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Focusing on top vs rear ground glass - why a difference?

 
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Bosaiya



Joined: 19 Oct 2005
Posts: 23
Location: Seattle

PostPosted: Sat Aug 01, 2009 3:46 am    Post subject: Focusing on top vs rear ground glass - why a difference? Reply with quote

I'm a little perplexed. The top ground glass (under the lid) should be equally distant from the lens as the rear ground glass (optional attachment) which lies along the film plane. That's what ensures that focusing on the top GG produces the identical image on film. And that's what happens on the standard supplied lens. Elementary.

I have a lens for which this does not happen. Viewing through the top produces the expected image - distant objects appear equally along with medium and close. There is quite a lot of vignetting at infinity focus, but still a normal looking image. Viewing through the rear ground glass there is nothing in focus at anything but extremely close macro distances - within several inches. Everything else beyond is a complete blur, nothing but a wash of color.

How can this be? Even if the troubled lens were not mounted the same distance as the original, what appears on one GG should be what appears on the other, shouldn't it? Why would there ever be a difference between the two? Under what circumstances would they ever diverge? I'd say that one of them was out of alignment, but they both appear equal with the standard lens and negatives with that standard lens come out as expected.

Is there some sort of voodoo afoot? Perhaps the missing link in so many crackpots' theories of non-Newtonian physics? Wormholes?
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Bosaiya



Joined: 19 Oct 2005
Posts: 23
Location: Seattle

PostPosted: Sun Aug 02, 2009 12:28 am    Post subject: Reply with quote

I did a few tests today and as I suspected the image on the film plane was correct. I'm still not sure why I would get the difference between the two.
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C. Henry



Joined: 13 Dec 2005
Posts: 358
Location: North East Georgia, USA

PostPosted: Sun Aug 02, 2009 6:05 pm    Post subject: Reply with quote

B.

While I do not own, nor have I ever used a Graflex Reflex camera, it occurs to me that the lens that is giving you trouble might somehow interfere with the mirror returning to its proper position. This will cause focusing errors on the top focusing screen of any single lens reflex camera, not a Graflex specific problem. Just a suggestion of something to check out.
Wishing you success in solving your problem.

C. Henry
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Bosaiya



Joined: 19 Oct 2005
Posts: 23
Location: Seattle

PostPosted: Mon Aug 03, 2009 10:43 pm    Post subject: Reply with quote

That had briefly crossed my mind. The lens body is too long to operate at infinity - it interferes with the mirror function. Using it on more closely-focused objects (extending the bellows) solves this. I made sure that the mirror was not obstructed when I did my crude tests.

I do wonder if that has some implications, but again, can't see why it would look different between the two equidistant ground glasses.
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Bosaiya



Joined: 19 Oct 2005
Posts: 23
Location: Seattle

PostPosted: Fri Aug 07, 2009 3:13 am    Post subject: Reply with quote

Figured it out. A day late and a dollar short.

I had cracked the top ground glass a little while back and repaired it with tape. The tape, for whatever reason, made it look like everything was in focus - near or far. My replacement glass came today and once in, everything was well.

Unfortunately I ended up blowing a good 30+ shots today, judging by the first batch I developed. Ouch. Fortunately I took a MF TLR along and those shots came out great.

Lesson learned - the hard way.
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