How do I blur the background around an object in post-production?

Question

In Photoshop (current version is CC 2014 (2014.2.1 20141014.r.257 x64)) I did the following steps of interest:

1. duplicate background w/subject masked
2. apply a blur filter of some kind to the background, heavily.

Now since the subject was carefully masked (thanks to Select Focused for doing most of the work!) I have an effect that looks like a very shallow depth of field. The uninteresting background can be abstracted.

However, I get an unwanted border around the subject. The blur of the background includes the subject which spreads out to affect the color of neighboring pixels, and that shows up as a fringe when I drop the sharp version of the subject in.

I tried adding an inverse of the mask to the filter layer's mask, but it didn't do anything.

Is there an easy way to address this? I'm pretty sure that the overall approach is the classic way of doing this, so I must be missing something.

(I used the Blur Gallery, with Field Blur or Iris Blur. It doesn't matter since the mask generated is applied after blurring the entire image)

Answer 1

Use the "Lens Blur" filter, which is basically a variable diameter blur function that a) avoids the effect of the focused object bleeding into the background and b) uses a more realistic kernel which more closely resembles an out of focus background.

You can select the area you want to remain sharp in advance, and then tell the filter how much the rest of the image should be blurred. Alternatively you can create a depth map which allows you to have more distant parts of the image even more blurred for the most realistic effect.

Answer 2

I think you need to remove the subject from the copied background layer, before applying the blur. Use the selection you are using to create the mask - and use it to cut out the subject from the copy. Then apply the blur. Then apply the mask.