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