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Article on Berthas (long lens SLRs built around a Graflex)

 
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Dan Fromm



Joined: 14 May 2001
Posts: 1891
Location: New Jersey

PostPosted: Wed May 01, 2013 10:26 am    Post subject: Article on Berthas (long lens SLRs built around a Graflex) Reply with quote

I made a Baby Bertha (long lens SLR built around a 2x3 RB Series B).
An account of the adventure has just been published on the French large format forum in English and in French. See:

http://www.galerie-photo.com/baby-bertha-6x9-en.html (English)

http://www.galerie-photo.com/baby-bertha-6x9-fr.html (French)
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45PSS



Joined: 28 Sep 2001
Posts: 3245
Location: Mid Peninsula, Ca.

PostPosted: Fri May 03, 2013 4:06 am    Post subject: Reply with quote

(http://www.mcmahanphoto.com/lc380.html)
Camera being used in portrait position by the man on the right is a Top Handle Speed Graphic;
(http://www.avlispub.com/garage/apollo_1_launch.htm)
Camera in the side bar being held by Arch Smith is a RB Auto.

Dan,
a recent Pacemaker Speed CLA shutter produced accurately exposed negatives at the 1000 and 500 settings even though a shutter speed tester showed them to be slow. Leaf shutters that tested slow produced very dense negatives compared to speeds that tested accurate.
I suggest testing FPS with film at each or questioned speeds against a leaf shutter that test or is known to be accurate.

Interesting article.
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Dan Fromm



Joined: 14 May 2001
Posts: 1891
Location: New Jersey

PostPosted: Fri May 03, 2013 7:12 pm    Post subject: Reply with quote

Charles, as I've told you several times the shutter speeds don't respond to the tension setting. They respond as expected to slit width.

The problem with my RB's shutter speeds isn't that they're slow. It is that there are only four of them.

At this point the shutter speed issue is moot. Until I reconcile myself to shooting 2 1/4 x 2 1/4 (or less) instead of 2 1/4 x 3 1/4, the project is dead. Vignetting that seems to be inherent in the setup is a killer.

What bothers me about most this is that I've seen no complaints about vignetting with 4x5 or 5x7 Berthas. It could be that I don't understand what causes the vignetting. Or it could be that there's something in the geometry of larger Graflexes that's different from that of the 2x3 RB Series B.
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45PSS



Joined: 28 Sep 2001
Posts: 3245
Location: Mid Peninsula, Ca.

PostPosted: Fri May 03, 2013 8:11 pm    Post subject: Reply with quote

Suggestion: switch out the 4 inch PVC to 6 inch or 8 inch black ABS.
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Dan Fromm



Joined: 14 May 2001
Posts: 1891
Location: New Jersey

PostPosted: Fri May 03, 2013 8:27 pm    Post subject: Reply with quote

Charles, thanks for the suggestion. The tubes' diameter isn't the problem. A 2.25" x 3.25" rectangle will fit in a 4" tube.

The mirror box is the problem. It is low and close to the film plane. That's why I get worse vignetting in portrait orientation than in landscape.

It isn't a problem with the short lenses normally fitted to the 2x3 RB Ser B. It is a problem with long lenses.

Here's how to think of it. The lens projects a cone of rays. To cover the format, the cone of rays has to be 4" in diameter at the film plane. If you have a 2x3 RB Ser B, measure the mirror box and its distance from the film plane. If you don't, accept my measurements. They're in the article. Now calculate how tall a cone whose base is 4" in diameter and that just clears the mirror box can be. And then you'll understand why long lenses can't be hung far out in front of a 2x3 RB Ser B and cover the format.
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45PSS



Joined: 28 Sep 2001
Posts: 3245
Location: Mid Peninsula, Ca.

PostPosted: Sat May 04, 2013 1:34 am    Post subject: Reply with quote

I am aware that a lens projects light in a cone shape, that camera bellows are larger than the format of the camera and some are tapered from the front standard to the rear of the bellows. Some of this size difference allows for movements. I did pick up a 2x3 Series B and some RFH's for a reasonable price but it will be a few days to weeks before I can get around to measuring, calculating ratios, and making further constructive suggestions.

More to follow someday.
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