In rec.photo.equipment.35mm arnold ziffendorfer wrote:
On Sun, 18 Nov 2007 01:42:40 +0000 (UTC), Peter Irwin wrote:

But their definition of "zoom lens" is just wrong.
There are many lenses of adjustable focal length
which are not zoom lenses. For instance, a front cell
focussing lens is focussed by changing the focal length,
and would not be called a zoom lens by anyone.

A zoom lens is one which allows the focal length to
be changed and remains in focus when the focal length is
adjusted.

Peter.

That's a parfocal zoom-lens. A zoom-lens need not be parfocal to be called a
zoom-lens.

OK. How would you class a front cell focussing triplet?
It is designed to be variable in focal length in order
to focus. I sure wouldn't call it a zoom.

How would you class a variable focal length Dallmeyer Adon?
It can adjust to a pretty huge range of focal lengths
for different magnifications, but not only do you have to
refocus it, you have to calculate the aperture each time
you change it. I would hardly call it a "zoom"; it is a fairly
difficult lens to use.

Though called parfocal zoom-lenses none are truly
parfocal.

Any zoom lens worthy of the name should be close enough
that you can get away without refocussing.

Peter.
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