I'm fairly new to DSLRs and I've been exploring the functions on my new Canon 60D.

There's a button to the side of the lens described as the "depth-of-field preview button". Based on the limited description in the manual, I thought this would give me a preview of how the photo would look in terms of what's in and out of focus when I'm using Live View. 

When I press it though, despite jerking as if it's processing something, I see no difference in the LCD screen. I've played with the aperture with no discernible effect.

Am I missing something? What should I be seeing?