Online Depth of Field Calculator. Ok, if you end up spending more than a hour playing around with this thing, you can classify yourself as something of a nut. If you grab the code and try to improve it, you’re nuttier still. Right now I’m having an “Almond Joy” moment 
Seriously, this is a fun tool. Did you know that, if you used a 50mm lens @ f1.4 on a D20 focused on a subject 4 feet away, that the depth of field would be scant 1.21 in. ? Well now you do. A very cool feature of this calculator is that it includes predefined data for many popular digital cameras (such as sensor size and actual lens focal lengths) that factor into DOF calculations. Heck, it even had info for my old Canon A20.