" Another creative use of depth of field is to use a shallow depth of field when taking a portrait if you're in a less-than-ideal setting. Tightly focus on the subject's face and open up your aperture to blur that city street or whatever is in the background and you'll have a portrait that jumps out at you. Finally, don't take this to mean that stopping down your aperture and throwing your focus on infinity will give you a completely in-focus landscape photograph. We've talked about shallow depth of field, but when you want deep depth of field in a landscape image, you should use something called "hyperfocal distance". If you go back and refer to the lake image used earlier in the lesson where I noted the rocks in the foreground, that sharp focus throughout the image was achieved using this method. It's the best way to assure your foreground is tack sharp as well as the rest of the photo.
First, you need a lens with a depth-of-field scale imprinted on it. A lot of lenses don't have this on them, so if you're selecting a lens to buy, this is a good thing to look for. It's a bunch of numbers written on the lens that coincide with the apertures on the lens. Each number is printed twice - once on the left of the center position, once on the right.
If your lens has apertures running from f/2.8 through f/32, you will see a "2.8" written in the center and a "32" printed way to the left and then again way to the right. I know, you're saying, "Man, all those little numbers look so CONFUSING! What the heck am I supposed to do with those?" It's not as hard as it looks, and I promise you'll be happy you learned this. This is what you do:
Set your lens to its smallest possible aperture. That's f/32 on the fictional camera I'm talking about, so we'll stick with that. So now find the "32" markings on the depth-of-field scale on the lens. Got Ôem? Okay, now instead of the normal procedure of positioning the infinity distance mark at the center, position the infinity symbol above the "32" mark on the right. Now you are now focusing most sharply on a distance of somewhere around ten feet or so, but infinity is just within your depth of field, also sharp. Okay, now look at the left-hand marking that says "32" and it will tell you at what distance your depth of field starts - from about three or four feet out all the way to infinity."
Taken from:
http://www.morguefile.com/archive/classroom.php?lesson=3&MORGUEFILE=i55680t0h2bl8u1m2hd27q2rf0
Also heres a snapshot of a lens with the DOF scale if you dont have one:
http://i19.photobucket.com/albums/b172/sik66/100_3322.jpg