is most easily achieved in the taking stage of digital cameras using the RAW setting.
You will need 2 images – either 2 slides or negs – or one RAW image taken for mid tones.
The RAW image you will need to make 2 conversions, one being for the shadow areas and one for the highlight areas, before converting to a TIFF.
Select the dark image – Select >All > Copy
Select the light image and paste the dark image on top (makes 2 layers)
Add layer mask on the dark layer (2nd button from left on the layer palette).
Select the light layer – Select > All > Copy.
Now Alt + Click on the white rectangle on the top layer and paste.
This gives a black and white version of the image on the layer mask.
Filter > Blur > Gaussian Blur and select 50 pixels > OK. (This looks blurred).
Click on the background layer and you will see the changed image, this can be improved more by using the brightness and contrast adjustments.