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7-1/2" #33 focus shift?

 
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troublemaker



Joined: 24 Nov 2003
Posts: 715
Location: So Cal

PostPosted: Tue Mar 18, 2008 4:39 am    Post subject: 7-1/2" #33 focus shift? Reply with quote

I finally got my RB running again and now have a Graflex focussing panel for it so I can more easily use a loupe when i want. And what I saw when I tested it out surprized me. A ton of focus shift, enough to ruin a lot of images trying to shoot SLR style.
It's the old RB series B with the standard 7-1/2" 4.5 Anastigmat and very clean. I am wondering if this was common to these lenses and if so, how did folks cope with it.
Lens issharp wide open, and then when I stopped down to f~8 to 11 it really throws the focus out. I have to wheel the lens out closer to get sharpness back, quite a bit at 6-15 ft. Not a problem with a dark cloth and usingthe GG on the back, but not so easy using the hood without a focussing aid.
Surprise, I now know why I didn't like the crappy images I made with it a couple years back.
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45PSS



Joined: 28 Sep 2001
Posts: 4081
Location: Mid Peninsula, Ca.

PostPosted: Tue Mar 18, 2008 8:25 am    Post subject: Reply with quote

I had a 10 inch anastigmat that was razor sharp from wide open to stoped down and had no focus shift. I suspect that an element or group is out of alignment (mispositioned in the barrel) causing the focus shift in your lens. According to Graphic Graflex Photography 1940 the closest focusing distance for the 7 1/2 lens in a Series B is 27 inches and there was a 15 inch telephoto available. There is no mention of focus shift for any Kodak lens.
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troublemaker



Joined: 24 Nov 2003
Posts: 715
Location: So Cal

PostPosted: Tue Mar 18, 2008 7:53 pm    Post subject: Reply with quote

I'll take it all apart after work and see, but IIRC I already checked all that two years ago. anyway, I'll get back this evening.
Right now the thing is plenty sharp edge to edge and corner to corner wide open. More so than my Cooke 7" 5x7 which wont even cover 4x5 unless stopped down. So I think it is a shifty lens...
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45PSS



Joined: 28 Sep 2001
Posts: 4081
Location: Mid Peninsula, Ca.

PostPosted: Wed Mar 19, 2008 12:40 am    Post subject: Reply with quote

The only info I can find on Kodak Anastigmat lens is in the Vade Mecum and that is a quess. The lens diagram suspected of being used is:
Front element: Convex front, flat rear slight clip on edge at front /rear; air gap; double concave edge of rear side has cliped edges; aperture; double concave, cliped edges on side facing aperture possibally cemented to a double convex rear element.
Things to look for are the retaining ring lockscrews for an element or group usually enguage the lock ring at the center of the ring not the edge and the retainer screw/pin may go thru both the barrel and retainer ring (trying to remember how the ones I've had apart were assembled); discoloration of the barrel where not covered by an element or retainer.

Someone before you may have had the lens apart and reassembled it incorrectly.
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troublemaker



Joined: 24 Nov 2003
Posts: 715
Location: So Cal

PostPosted: Wed Mar 19, 2008 6:45 am    Post subject: Reply with quote

I took everything apart and all looks as it should be.
The front group has removable back element and standard front lock ring, and the rear has a lock ring on the inside facing forwardlike the older type 135 Optars. Not much to mess up, and all the elements appear to be seated properly. But I'll add this, when the lens is focussed it is nice and sharp unlike when one partially unscrews a back element etc.. to purposely soften. I'll go through it again tomorrow as time allows. And then I can take the whole camera apart again and see if I can get the mirror to lock all theway down as I found that to be out of whack also.
Thanks for the info.
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