> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Russell McMahon
> Sent: Monday, April 10, 2006 5:21 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Sigma camera lens repair.. .
> 
> 
> > >The hardest part will be to ensure calibration at or near
> >>the ends (infinity and closeup). Full testing of the lens
> >>may be required before you waste a film on testing (and
> >>you can't see much in the viewfinder, you have to use a microscope 
> >>which is a part of the infinity jig).  Do you have an infinity test 
> >>jig ?
> 

You mention this is a DSLR so this trick might not work.  But, for a
film SLR you can make an infinity focus rig from an empty GLASS slide
mount, some frosted scotch-tape and a second camera with a known good
infinity focusing lens (preferrably 50mm or at least something with a
similar diameter to the lens in question).  You will also need a
reversing ring compatible with both sizes of lens.

I may be skipping some detail here (or possibly getting something
ass-backards), I can look it up later.

1) Make a target of the slide mount by carefully scratching an X on the
surface of the glass.  You might want to put the frosted tape on the
opposite side of the glass to give it some visible texture.
2) Tape the slide to the film rails inside the camera.  You will
probably need to remove the camera back and also part of the slide mount
so that the X is centered in front of the lens and in the film plane.  
3) Attach camera and lens in question to tripod, point at difuse light
source (open sky, well lit concrete, etc).  Attach known good camera and
lens to front of lens in question.
4) Focus known good camera to infinity.  Lock both shutters open (B or T
setting).
5) Looking through known good camera, rotate questionable lens until X
comes into sharp focus.  Now questionable lens should be focused at
infinity.


I have never done this with an SLR lens (let alone a DSLR) but I have
used this trick to adjust the infinity focus on a 35mm range finder.
Got the lens focused at infinity, rotated an adjustable ring on the lens
to re-align its marks with the focus mark on the camera body then
adjusted the mirrors in the range finder.  Got lucky and didn't need to
add any shims between the lens and body.  Once focus was ajusted for
infinity, near focus was good.  Checked near focus with tape measure,
pencil pushed into modeling clay (vertical target) and small piece of
frosted glass held to the film rails.  Watched focus on ground glass
with x10 loupe then checked range finder.  Then checked view through
range finder and looked at focus ring marks compared to tape measure.

Rob
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist