[rollei_list] Re: 2.8F models...the 3.5/75 six elements (again), the answ...

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Tue, 3 Jan 2006 15:46:21 -0800


----- Original Message ----- From: "Carlos Manuel Freaza" <cmfreaza@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, January 03, 2006 2:25 PM
Subject: [rollei_list] Re: 2.8F models...the 3.5/75 six elements (again), the answ...


Peter,I don't remember details now, but I'm 99 % sure
that the six elements was used for the 3.5E3,I could
confirm it tonight.
I agree with you that the point was to obtain constant
quality more easily and more cheaply, HOWEVER this
problem existed for CZ and Schneider, not for Rollei
that only received the lenses that accomplished their
requirements, I think that Carl Zeiss and Schneider
lost time selecting the adequate glass and adjusting
each lens to reach the technical requirements with the
3.5 five elements,it was easier to cut two thin
glasses to form a negative element than to select and
to cut a thick pure glass (more thick the element,more
difficult to obtain pure glass from the block)   for
the original divergent meniscus (second element for
the Planar and fourth element for the Xenotar), I
think this is the key for this issue, if you compare
the 2.8/80 Planar and Xenotar divergent meniscus with
the original Planar and Xenotar 3.5/75 divergent
meniscus, you can see that the 2.8/80 divergent
meniscus are three or four times thinner than the 3.5
meniscus...
BTW, your reference about problems with  sharpness for
the five elements are your speculation,perhaps it was
more difficult to obtain the required contrast and/or
color transmitance with the five elements,lens that
was excellent if approved by Rollei...

All the best
Carlos

I am not sure I buy this. None of the Planar or Xenotar lenses uses elements which are particularly hard to make other than the thin element in the 5 element Planar and that is not an extreme example. All of these lenses appear to use glass types which were conventional for the time. The size of the elements is not unusual in either size or thickness. At one time it was difficult to obtain chunks of glass large enough for very large lenses but modern practice (modern meaning since WW-2) has been to hot mold the element blanks to approximate shape. In the old technique a "pot" of glass was allowed to cool after refining and then broken up to remove it from the pot. One took the glass as it came. I don't think this system applied to lenses of the Planar/Xenotar era although I can't be sure. In any case, as I stated above, none of the elements in any of the lenses would have been in any way unusual in size or shape or have used truely exotic glass. Further, computer analysis was already in use in the mid 1950s when these lenses were designed. While modern lens design programs are a lot more poweful than the ones then nonetheless calculation of a modified or ammended design would not have taken a long time.
Two elements with a cemented surface can be used to emulate some glass type which is not available but I don't think this was the case with these lenses. I wish I could find the prescriptions for the six element Rollei lenses but they do not appear in any of the sources I have available. The reason for the change is still a mystery to me.

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

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