Originally Posted by
Michael Rosenberg
I had posted this information a few months ago:
Quote:
Re: Focusing problems with the fresnel groundglass on the Chamonix 45N-1?
I was a little baffled with this thread. Two people with the same camera having identical problems with a particular focal length lens. I would have thought that the Chamonix being a new camera the position of the gg/fresnel combination would have been tested. So I thought I would test my own Chamonix, and using mm ruler verify any small differences if they existed.
For a target I used a window mat on a mat board that would provide a bright line (reflecting the overhead light), and a dark line that would be a shadow from the overhead light. Both lines would be parallel to each other, and the target would also test flatness of the gg (i.e. there should be no change in focus of the parallel lines).
I checked three lenses, my 58mm Schneider XL, 90mm Linhof Scneider Angulon f5.6, and my Apo Sirornar S 135mm. All were tested with either the fresnel in the proper position, with it on top of gg, and no fresnel - just the Chamonix gg. I used a Scneider 6X loupe to check for focus.
With the 58mm lens the focal shift +0.5 mm with the fresnel in the proper place. With the 135mm lens the focal shift was +1 mm with the fresnel in the proper place. This should not make a big difference in depth of field unless shooting wide open I would think. However, with the 90mm lens the focal shift was +3.5mm with the fresnel in the proper place. There was no focal shift when the fresnel was placed on top of the gg. I have tried 2 other gg that I have, and made the same observation.
One possible explanation is the focal length of the fresnel is affecting the focus of 90mm lenses. I do not have another fresnel to test. But I think hereafter I will keep my fresnel on top.
End Quote:
I also followed up that I had put a Maxwell screen from another camera (that I had sold) on the Chamonix and saw no focus shift. I had a thicker cover glass so I cut shims (hose washers) for the gg hold-downs.
I honestly cannot see in any of my images that there is a focusing issue, but I generally stop down f22 or greater. I have had my camera almost two years. I think the only way this issue affects me is maybe where dof was critical and objects closest to me weren't as sharp as I may have thought - but that maybe operator error.
Mike
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