MFD is the distance between the sensor and the subject. Minimum Working Distance is the distance between your lens' front element and your subject when the lens is set at 1:1 magnification.


For longer lenses, MWD might indeed be half the MFD, because the longer reach of the lens will allow you to get further away from your subject.


For example, my Sigma 50mm Macro MWD is about 2 inches, while the MFD is around 9 inches (all of these figures at 1:1 magnification). I won't be able to snap a bug's picture with such a small MWD, but it is more than enough for flowers and electronic circuits (the reason why I bought the lens in the first place, as I am an Electrical Engineering student).


Take for example this macro shot of a tulip :


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f/8, 1/400 s, ISO 200, Sigma 50mm f/2.8 DG EX Macro on Rebel XTi


Even if this was taken at about 1:2 magnification on a 1.6x body (that makes the 50mm behave like a 80mm), I was CLOSE to the flower. No insect would have stayed there with me coming this close to it with the lens.