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Jayd
Tinkerer
Username: Jayd
Post Number: 60
Registered: 06-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 30, 2009 - 02:40 pm: | 
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When doing the infinity setting back sighting lens test Ala Ed Romney should I be able to see the far scene in focus through the camera lens when back far enough that it is seen in focus around the camera? The scratches on the glass at the film plane are perfectly sharp and I do have them on the lens side, I used my best lens a 100mm RE Topcor and a 50mm Canon with the same results. Unfortunately Romney’s book’s illustrations are not clear enough for me to make out, but it seems he is saying I should see sharply through the lens when I see sharply around the camera.
Thanks
Jay |
Jayd
Tinkerer
Username: Jayd
Post Number: 61
Registered: 06-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 30, 2009 - 02:46 pm: |
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I might add with my glasses + 2.25 on I don't even need a secound camera to see that the patterns scratched in the glass on the film plane are sharp, with the secound "viewing camera I don't really need my glasses. The camera being checked is my Koni Omega with 90mm lens.
Thanks
Jay |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 893
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 30, 2009 - 02:54 pm: |
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You DO need a second camera. That is your measuring instrument. If you're just looking in through the lens, with or without glasses, the test is meaningless.
Here are some clearer illustrations: http://rick_oleson.tripod.com/index-123.html
Now: Looking in through the front, viewing with an SLR known to be correctly in focus at infinity, the scratches on the target glass in the film plane need to be perfectly sharp ("Perfectly sharp" in this case means, your viewfinder indicates that they are in perfect focus with the lens set at infinity. A split-image rangefinder in the viewing camera is very helpful for lining up the scribe marks on the target glass). Nothing else matters, it doesn't matter if you can see anything in the background or not. |
Jayd
Tinkerer
Username: Jayd
Post Number: 62
Registered: 06-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 30, 2009 - 07:50 pm: |
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Thanks Rick that makes sense to me.
Now a couple more Newbie question about back sighting:
1. On a Foldex 30 I'm restoring true infinity by back sighting is about half a turn out from the original position maybe slightly more I have adjusted the focusing ring? Stop accordingly but wonder if I should replace the shim behind the lens/ shutter assembly with a thicker one instead increasing the distance from the film plane to the lenses rear element instead simply adjusting the front element?
2. This being a view finder camera and my doubts about the best way to adjust the lens: can I use the back sighting method to check the focusing at other distances I.E. mid and close? I assume one would set both cameras to the same distance of course.
Thanks again
Jay |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 895
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 30, 2009 - 09:47 pm: |
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It should be fine to adjust the front element. Unless the front standard has been bent, the most likely cause of it being off focus in the first place would be that someone had worked on it at some time in the past and didn't get it back quite at the correct position.
The focus helical is robust and will not change over time, and the distance settings will have been correctly engraved at the factory. If you get it correct at any one setting, it will be correct at all of them, you need only adjust once. Infinity is the easiest point to establish with complete certainty, that's why I adjust to infinity rather than at some closer point that would be subject to measurement errors etc. |
