When I earlier saw discussion about refractors with a Petzval design, I
thought the idea was rather silly. Why so many elements in a refracting
telescope... and in a design known to be flawed.

But the Petzval lens did produce the earliest camera lens capable of an
f/3 focal ratio. For comparison, a Cooke triplet usually tops out at
f/6.3.

Of course, a four-element Tessar can go down to f/2... nowadays, with
the use of expensive glasses, such as are used in fancy eyepieces. The
Petzval lens has the *flaw* of curvature of field, negated by
astigmatism (so that at least the tangential field is flat) because one
can't do better with ordinary glasses.

And I find that Tessar-based telescopes are becoming common *now*
because a patent has expired... U.S. patent 4,400,065, from 1983...
issued to one Al Nagler. If it's *his* idea, it probably _is_ a good
one!

Plus, of course, a double-Gauss camera lens design needs at least *six*
elements.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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