If you're confused by figuring out how to control depth-of-field, take a look at Expo Imaging's ExpoAperture 2 Depth-of-Field Guide ($30, direct).
Here's how it works:
1) Choose a distance to your subject and range of focus that you would like around that subject.
2) Choose your sensor size or format of film.
3) Then turn to the particular focal length that you want and the EA2 gives you your aperture.
You'll have more accuracy in your f-stop choice and less hunching over like a lunatic, cradling your camera in your hand trying to view your LCD to check your depth-of-field.