How does changing focal length affect depth of field?

Question

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.

Answer

There is tool to calculate DOF by putting Focal length and f-stop here: http://www.dofmaster.com/dofjs.html
With result you can create an interesting graph

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