Photoshop Tip: Soften your layer masks
Tips & Tutorials, by Mark White
September 4, 2012
The Gaussian Blur filter, set to 1-2px, can be used to soften a layer mask
This works in just the same way as the Feather command, but can be used at any stage through the editing process.
Use this technique to help blend the transition between two separated areas. The effect is only slight, however will remove the pin-sharp edge that makes objects look unnatural.