[rollei_list] Re: rollei focus adjustment questions

  • From: chatanooga@xxxxxxxxx
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Tue, 6 Jun 2006 10:39:03 +0100

Richard
Fascinating insight. Sounds like a fairly non-trivial procedure!  So it
looks then that there is only adjustment (at inf) and if this is exactly
done all should be well across the whole focus range? What I find curious
though is that one would think  the error is most obvious at the
closest  distance
- ie this would be a natural place to do the calibration?. In my camera's
case  I have no focus issues at infinity - in fact there is no sharpness
difference in negs taken at exactly infinity and those taken with the focus
pulled back a small bit.

On 6/6/06, Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote: 


----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, June 05, 2006 4:43 PM
Subject: [rollei_list] Re: rollei focus adjustment questions


> Richard may have good eyes but I can't begin to really
> check focus
> without a 20-30X well-corrected magnifier... they are not
> easy to come
> by...
>
> Eric Goldstein
>
   Well, I said 10X _minimum_ and I agree that stronger
magnifiers are hard to find. One really needs a low power
microscope. If you have that you can check using a clear
glass reticule with a mark on it for the focal plane
surface. However, a 10X glass will tell you if things are
way off or if the lens has a problem.
   The method used by Rollei is to place a first surface
mirror at the focal plane and use an auto-collimator with a
telescope looking back through the lens. This is a very
accurate method but requires special equipment.
   In principle, this is not hard to do, but in practice it
needs some sort of mechanical fixture to hold everything in
place. The system consists of a viewing telescope at
infinity focus, a collimator with target, and a beam
splitter mirror or prism block. The target in the collimator
appears to be at infinity. When the taking lens is exactly
at infinity focus the light from the collimator is reflected
from the mirror back through the lens and the light is again
parallel, i.e., collimated, so it again appears to be coming
from infinity. If the telescope is accurately focused for
infinity the image seen through it appears to be in focus.
The telescope can provide whatever degree of magnification
is desired.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx

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