It can be quite difficult. It's hard to comment on someone else's work without knowing the FOV of the shot. If you pay a lot of money for a fast telephoto lens with a wide aperture (or you shoot large format film like 4x5 or 8x10) you can get this sort of effect. The viewing distance will also be a factor. If you're too close, you're artifically increasing the fov of the image that your brain has to interpret. If you move back from the screen you may find it looks a lot better. The second image looks more like a standard FOV, especially when you compare it to the previous image in his gallery.
I'm not sure how Luc did this as I couldn't get the shader to work on object (fake stones were OK). I can think of a hack involving multiple renders, but that would be a simulation only
The depth of field link I posted was for reference to assist with creating more realistic masks, although you would also have to figure out a way to calculate the outer limits of the blur mask on either side. I'll have a look at a later stage.