There are several online depth of field calculators around but I do like this one from Don Fleming. It’s bang up to date with all the latest D-SLR models, and makes the proper adjustment for a full frame sensor compared to an APS-C sized sensor. Did you know that shooting at f/5.6 on a D300 gives the equivalent depth of field of f/8 on an older Hasselblad? Read David Ziser’s post at Digital Pro Talk for a great argument on how using lower f stops is possible with digital equipment.

I like the DOF calculator as a quick reference – for example here I can see shooting at a subject 10 feet away using a 50mm lens at f/2.8, objects just under a foot in front of the subject and just over a foot behind will also be in focus. A graphical representation is shown as well, example below.

Try the online depth of field calculator here, hope you find it useful!