Re: [Rollei] Re: Coating and Collimating

[Richard Knoppow <dickburk >]

At 10:56 PM 06/19/2002 -0400, you wrote:
>
>> I had JVS do the recoating on one of my E2.8 Xenotars.  It will look great,
>> but bear in mind the lens will have a slight color shift which is only
>> noticeable on transparencies. To get around this I use a R1,5 which
>> compensates when shooting trannies.
>
>
>This surprises me -- John made a point of explaining to me that he closely
>replicates the original purple, but applies the coating at a much higher
>temperature, which makes it harder.  I was also under the impression that
>coating colors don't really affect the color of transmitted light (although
>that does seem a bit counterintuitive).  How does the color shift manifest?
>
>Also -- and this is a delicate matter -- Bill Maxwell has noted that a few
>Rolleis have come back to him for recollimating after being recoated; there
>is some concern that Focal Point's collimator is a touch off.  I wouldn't
>want to be the one to point this out to JVS, who is cl
> 
>
>Douglas Cooper
>http://www.dysmedia.com
>
>NO ARCHIVE
>early a (deservedly) proud guy.  Also, the problem may well have been fixed.
>Worth checking, however, to see whether you have any focusing errors.
>
>
 Collimating, as used here, means setting the infinity stop of the camera.
A very small error here probably does not affect overall performance. 
  However, mismatching of the finder to taking lens will make all images
out of focus. If the taking and viewing lenses are not in coincidence it
may be the result of different assumptions of where the film plane actually
is. 
  Generally, I set focus using a ground glass placed against the film
rails. This may not be entirely accurate since the film actually runs in a
very narrow channel between the side rails and the back plate. It may
buckle slightly one way or the other. One could measure the actual film
position using a prism autocollimator which projects a target through the
taking lens onto the film and allows viewing it by means of a beam splitter
and telescope. A laser interferometer would also measure it. 
  A collimator is simply a target placed exactly at the infinity focus
plane of a lens. The image is projected to infinity and appears to be there
to any other optical system looking into the system. 
  One can make a makeshift collimator from a view camera by autocollimating
the lens to infinity focus and placing a small light source or target at
the ground glass surface. 
  The autocollimating is done with a small mirror, large enough to cover
the front of the lens. Preferably a first surface mirror but an ordinary
shaving mirror will work. Just make sure the plane side is facing the lens. 
  At its simplest place the tip of a pencil flashlight on the ground glass
near, but not at, the center of the glass. The mirror and lens will project
the image of the light back to the ground glass. Focus it sharply. The lens
is now focused _exactly_ at infinity. A light or target in the focal plane
will appear from the front of the lens to be at infinity. This can be
useful for checking the infinity focus of other cameras. I have an ancient
B&L 12", f/4.5 Tessar which works well for this because it presents a large
window. 
  A little thought will show that two things happen when the light path is
folded. One, the light must pass throught the lens twice. Effectively its
the same as if two lenses were used in series with no distance between
them. Since the total focal length for two lenses of the same focal length
in series is half the focal length of either one has effectively halved the
focal length. Since the system is exactly symmetrical it is operating at
1:1 image to object size. At 1:1 the distance between object and image is
exactly four focal lengths. Since the path is folded the actual path is
here is double. Since the effective focal length of the lens is halved we
can devide by two again. The result is that the lens is exactly at its
infinity focus position. 
 This trick is also useful for measuring the focal length of the lens and
finding its principle planes when the focal length is known. 
 To find focal length set the lens up for exactly 1:1 (without the front
mirror of course). The displacement of the lens from its infinty focus
position will be exactly one focal length. Also, as mentioned above, the
distance from image to object will be four focal lengths. 
  The principle planes are found by placing the lens again in its infinity
focus position and measuring exactly one focal length from the focal plane
to the lens. The lens is turned around to find the other principle plane. 
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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