[LargeFormat] Double Protar Lens

[Richard Knoppow]

----- Original Message ----- 
From: "Clive Warren" <Clive.Warren@megacycle.co.uk>
To: <largeformat@f32.net>
Sent: Sunday, August 31, 2003 10:19 AM
Subject: [LargeFormat] Double Protar Lens


> Have just picked up a Bausch and Lomb Series VII Protar
for a 5x7 camera
> that appears to be a double convertible rather than a
triple. Without
> looking at the books, I thought they were all triple
convertibles. Does
> anyone have one or used one?
>
> Cheers,
>             Clive
>
  Protars came in all sorts of combinations. When the both
cells are of the same focal length the lens has its maximum
speed and probably maximum coverage.
  The quality of the image from a single cell is
surprizingly good, the combined lens is excellent although
it has the usual problem of thick meniscus lenses of having
a large amount of zonal spherical aberration, just as in the
Dagor.
  Single cells should be used in back of the stop. The stop
position affects field curvature among other things.
However, bellows draw can be significantly reduced by using
the lens in front of the stop. For the narrow coverage of
lenses long enough to require this there isn't any visible
difference in quality.
  The Series VII Protar cells are individually corrected for
coma, which Dagor cells are not. The performance is quite
good and the achromatization also seems to be good. Many
convertibles have a lot of color fringing. My Protar doesn't
seem to.
  I have a Zeiss triple convertible. Its an excellent lens.
I don't know how much, if any, difference there is between
the Zeiss and Bausch & Lomb versions. They came is slightly
different focal lengths. If you post the focal lengths of
the cells I can look the lens up in my ancient B&L catalogue
and give you details.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com