The depth of field preview button is located right next to the lens mount, and is sometimes looked at as one of those buttons you just dont need to touch! So, most people just leave it alone…in fact it is a handy tool that allows you to see the depth of field just as it would look in your final image. Yes, what you see in your viewfinder is not the final depth of field you would get!
The reason for this is, its better to have a nice bright view finder than to see your lens stopped down to say f8, if you were shooting at f8. Instead, your camera shows you the largest aperture possible for your lens (f 2.8 if its a 2.8 lens), and the aperture ring closes down only for the short duration the shutter is being fired.
The reason most people dont bother with the depth of field preview button is, the viewfinder really darkens up and you are left with little to judge by. However, there is a distinct advantage using a digital as opposed to film cameras of yesteryear – you can look at the preview on live view. The camera’s LCD automatically brightens up the image, and you are given the truest possible rendition of DOF preview. This is a very handy tool if you are a serious photographer, taking the trouble to get your depth of field just as desired.
Here is a visual (video) explanation of it by ZY Productions –