Today’s Question: When one makes a selection or layer mask and then feathers it, does the feathering extend only outward from the masked area? Or does it also extend into the masked area?
Tim’s Quick Answer: When you apply feathering to a selection or layer mask, that feathering extends both outward and inward relative to the existing edge of the selection or layer mask.
More Detail: If a selection or layer mask does not have any feathering applied, the edge of that selection or layer mask can be thought of as having a crisp edge with no transition. The selected area can be thought of as white, and the non-selected area as black, which happens to be exactly how Photoshop presents a layer mask or saved selection.
When you apply feathering to a selection or layer mask, you are quite literally applying a blur to the selection or mask. That blurring extends in both directions from all contrast edges in the selection or mask.
In other words, when you feather a selection, the selection edge will be enlarged (by virtue of the blurring), and that enlargement will extend in both directions. So the area you selected will be a little less selected within the selection area, and a little more selected in the area outside that selection area.
Because of this aspect of feathering, it is very often necessary to shift the edge of a selection or mask outward slightly after applying feathering. This is one of the reasons I prefer not to feather selections, and instead save that feathering for after a mask has been created based on a selection.
Furthermore, this is one of the reasons I prefer to use the Select and Mask workspace in Photoshop to apply feathering, so that I can also take advantage of the Shift Edge control that enables you to shift the mask edge inward or outward as needed based on the subject area, the amount of feathering applied, and the strength of the adjustment (if applicable) being applied to the area you had selected.