[rollei_list] Re: Planar vs. Xenotar test
- From: Peter Kotsinadelis <peterk727@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sun, 9 Jan 2005 18:12:07 -0800
You left out your usual epistle about the Opic. ;-)
On Sun, 9 Jan 2005 16:59:24 -0800, Richard Knoppow
<dickburk@xxxxxxxxxxxxx> wrote:
>=20
> ----- Original Message -----
> From: "Peter Kotsinadelis" <peterk727@xxxxxxxxx>
> To: <rollei_list@xxxxxxxxxxxxx>
> Sent: Sunday, January 09, 2005 12:26 PM
> Subject: [rollei_list] Re: Planar vs. Xenotar test
>=20
> > Hi Mike,
> >
> > We joke a bit about Planar v. Xenotar and Tessa v. Xenar.
> > It has been
> > an ongoing battle here on this list and another. Both the
> > Planar and
> > Xenotar are excellent. The major difference between the
> > two is the
> > Planar is slightly better wide open. This is arguable and
> > those who
> > prefer the Xenotar will say otherwise. The Xenotar and
> > other Schneider
> > lenses are slightly more contrasty than the Planar. I own
> > both.
> > As to the Tessar, it is soft until about F5.6 then at F8
> > and F11 its
> > resolution is equal to that of the Planar and Xenotar and
> > sometimes a
> > tad better in the center than the edges. Same for the
> > Xenar, a
> > Schneider copy of the Tessar. That is the primary
> > difference with
> > Tessar v. Planar in that the Planar is more even in
> > sharpness
> > edge-to-edge.
> >
> > Peter K
> >
> I will have to arrange to have two lady friends, Tessa
> and Xena, to carry my equipment around. Both will have nice
> boquets but one will be flowery and the other spicy. All
> four of their elements will be well designed.
> The Tessar as a generic type suffers from oblique
> spherical which can not be completely corrected for. Modern
> glass does not make a significant difference. The Xenotar
> type, despite looking as though its asymmetrical, actually
> has a lot of symmetry which makes correcting it for coma,
> lateral color, and geometrical distortion much easier. These
> three aberrations are cancelled by symmetry. The lack of
> coma and better oblique SA make these lenses much better
> performers than a Tessar when wide open. SA and coma are
> proportional to the stop so stopping down a lens reduces
> these aberrations rather quickly. Oblique SA is also
> proportional to the stop. The _usable_ speed of a Xenotar is
> significantly greater than that of a Tessar of the same
> nominal speed.
> Bokay seems to be a function of the balance of higher
> order aberrations. Lenses can be perfectly sharp for in
> focus images but have somewhat odd looking out of focus
> images. Fast lenses, where the high order aberrations are
> more promenent than in slower lenses, probably have more
> trouble with this.
> Arguing about Tessar vs: Xenar is hopeless since they
> share certain characteristics and neither is a unique
> design. Zeiss has made many variations of the basic Tessar
> design as Schneider has for their version of it. Its pretty
> hard to know exactly what prescription applies to which
> production lenses. The same for the Xenotar vs: Planar.
> Zeiss made at least two variations of the five element
> Planar. The original appears to have been very difficult to
> make. The later version is constructed more like the
> Xenotar. The very limited data in the lens survey program
> LensVIEW suggests that the original Planar (with cemented
> front element) is the best of the group, but not by very
> much. Its steeply curved, closely spaced, elements would
> have made it a fussy lens to manufacture. I suspect the
> change to the Xenotar type, which has thicker elements and a
> plane cemented surface, was for reasons of economical
> production rather than performance.
> Sometime someone, with time to burn, will obtain several
> samples of each of these lenses and test them with modern
> lens testing equipment. This will generate genuine MTF
> curves, defocus curves, etc., to compare with the computer
> analysis of the prescriptions, provided one can be sure of
> published prescriptions.
> There are indictions in the Zeiss Index that back in the
> old days Zeiss obtained competitor's lenses and measured the
> actual prescriptions. This would require disassembling the
> lens, measuring the curvature and thickness of the elements,
> and, their glass characteristics. This last is no easy
> chore.
> Measuring lens performance either by visual inspection of
> the aerial image or in film, is fraught with potential
> errors. I suspect that most of the tests in popular
> magizines would leave much to be desired in the way of
> design and control of the experiment so I am always a bit
> sceptical of the published results.
>=20
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>=20
>=20
--=20
Peter K
=D3=BF=D5=AC
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