Strange results while trying to collimate / adjust a lens...

[Christoph C. Feldhaim]

I had a strange result from an experiment which I don't understand.
I tried to use a second camera - a SLR - to check the infinity setting of my Mamiya Universal 250 mm lens.

I had a 35-70 mm zoom on the SLR and set it to 70 mm, open aperture, Shutter open, infinity.
I put a transparent plastic on its film plane with one half of it matted with sandpaper,
on the other half I had painted a cross, both on the same side and this side pointing to the lens.
I put a mirror in front of the lens and illuminated the cross on the film plane which got reflected and projected on the matte part of the plastic.
It was sharp at infinity setting. So I checked the infinity setting of the lens was okay by autocollimation.

Then I put the SLR right in front of my  250 mm lens (strictly parallel) on the Mamiya Universal, set both lenses to infinity and
expected the illuminated cross on the SLR film plane to be projected sharp on the matte screen of the Mamiya Universal.
 
Strange enough that wasn't the case ... was the 250 mm lens not adjusted ? At a setting of 7 meter on the 250 mm lens the cross appeared sharp.
I did the same autocollimation procedure I had done for the 35 mm SLR on the Mamiya Press and it told me: "Yes - at 7m it is sharp - there is your infinity ..."

Now: I checked the lens double by pointing it on a target exactly 5 meter away and at the 5 meter setting it was sharp which would suggest the 250 mm lens was not wrong.

I double checked everything and simply have no clue where the error might be.
Both lenses seem to be well adjusted by optical checking (looking through),
but the 250 mm lens seems to fail the autocollimation test while
it appears fine at manual checking by looking through it and watching the matte screen.

How is that possible ?

[Rod.Klukas]

I had a strange result from an experiment which I don't understand.
I tried to use a second camera - a SLR - to check the infinity setting of my Mamiya Universal 250 mm lens.

I had a 35-70 mm zoom on the SLR and set it to 70 mm, open aperture, Shutter open, infinity.
I put a transparent plastic on its film plane with one half of it matted with sandpaper,
on the other half I had painted a cross, both on the same side and this side pointing to the lens.
I put a mirror in front of the lens and illuminated the cross on the film plane which got reflected and projected on the matte part of the plastic.
It was sharp at infinity setting. So I checked the infinity setting of the lens was okay by autocollimation.

Then I put the SLR right in front of my  250 mm lens (strictly parallel) on the Mamiya Universal, set both lenses to infinity and
expected the illuminated cross on the SLR film plane to be projected sharp on the matte screen of the Mamiya Universal.
 
Strange enough that wasn't the case ... was the 250 mm lens not adjusted ? At a setting of 7 meter on the 250 mm lens the cross appeared sharp.
I did the same autocollimation procedure I had done for the 35 mm SLR on the Mamiya Press and it told me: "Yes - at 7m it is sharp - there is your infinity ..."

Now: I checked the lens double by pointing it on a target exactly 5 meter away and at the 5 meter setting it was sharp which would suggest the 250 mm lens was not wrong.

I double checked everything and simply have no clue where the error might be.
Both lenses seem to be well adjusted by optical checking (looking through),
but the 250 mm lens seems to fail the autocollimation test while
it appears fine at manual checking by looking through it and watching the matte screen.

How is that possible ?

It is possible because the 250mm lens is not optimised for the same near distance that your SLR is.
Lenses are designed to be at their best in a certain distance range.  The 35-70mm can focus in an SLR camera
to maybe 12-18".  the 250mm for mamiya press was optimized for maybe 3' to infinity.
That is why Macro lenses are even in existence.  They are designed to be at their best from 1/2 lifesize to usually 2-3 times lifesize.
A normal 50mm will be much better at 50 feet than your macro would be at this distance.
Hope this helps.
Rod

[Christoph C. Feldhaim]

Thanks for answering.

But I'm still wondering.
However - I decided to send the lens to Mamiya for adjustment,
since they have the proper tools to adjust rangefinder and camera to the standards thus,
that any lens should fit to any camera case. Unfortunately there are no numbers published
about the distances to adjust, so an individual adjustment would always not guarantee, that other lenses fit.