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[Phys-L] Re: optics terminology



It is true that practitioners often refer to the focal plane/point as the
image plane/point which is conjugate to whatever object is being imaged.
They may also refer to the focal length as the distance from the adjacent
lens surface to this focal point (some distinguish this as the back focal
length).

This may be dismissed as semantics, but it becomes important to adhere to
the definitions inherent in optical design/evaluation equations when
employing them for calculational purposes.
In this context, JM's definitions are correct. The focal point is the image
location of an infinitely distant axial object (equivalently, the point to
which an incident parallel beam is focused). The focal length is the
distance from a focal point to the corresponding principal plane. Note that
this focal length is a property of a given lens system, can be stamped on
the lens and does not vary with one's choice of the object of interest.
This is standard with all optics texts I have seen, and with the AIP
handbook.


Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com|
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