In this discussion on finding the focal length for combination lens, the image described in the answer shows the measure of focal lengths.

Description of the Gullstrand equation also talk about front and back focal lengths.

DSLR camera lenses are described as retrofocal.

I wish to know how to find the front and back focal length of a combination of a lens if at all they exhibit this property.


1 Answer 1


Yes, the definition of the effective focal length for a pair of lenses is symmetric whether you send the optical beam from the front or from the back of your combination of lenses, i.e., symmetric in $f_1 \leftrightarrow f_2$. The symmetry is broken by the different front and back working distances of the combination of lenses. In the link you provided of the Gullstrand equation, the front and back vertex powers are just a complicated way of expressing the reciprocal of the front and back working distances.

In general, almost any optical arrangement of lenses can be replaced by one equivalent lens with an effective focal length that depends on the focal lengths of the lenses and their distances, independent of whether you look at it from the front or the back.


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