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Steve
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 04, 2005 - 02:03 pm: | 
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On my father's Praktica BC1, infinity focus in the viewfinder occurs at just over the 20 feet mark on the focus ring on the standard 50mm lens. He also has a zoom lens which shows a similar result. Infinity focus occurs at about 18 feet at the 70mm end of the zoom, and about the 50 foot mark at the 210mm end of the zoom.
I assumed this was due to an error in the mirror position (I seem to specialise in SLRs with mis-set mirrors!), but I checked focus at the film plane using a viewfinder screen from a scrap camera, and the focus agrees exactly with the viewfinder, which must mean either that the lens position is wrong, or that both lenses have exactly the same fault (not too likely!).
As you might expect on a "modern" mass-produced camera, there are no shims behind the lens mount. The spring ring for the bayonet fits within a machined diameter in the mount and does not affect anything. I checked the movement of the standard lens between the "just over 22 feet" and infinity positions, using a dial-test-indicator, which shows that a shim of about 10 thou (.01inches or .25mm) would be needed to correct the error.
My father bought the camera new, and it has had no accidents. It has been to a repairer once, to correct a sticking shutter. Is there anything that could have been left out during repair to account for the fault? |
Paolo Amedeo
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 04, 2005 - 02:34 pm: |
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Hi Steve,
Probably, there was a shim between the body and the mount... I am afraid that installing washes of the proper thickness under each screw it wouldn't be enough, since the ring with the spring ring would become loose and, potentially, some light could come in, reducing the contrast of the picture...
I would suggest cutting a ring of the proper shape and thickness from black cardboard or plastic...
Paolo |
Roberto Rossi
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 04, 2005 - 07:27 pm: |
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Not seen any BC1 with mount shims,although some might have them I suppose.However are you sure the screen was not disturbed during the repair?If fault occurred after repair I would take it back and let the repairer sort it. |
Steve
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 05, 2005 - 01:59 am: |
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Since writing my original message I have noticed that the instruction book gives a figure for the "flange focal length" of 44.4mm. Presumably this is the flange to film-plane distance. I have checked this, and make it 44.245mm, give or take .01mm, a difference of .155mm - not quite as much as my previous estimate, but in the same ball park. Maybe there should be a shim behind the lens mount after all. It would be a bit complicated, with cutouts for the bayonet spring and the electrical contacts, presumably.
The repair was many years ago, so I'm afraid return isn't an option! |
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