[rollei_list] Re: xenotar 2.8f vs planar 2.8f

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Thu, 11 May 2006 14:48:37 -0700


----- Original Message ----- From: "Jeffery Smith" <jls@xxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, May 11, 2006 12:02 PM
Subject: [rollei_list] Re: xenotar 2.8f vs planar 2.8f



Jerry branded me a troll about 4 years ago when I asked the exact same question. I thought that it was a completely legit question (then and now), but apparently it is a "hook" question on this forum.

By the way, I never bought a Rolleiflex with a Tessar because I was reluctant to ask about the lens quality compared to the others.

Jeffery

I suspect the variations among lenses of one type are larger than the differences between the two. I have a program called Lens View which is a collection of lens design data for perhaps 30,000 lenses. Incuded are the prescriptions for the original f/2.8 Planar and for the Xenotar. I am guessing a bit on the Xenotar because Schneider has about three variations. I think I know which was the production lens. LensView will generate files which can be opened with all the standard lens design programs. I have the free version of Oslo. The analysis of the two lenses in this program shows little difference. The Planar has slightly less zonal spherical aberration. I have not check them for energy distribution (spot diagrams)which can indicate differences in edge sharpness. The Zeiss design has some features which probably made the lenses hard to make and expensive, the Xenotar is better from a manufacturing standpoint, i.e., no very small spacings, no extra thin elements, no highly curved cemented surfaces all of which are in the Planar.
LensView does not have enough information on the glass types to do a good analysis of color correction. Very few published lens prescription give the complete glass specifications.
Over the years the Xenotar has gotten a reputation for being sharper than the Planar. The computer analysis does not show any reason for this. I suspect it may have something to do with the edge contrast of the two types.
In general, both lenses are based on a design by C.G.Wynne in the mid 1940's. By combining two elements of the basic Planar/Biotar type he was able to eliminate one element and one cemented surface. While the schematic of the lens would make you think otherwise it is essentially a symmetrical type with the advantages of cancellation of coma, lateral color, and geometric distortion which come with symmetry or partial symmetry.
For the f/3.5 version both Zeiss and Schneider reverted to the conventional six-element Biotar type lens. It may be that despite the extra element and additional cemented surface, the six element lens was cheaper to build than the five element version given a certain quality requirement.
My choice would be based on the condition of the individual camera and lens. Lenses should be as near perfect as possible. So called cleaning marks are not acceptable. They are fine scratches and result in some diffusion of the image and loss of contrast. A single large scratch does less damage to the image.
The Tessars in Rolleiflex's are of very good quality. Even the old ones are very good lenses. They are not as good as the Planar/Xenotar for a couple of reasons: one is the lack of symmetry. While non-symmetrical lenses can be made to have little or no lateral color (the Kodak Commercial Ektar is an example) such correction is difficult. Also, all Tessar types have inherent oblique spherical aberration, a fault which looks much like coma and results in directional blur away from th optical axis. This fault, like coma, is proportional to the stop, so is cured by stopping down. The f/3.5 Tessar and Xenar are not sharp in the corners until stopped down to around f/8. An f/3.5 Planar or Xenotar also has some blur when wide open but will be sharp everywere at about f/5.6. At small stops there is not much difference between the Tessar and Planar other than the Planar type probably has better color correction.
From my experience the Tessars and Xenars used in Rollei cameras are a toss-up. The Xenar was cheaper but Schneider lenses were in general cheaper than Zeiss. Perhaps this was because of production efficiency, perhaps because of differences in quality control. In practice, at least or the lenses used in Rolleis there does not seem to be any significant difference.
I am not sure this is true of other Tessars and Xenars where I think the Tessar may have the edge.
While there seems to have been a wide gulf between lens quality between Zeiss and Schneider pre-WW-2, post war Schneider lenses are a different story. Mostly very well designed and made with decent QC.
As far as older vs newer Tessars, the old ones were not coated. For a lens with six glass-air surfaces coating makes a difference but is not essential. For the five or six element Planar/Xenotar, with 8 glass-air surfaces, flare becomes significant so coating is important. Flare increases geometrically with the number of surfaces.


---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx



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