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Re: [Rollei] OT: Schneider-Kreuznach ALPA Apo-Helvetar 5.6/48mm

- ----- Original Message ----- 
From: "Fox, Robert" <RFox  >
To: <rollei  
Sent: Thursday, January 29, 2004 11:08 AM
Subject: [Rollei] OT: Schneider-Kreuznach ALPA Apo-Helvetar

> Cool new Schneider lens. Apparently it does very well wide
open at f5.6:
> http://www.alpa.ch/alpa/helvetar.htm
> http://www.helvetar.ch/helvetar/Helvetar-Tour1.html
> Does the lens name have anything to do with cheese? ;-)
> R.J.

  From the Schneider data sheet this appears to be a
conventional Super Angulon type. The Super Angulon is based
on the lens invented by Michael Roosinov, this is the basis
for most of the modern non retrofocus wide angle lenses.
Roosinov invented the "tilting entrance pupil" principle,
which allows increased illumination at the margins of the
  I doubt if this is actually an apochromat despite the
name. Unlike Rodenstock, Schneider does not show a graph of
longitudinal chromatic aberration, which would show the
trueth instantly.
  I think the puzzling statements on the Alpa page about the
coverage for digital backs is concerned with lens resolution
vs: magnification. The smaller the sensor the more the image
is magnified so the greater the lens resolution must be for
the same sharpness. Since this lens is likely diffraction
limited at the center of its field, and since diffraction
increases as the lens is stopped down, the center resolution
of a very well corrected lens will get worse as it is
stopped down.
  However, the lens behaves like any other lens for a fixed
size image circle, that is, it gets better as it is stopped
down up to a certain point. This lens is rated as having a
maximum image circle of 123 mm at infinity focus and f/22.
The image circle at f/5.6 is only 89mm. The MTF curves on
the Schneider data sheet show that the lens is optimised for
infinity and that it has its best performance at f/22.
Actually, the resolution at the center does not change from
f/5.6 to f/22. Since the maximum image circle is 123mm the c
overage angle is about 104 degrees. This is very good
coverage but not unusual compared to other current wide
angle lenses. I have no doubt this is an excellent lens but
its not worthy of the hype on the Alpa site, some of which
is rather misleading (like the distances its optimised for).

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