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Some digital SLRs also have depth of field calculation built in, as a special function. My 30D has it. You focus lock on point 1, then on point 2, and then on your final framing choice. The camera computes the appropriate aperture so that both point 1 and 2 are in focus, given your final composition choice.
Depth of field is a function of two things, magnification (not focal length) and aperture. There is a wiki article that gives the appropriate equations. Most simply, the greatest your magnification (the closer the focus point is) and the larger the aperture, the shorter the depth of field.
It’s not a bad idea to commit some depth of field numbers to memory. For example, depth of field for your 50mm lens, at 10 feet, 20 feet and infinity, for a small range of f-stops. If you have a few numbers in your head, you can get a better idea of your results, even for different lenses (magnifications) and f-stops.