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Re: [Rollei] Why the 6-element lens for 3.5Fs

- ----- Original Message ----- 
From: <ll.clark  >
To: <rollei  
Sent: Thursday, May 01, 2003 7:16 PM
Subject: Re: [Rollei] Why the 6-element lens for 3.5Fs

> In <  >, on
>    at 10:02 PM, Marc James Small <msmall  > said:
> >From five to six elements, of course:  look at the
subject heading!
> Things don't appear to me to be quite so transparent. No
one can seem to
> agree whether the change in the Planar was to correct
falloff in
> illumination in the corners, or to improve sharpness.
> What was the reason or reasons for changing from five to
six elements in
> the Xenotar?
> ----------------------------------------------------------
- -
> les clark / edgewater, nj / usa
> ----------------------------------------------------------
- -
  It virtually has to be to improve sharpness. Fall off is a
matter of the geometry of lenses. It does vary somewhat from
the strict cos^4 theta law, but not by much. There are
special designs which eliminate one factor and reduce the
fall off to cos^3 theta. This is used in some wide angel
designs. Its known as the tilting entrance pupil principle.
Retrofocus lenses also have less fall off than a "normal"
lens for a given angular coverage. That's because the half
angle, the "theta" above, is less. A lens which is not
rectilinear, for instance, a fish-eye lens, also has less
fall off, providing it has barrel distortion. Fish eye
lenses have lots of barrel distortion so have less fall off.
A fish eye lens with a tilting entrance pupil can have cos^2
theta fall off and accept light from greater than 90 degrees
half entrance angle.
  While many retrofocus lenses based on the Planar type
exist I do not believe any Rollei TLR lens is of this type.
Its possible that some 80mm lenses for SLR 6x6 cameras are
retrofocus. The extra back focus is needed to clear the
mirror box.
  OTOH, an additional element might make it much easier to
control the rim rays in a design. Further, a symmetrical or
semisymmetrical lens has a certain amount of automatic
cancellation of coma, geometrical distortion, and lateral
color. Coma is a particularly ugly aberration so reducing it
is important. By going to a six element conventional
Planar/Biotar design from the five element design symmetry
is gained making correction easier. I think this is clearly
the reason for the change.
  While the true Planar has one more element and one more
cemented surface than the five element type it may very well
be less sensitive to manufacturing variations, making it
cheaper to manufacture, or it may be able to use less
expensive glass. One would have to have an intimate
famialiarity with the design to know if either of these
speculations are true.
- ---
Richard Knoppow
Los Angeles, CA, USA