Re: [Rollei] H1 filter for Tessar and Xenar only, why?

["Richard Knoppow" <dickburk >]

- ----- Original Message ----- 
From: "Jerry Lehrer" <jerryleh  >
To: <rollei  
Sent: Saturday, January 24, 2004 2:47 PM
Subject: Re: [Rollei] H1 filter for Tessar and Xenar only,
why?


> Carlos
>
> Correct.  That was the reason given for the "Sixth
Element",
> many years ago (sounds like the title of a sequel to a
Bruce
> Willis film).  Evidently, they said, the differences only
showed
> up with color transparencies like Kodachrome.
>
> Now they say it was for increased correction.  I'll have
to check
> with Hr. Prochnow's book.
>
> Yep!  It's there on page 16-376.
>
> Jerry
>
  Increased correction may mean something like lateral color
rather than UV suppression. UV filtering can be done with
the lens cement. The six element Planar/Xenotar is a
semi-symmetrical lens which will automatically have better
lateral color corretion. The extra element is not just a
filter, the whole lens is of a different design with little
in common with the five element designs other than the
names. the six element Planar/Xenotar is a conventional
Biotar type lens where the five element design is a
derivation of a lens which Kingslake attributes to
C.G.Wynne. It is derived from the six element Planar by
combining powers of two of the elements and eliminating one
of them and a cemented interface. This _should_ make it
cheaper to make but an examination of the Zeiss version
shows that it was probably difficult to make because the
cemented interface was steeply curved and the positive
element very thin. The Xenotar had a plane cemented surface
and no very thin elements. All of the six element versions
have planar cemented surfaces.
  One could get a better idea of relative cost by looking at
the glass types involved and finding out their cost.
  Unfortunately, the patent data does not usually specify
the anomolous dispersion of the glasses used so complete
analysis of the chromatic correction from this data is not
usually possible. At least not without a lot of guessing.
  It _is_ possible to make very highly color corrected
Tessars, as for example the Kodak Ektar series. These lenses
are advertised to have virtually no lateral color and very
little longitudinal color. Commerical Ektars are virtually
apochromatic, Kodak advertised that they could be used for
photomechanical color separation work. I think the fifth
element in the Rollei Planar/Xenotar were used to get better
rim ray correction, probably to get rid of the residual coma
in Tessars at large stops. The Planar type lens has very low
coma and spherical aberration, one reason they are widely
used for fast lenses.

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

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