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Re: [Rollei] Planar Sharpness

At 06:20 PM 04/03/2002 -0500, you wrote:
Hi Peter,
Thanks for the reply.
I do not have this camera anymore but your letter brings up a point to me.
You said that a CLA does not include a check with a collimator.
What is this?  I would like to have my 2.8F CLAed and one of the things I
expected to be done was to make sure that what I see in the view finder is
what will be on the film plane.  Is this what this Collimator checks?
 ----- Original Message ----- 
 From: Kotsinadelis, Peter (Peter) 
 To: 'rollei   
 Sent: Monday, April 01, 2002 10:52 AM
 Subject: [Rollei] Planar Sharpness
  A collimator is a devise which makes light parallel, as though it were
coming from an infinite source. 
  What is meant here is an autocollimator, where the camera lens becomes
part of the system. The type of autocollimator used for a camera has a
collimated light source and a prism with a telescope attached so that it
can simultaneously project a beam of collimated light into a lens and look
through the lens. The target is the film. the projected spot is adjusted
for best focus while looking through the telescope. That puts the lens
exactly  at infinity focus for the surface of the _film_ or whatever target
is used. Focusing can be done with a bit of ground glass at the film gate
but there is no guarentee it will be exactly where the film is. 
  If the film is replaced with a plane mirror the autocollimator can be
used for aligning the tilt of the lens. Older Rollei cameras use shims for
this. The F series has threaded adjustments. Either camera can be adjusted
for very accurate parallelism of the lens. 
  This adjustment can be made with a reference flat and a depth gauge
(shown in factory manuals) but the autocollimator, if available, is more
  I believe Bill Maxwell uses an autocollimator for setting up cameras.
National Camera used to offer an autocollimator complet with mirror flat
for Rollei and similar cameras. 
  BTW, its easy to set up a view camera as a makeshift collimator by
autocollimating it with the aid of a small mirror. This is also a good
method of finding the exact infinity focus point. You need a pencil
flashlight or some other small source. Youn can also draw a cross near, but
not at, the center of the ground glass. Put the flashlight against the
ground glass, near but not at the center. Put a mirror over the front of
the lens. A first surface mirror is preferable but a shaving mirror will
do. An image of the lightr will be projected back to the ground glass.
Adjust it for best focus. The camera is now focused exactly at infinty. If
the groudn glass has a cros-hatch on it and is illuminated the pattern as
seen from the front of the lens will appear to be at infinity and can be
used for checking the infinity focus of other cameras. 
  This is also useful for setting infinity stops where a sufficiently
distant object is not available. It also provides a reference for finding
the exact focal length of lenses and their principle points. 
- ----
Richard Knoppow
Los Angeles, CA, USA