Today’s Question: How do you know how much to feather a selection [in Photoshop]?
Tim’s Quick Answer: The short answer is that you don’t know how much to feather a selection by in Photoshop, which is why I recommend applying feathering to the layer mask created based on a selection rather than the selection itself.
More Detail: In many cases when you create a selection in Photoshop the selection edge will have a non-feathered border. In other words, the selection will transition from the selected area to the non-selected area with little to no transition in between.
In many of the situations where you would put a selection to use, you want to have a bit of transition between the selected and non-selected areas of the image. For example, if you’re using a selection as the basis of an adjustment that only affects the selected area of the photo, you’ll generally need to have at least a little bit of blending between the area being adjusted and the area not being adjusted.
With experience you can of course anticipate approximately how much feathering might be needed for a given selection. However, even with that experience it is much easier to apply feathering once you can actually see the effect of feathering in the context of the actual image.
So, I recommend that you not apply feathering to a selection, since it can be challenging to select the optimal value for that feathering when all you can see is the selection itself. Instead, I recommend not applying feathering until you’ve used the selection as the basis of a layer mask for either an adjustment layer for a targeted adjustment or an image layer for a composite image.
Once you have created a layer mask based on a selection, you can use the Feather control on the Masks tab of the Properties panel to feather (blur) the layer mask, or use Select and Mask mode to exercise greater control over the refinement of the edge of that layer mask, including feathering.