(1) If I have an assemtrical diverging glass lens, i.e. a lens with 2
different radii of curvatures, say R1 = +10 cm and R2 = +20 cm or R1 =
+10 cm and R2 = -20cm, I can calculate the focal length of these lenses
using the lens makers equation. But on a ray diagram, where,
geometrically is the focal length measured from?
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Tony,
Beginning at an off-axis point in object space, trace a
horizontal ray through the lens; the point where it intersects
the optical axis is a focal point. Extrapolate the beginning
(in object space) and ending (in image space) straight line
segments of this ray; their point of intersection defines
a "principal plane" (through this point and perpendicular to
the optical axis). The focal length is measured from this
principal plane to the focal point.
This procedure defines a principal plane (there are two)
for any optical imaging system. For more on "principal planes"
consult Jenkins & White or any Junior-Senior level Optics text.