I'm curious about how my "area of acceptable focus" changes when the focal length of the lens I'm using changes as I zoom (or switch lenses). In particular, I'd like to know how the front & back focal planes change, thus changing the depth of field and the minimum focus distance.
While it is a fact that changing focal length from shorter to longer reduces DOF and using a smaller (less light) aperture in will increase DOF (providing format is identical) however there is a simpler way to think of it.
DOF decreases the larger the subject is in the frame regardless of the lens and increases with smaller apertures.
Example: If you shoot the same photo, say a headshot, with a 200mm lens and, at the same distance, with a 35mm lens. Then take the image from the 35mm and crop it to match the image from the 200mm you will find the DOF/image identical.
Of course this is an example assuming that the resolution would not be factor. Which is WHY we change lenses and don't just crop.
There is tool to calculate DOF by putting Focal length and f-stop here: http://www.dofmaster.com/dofjs.html
If you stand still
The depth of field quickly gets narrower as you zoom in.
If you keep subject magnification constant
If the depth of field is large (comparable to the focusing distance), then it gets somewhat narrower when increasing focal length. If it is already narrow, then it is practically independent of focal length.
Front and back depth of field
When it's narrow, the depth of field is practically symmetric relative to the plane of best focus. As it gets wider, and specifically as it reaches the order of magnitude of the subject distance, it gets more and more asymmetric (more depth of field behind the subject than in front of it). At one point it reaches infinity, then things are sharp from half the focus distance up to infinity.
A simple rule that is probably more useful than my previous paragraph: the depth of field is always practically symmetric when read from the lens' focus scale.