One of the option is to place a big plane geometry behind the object and make it semi transparent.
If planning lot of camera movement or if you are going to use orbit controls then attach this plane to camera.
Eg: with plane/ with blur
Thx Vasanth, this is a nice hack, but it does not work when you have left and right other objects… or when you have orbit controls (ok you could add the plane to the camera as a child).
Any other solutions? I think it must be done with post processing and masking.