One of the nice aspects of using adjustment layers within Photoshop is that is automatically creates a white mask. This mask can then be used to refine the adjustment you make from within the chosen adjustment layer.
Curves adjustment layer.
Let’s take a curves adjustment layer for example. In this example, I want to make an adjustment to the bottom of the image so the first thing I do is create a selection around the areas I want to adjust.
To make the selection, I chose the lasso tool. The initial selection does not need to be perfect, just ensure you make a selection of the area of interest.
Click on the image to see the selection.
Now the selection has been made and is active, I created the Curves Adjustment layer which places the curves on-top of the layer stack. Notice that the selection I made has automatically been added to the mask adjacent to the curves.
I lift up the curve to brighten only the area which was contained within the selection I made. As the selection was only a rough selection, areas just beyond the selection will also be brightened but we are going to pull those back using the Mask Feather option.
We can now fine tune the mask by feathering it slightly to make the transition more smooth. To do this click on the adjustment layer Mask and then click on the small mask icon in the properties panel.
Adjust the "feather" slider until you see the desired effect you are looking for.