GoofyNewfie wrote:
What Deej was referring to is called the depth of field preview button. It stops the lens down to the selected aperture so you can see what effect that aperture has on focus depth. When viewing through the viewfinder, the image gets dark, but you can see the effect. Someone correct me if I'm wrong, but on some cameras, if you use live view, the screen stays bright and the effect is easier to see.
On the Canon 60D in live view pressing the DOF button will make the screen go dark as well. I have never found any good use for this since if you have the lens stoped down too far, its to darn dark to see what you you are taking a picture of, much less seeing what is in the DOF range.