Re viewer interocular

[P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

I was a little puzzled by Greg Kanes use of the term "Optical Centre"
simply centre would seem to suffice. Optical centre usualy refers to a lens
centre through its thickness not its width. You choose a ray passing
through the lens which after refraction emerges parrallel with itself on
entering it .So at this point the lens is behaving as if it had parrallel
sides. You then choose a second ray on the other side of the lens that does
the same the point where they intersect is the optical centre . These rays
are merely displaced slightly in a lateral direction and all rays with the
incident ray parrallel to the emergent ray pass through the optical centre.
This will only correspond to the physical centre when the lens is
symetrical. Focal lenth is strictly speaking measured between this point
and the focal point which is why asymetric lens combinations
with retrograde negative rear elements can have their optical centres in
front of      the front elements. This is made use of in Telephoto and Zoom
lenses enabling them to be more compact than their focal lenths .
P.J.Homer



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