Re: lens

[Richard Knoppow <dickburk@ix.netcom.com>]

At 12:35 PM 08/26/2001 -0700, you wrote:
>I purchased for $1.00 a Kodak Anastigmat f7.7 lens with a serial #
>38931. On the lens cap it has the #33. In my Kodak Data book dated 1943
>it says that this lens is a 7 1/2" lens, yet when I compared it with my
>Sch.210mm lens, the Kodak lens must be refocused about 2 1/2 inches
>back. Can someone tell me about this lens. It covers my 4x5 with no
>problem with pretty good movements.
>
>David
>

  I wonder if this lens is not off of a Kodak folding camera. Kodak made a
series of Kodak Anastigmat lenses in numbered series. The 70 series was of
the four element "dialyte" type, the 30 series Tessars. All the 70 series
were f/4.5 lenses except for the No.70, a 203mm, f/7.7 lens later sold with
hard coatings under the Ektar name. 
  The 30 Series lenses were all f/4.5
  f/7.7 Kodak Anastigmat lenses were offered on several Kodak folding
cameras. I don't have a list of focal lengths but there would have been
several since the cameras they came on were of several formats. 
  These are almost certainly Dialyte types, i.e., four element air spaced
lenses. I don't think any K.A. Tessar types were made with this speed. The
shutter type may tell the tale: lenses from folding cameras are typically
in Kodak Ball Bearing shutters or Ilex Universal shutters. 
  My 203mm, f/7.7 KA is in a Flash Supermatic shutter and has no serial
number whatsoever.
  Its easy to get an approximate measure of focal length. You need to find
the infinity focus position and the position for 1:1 image magnification.
The difference between these two positions is the focal length. If you
don't have a convenient object far enough away to approximate infinity
focus you can find it by autocollimating. This is easy on a camera with a
ground glass screen but can be done without it. 
  You need a mirror large enough to cover the front of the lens. Front
surface is best but an ordinary shaving mirror works well enough. 
  For a ground glass camera mount the lens and open it to full aperture.
Place the mirror over the front, distance is not critical but it should be
square with the front. 
  Shine a small light, like a pencil flash near, but not at, the center of
the ground glass. You might want to make a pencil mark on the glass at this
point. The spot will be reflected by the mirror back to the ground glass.
Adjust the lens for sharpest focus. Its not focused at infinity. A small
ruler is useful for getting 1:1 magnification. 
  Once you have determined the focal length you can also find the rear, or
second, principle point, sometimes useful to know.  Return the lens to its
infinity focus position and measure exactly one focal length from the focal
plane toward the lens. By definition this is the second principle point. 
  For lenses of this type, and most symmetrical or semi-symmetrical lenses,
the principle point will be about in the rear air space behind the diaphragm.
  
----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com