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Re: [Rollei] Oh, no, not Planar vs. Xenotar!
- Subject: Re: [Rollei] Oh, no, not Planar vs. Xenotar!
- From: Richard Knoppow <dickburk >
- Date: Wed, 21 Jun 2000 18:03:06 -0700
At 08:25 PM 06/21/2000 -0400, you wrote:
>----- Original Message -----
>From: Bob Shell <bob >
>Sent: Wednesday, June 21, 2000 10:13 AM
>> Zeiss says in their literature that the Planar is designed for a flat
>> field, thus its name.
>I remember that - but didn't Schneider also claim that the Xenotar had a
>flatter field than earlier lenses?
>(BTW - I should add, in case anyone is wondering, that I am not suggesting
>that the Planar is better than the Xenotar (or vice versa). I am just
>curious about what you might call the "signature" or "character" of the
Most lenses are designed to have as flat a field as possible. For
pictorial lenses sometimes a comporomise is made between field flatness and
astigmatism. Its possible to minimise astigmatism by allowing a little
field curvature. Lenses for copying or enlarging must have an absolutely
flat field but pictorial lenses are a little more tollerant.
Some caution is needed in measuring field flatness in a camera,
especially a roll film camera, where the film flatness may not be perfect.
Two cameras and two different rolls of film are involved with this
comparison so there are some other uncontrolled variables here.
It would be interesting to see what samples of the two lenses did on
bench tests with proper targets which could emulate infinity.
There are a large number of lenses analysed in _Modern Lens Design_
Warren J. Smith, McGraw-Hill Book co. Among them are a large sample of
Planar-Biotar types. The perfomance varies among rather similar looking
lenses so it is not surprizing that differences can be observed in very
similar actual lenses.
Field flatness does not vary with the stop. However, it its effects are
minimised by depth of focus as the lens is stopped down.
There are other effects which may be confused with field curvature. The
primary one is astigmatism. Probably coma is also a factor. Astigmatism
results in two points of focus. At one, a point object is reproduced as a
section of a radial line, at the other as a segment of an axial line. The
point is reproduced as a round blur spot at a point between these two.
Astigmatism in a modern high quality lens should be vanishingly small.
Coma is the reproduction of an off-axis point as a tear-drop shaped blur
spot. The narrow end of the spot can point either way along a radius toward
the center. Coma is eliminated by stopping down, and is cancelled in
exactly symmetrical systems. Lenses of the Biotar type have very little coma.