[LargeFormat] Anguloning for an Answer

[Richard Knoppow]

----- Original Message ----- 
From: "rstein" <rstein@bigpond.net.au>
To: <largeformat@f32.net>
Sent: Tuesday, December 02, 2003 4:47 AM
Subject: [LargeFormat] Anguloning for an Answer


> Dear Neices and Nephews,
>
>     I just realised that my old job as writer for a cheap
local newspaper
> has finally caught up with me. Anguloning for an answer,
indeed.... Next it
> will be puns about pulling teeth and such. Oh it just
makes you sick as a
> parrot, or over the moon. Whew, what a scorcher....
>
>      What I really want is some opinions about 90mm 6.8
Angulon lenses.
> There are a number of them on Ebay for lowish sorts of
prices and I was
> wondering if this would be a usable alternative for my
studio work as
> opposed to the Super Angulon or one of the Nikkor SW's.
>
>     I ain't a architectural photographer. I am a belly
dance photographer.
> That explains the sequins in the studio loo. What I need a
90mm for is large
> troupe shots on 4 x 5 colour neg. I have a 9 metre-wide
stage that I can
> back off 6-7metres from and I need to spread my coverage
out from the
> standard 150mm lens that I already have. I do not need
large movements up
> and down though a little side to side is quite common in
belly dancing.
>
>     Exposure will be via electronic flash and all I need
is 125 to stop most
> motion. The f.stop most commonly used is f.16. Focussing
is a one-time
> affair and then the dancers go for it and I just change
holders. I plan to
> use the central 6omm x 120mm section of the 4 x 5 neg as
the picture area.
>
>     So, has anyone used the old Angulon and can they
advise me? Thanks in
> advance.
>
>      Uncle Dick
>
>

     I have a very early Angulon, 120mm, f/6.8  The serial
number indicates it was made in 1929. The patent was issued
in 1930 and, indeed, the lens ring indicates a patent
pending.
     This lens is a dog, it has serious color fringing, a
fault symmetrical or nearly symmetrical lenses should not
have. I mentioned this to my lens designer friend who told
me that he had set up the patent precription in a comuter
design program and, indeed, it had some bad chromatic
errors.
     The lens must have been redesigned at some point
because later Angulons seem to be good performers. I have no
idea of when the redesign was done but, if my lens is
typical of production lenses, it would have gotten a very
bad reputation quickly.
     The Angulon is actually a variation of the Dagor. The
order of powers of the cemented elements is reversed and the
lens is made slightly asymmetrical to reduce coma for
distant objects. In fact, it was originally sold as a triple
convertible! The oversize outside elements are necessary
because the reversed Dagor is thicker for a given focal
length than the standard Dagor so it will vignette without
the large end elements.
     Two other old WA lenses are decent performers. One is
the Zeiss or Bausch & Lomb Series IV Protar, an f/18 lens.
This lens will cover nearly 100 degrees. It has some
astigmatism but is reasonably sharp. Its working aperture is
around f/36 to f/45. The ground glass image is not as dim at
f/18 as one would think. The main virtue of this lens is its
very small size and light weight.
  A better lens is the Wide Angle Dagor. Two versions of
this were made. An f/9 lens, designed by Franz Urban of
Goerz but also built by Zeiss, and an f/8 version built by
Goerz-American. The coverage is about 100 degrees for either
lens at f/45.
  Normal Dagors have a claimed coverage of 87degrees at
f/45. The Angulon had a claimed coverage of 102degrees at
f/45 when it was first announced but the iamge quality
beyond about 90 degrees is not good so it really has no
wider coverage than a Dagor.
  One must differentiate between the "circle of
illumination" and the "circle of good quality" of a lens.
Some lenses, the Dagor for example, have a very large circle
of illumination but a smaller circle of good quality. The
Dagor good quality circle increases in size as the lens is
stopped down. It is actually rather limited when wide open.
At f/22, about the optimum stop for these lenses, the
coverage is around 60 or 65 degrees. The Angulon has a very
large circle of illumination but the circle of good quality
never gets that large.
  Some lenses have circles of good quality which extend
about to the limit of the circle of illumination and do not
get significantly larger as the lens is stopped down. The
four-element air-spaced type lens sometimes called a
Dialyte, is an example. These lenses have pretty good
corrrection wide open and are optimum perhaps two stops
down. However, the coverage doesn't get get larger as they
are stopped down. The astigmatic correction of these lenses
is usually such that the two fields diverge from each other
very rapidly beyond the "stigmatic point" so image quality
becomes bad very quickly beyond a certain coverage angle
regardless of the stop. Apo-Artars, Dogmars, the 203mm,
f/7.7 Ektar, 70 series Kodak Anastigmats, and many others
are of this type. They can produce excellent image quality
over a fairly narrow field.
    None of the old type wide angle lenses is as good as the
modern ones. Those are based on a patent by Micheal Roosinov
for a lens which looks something like two back-to-back
reverse telephoto lenses. It has the advantage that some
coma can be introduced into the stops causing the stop to
become effectively larger as the angle increases. While a
"normal" lens has a fall off of at least cos^4 theta (where
theta is the half angle of the image) the Roosinov lens
reduces this to cos^3 theta. The Zeiss Biogon(second type)
and all of the modern WA lenses like the Super Angulon and
Grandagon are of this type. They can have wider angle
coverage than the older types and have larger working
apertures. Because they have many glass-air surfaces they
did not become practical until good anti-reflection lens
coatings became available and economical.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com