While reading about soft focus, I came across a method for applying soft focus digitally. One of the steps involved was a motion blur. After some experimenting, I discovered that the GIMP motion blur moves the image from where it previously was, by one half the blur length. The examples in the article lead me to believe that photoshop leaves the image where it was, without shifting it over, which seems to me like the correct behavior.

Here's an example of GIMP's motion blur. Before:

After motion blur:

I got around this by doing this:

1. Duplicate the layer
2. Let's say you wanted a blur of length 11, centered on the original image at 45°. In the upper layer, do a motion blur of length 5 at 45°
3. In the lower layer, do a motion blur of length 5 at 225°
4. Set the upper layer opacity to 50% and merge down

I compared that with this procedure:

1. Motion blur of length 11 at 45°
2. Move the layer down 5 pixels and right 5 pixels

The second procedure looks slightly better visually -- the first procedure produces an image with a more abrupt change in the blur where the original pixels were. I can imagine a process where you do two motion blurs of length 11 in opposite directions, move them both back to the middle and set the top layer at 50% opacity, but that leaves the upper right and lower left corner unblurred.