> From: Moose <olymoose@xxxxxxxxxxxxxxx>
>
> In optics, F-stop is, by definition, a purely mechanical number. it is
> the ratio of the opening of the front element of the lens divided by
> the
> focal length.
Actually, it's the ratio of focal length to focal width, which is
measured at the front nodal point.
Thus, a long telephoto tends to have a focal width that is very near
the size of its front element, but a retrofocal wide angle lens may
have a front element that is many times bigger than the focal width.
:::: Real peace in a petroleum-fueled world means rejecting petroleum
dependence in all possible ways. -- Jan Lundberg
:::: Jan Steinman (a fossil-fuel-free zone)
<http://www.Bytesmiths.com/Van>
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