Consistant cropping on similar product shots.

[The_Traveler]

Let say I wanted to alter the brightness/wb/saturation on one layer and I needed to compare it to another 'standard' without changing the position.
There are certainly several ways do do this but for one simple way:
Add a layer mask and draw a wide black line through the mask, thus being able to "see" through to layer I liked.

Now add the appropriate adjustment layer, perhaps 'brightness' and then right click on the adjustment layer and check the clipping mask option. The 'clipping mask' ensures that any changes made on the adjustment layer affect only the layer it was attached to.
Then make the changes and when satisfied, either discard the mask or color over the black line with a white one.