Like I said, depth of field
The opacity setting in the cap approximate what it could do. You just don't have as much control with opacity. In theory you could use a color filter for this, but we don't have decent controls for changing hue and contrast vs, just plain color. You could use the image manipulator, but it would be very slow.
Which reminds me, there might be a drawback given that this is a blur, so the drawbacks may out weigh the benefits entirely.