Distribution of DoF


hello ng,

my questions concerns the distribution of the depth of field around
the plane of exact focus.
This distribution changes with a) focal lenght (the longer the focal
lenght the greater the fraction of dof in front of the focus point)
and b) distance (the greater the distance the greater the fraction of
dof behind the focus point).
From the equations for front and rear dof ( Dfront = df^2 / f^2+Nc*(s-
f) and Drear = df^2 / f^2-NC*(s-f) ) I understand that this explains
itself due to the addition and subtraction of the Nc*(s-f) term which
enlarges or reduces the denominator. But if I look at one of the
graphics explaining dof (e.g. http://en.wikipedia.org/wiki/Depth_of_field:
DOF for symmetrical lens) I can´t visualize what´s going on. So can
someone please give me a geometrical or more visual explanation of the
arithmetic?

Best!
Monica

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