It is my vague understanding that the matched sets of finished prewar lens elements were separated to have T coating applied, then mismatched when the lenses came back for reassembly.
Richard Knoppow wrote:
----- Original Message ----- From: "James & Christiane Thompson" <jcthompson@xxxxxx>To: <rollei_list@xxxxxxxxxxxxx> Sent: Tuesday, November 14, 2006 2:17 AM Subject: [rollei_list] 2.8 80mm Opton-TessarHello all,I am curious whether or not the replacment lenses for the mis-matched Zeiss Jena lenses on the K7A were changed or redesigned in any way, e.g. re-computed or new types of glass. Or are the Opton versions basically the same as before, just correctly matched and coated?Thanks, JimI waited to see if anyone on the list had anything definite about this. The story as I understand it is that the lenses first supplied by Zeiss Jena to Rollei were originally made for the Ikoflex III and were made before the war heated up. One of the characteristics of the Tessar type is that most of the power of the lens is in the rear component (of two cemented elements) and most of the correction is in the front cell (two air spaced elements). If the wrong combination is used some of the correction will be lost. Evidently this, or something like it, happened to the old stock. On top of this f/2.8 is about the limit for the Tessar type even when everything is working right. I have no idea if the prescription for the Oberchoken lenses was identical to the Jena design but suspect not. The reason is simply that other people were making the lens. Tessars are not complicated to design and Zeiss had perhaps a hundred variations on the type. I doubt if the Oberchoken lenses had any sort of exotic glass in them or if they were of any sort of unusual design. I suspect they were simply new lenses properly quality controlled. f/2.8 is about the design limit for a Tessar. While Tessars of this speed can be satisfatory the inherent problems with Tessar types, namely oblique spherical aberration, will prevent the lens from having really good performance at near maximum aperture. Rollei seems to have realized this and changed from the Tessar to the Zeiss Jena Biometar (Rolleiflex 2.8B) and shortly after to the similar Schneider Xenotar. Both of these lenses, and the 5 element Zeiss Planar, have both more surfaces to work with and some degree of symmetry, factors which make the lenses capable of better correction than the Tessar, especially at large apertures. I will check my lens prescription program to see if there are any patents that look like they may be the f/2.8 Tessar but without additional information its difficult to tell if a particular patent design was an actual production lens.--- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org- Online, searchable archives are available at //www.freelists.org/archives/rollei_list
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