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Film flatness in RFBs

 
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Not MPP



Joined: 26 May 2002
Posts: 18
Location: I.O.W. England

PostPosted: Wed Jul 31, 2002 4:43 pm    Post subject: Reply with quote

I have 2 Graphic 23 backs on a Crown 23, and am having problems with film flatness when using a 35mm Apo Grandagon. I have bent the spring tensioners to try to increase tension but to no avail. The effect is kind of "pincushion" with bent verticals. It doesn't seem to happen with a 65mm. Does this mean that I will have to try a "Singer" RFB, or can someone suggest a fix? It does not happen when shooting the same subject "portrait", so it has got to be the film - hasn't it?
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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Wed Jul 31, 2002 7:23 pm    Post subject: Reply with quote

First off lets define the problem first. If the pincoushion effect is sharp, then I'd bet money it's the lens.

if the effect is that the image is soft on the edges in an area in the shape of a pincoushion then it's probably the back.

If the same back on the same camera gives different reslts with different lenses then I would start looking at the the lenses.

film curl happens at different rates depending on where you are in the roll.

I'd shoot two rolls of transparency film of the same subject. On roll one shoot the apo first for 4 frames, then te 65 angulon for the last 4. On the second roll reverse the order.

If the apo is consistanly showing pincoushion and the 65 is not. Then the next step would either to send the lens back or contact Rodenstock. To be absolutely sure I'd go to the extent of trying to some how shoot sheet film. Grandted I wouldn't expect it to cover 4x5 but you would see the entire image area and could judge the lens's capabilities. Now finding a 4x5 camera that can focus a 35mm lens will be a project in itself.

Now if the effect is intermittent some on the 65 some on the 35 then it's the back.
Try to borrow a Maymia back from an RB67 and test that
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Dan Fromm



Joined: 14 May 2001
Posts: 2119
Location: New Jersey

PostPosted: Wed Jul 31, 2002 9:37 pm    Post subject: Reply with quote

Quote:

On 2002-07-31 09:43, Not MPP wrote:
I have 2 Graphic 23 backs on a Crown 23, and am having problems with film flatness when using a 35mm Apo Grandagon. I have bent the spring tensioners to try to increase tension but to no avail. The effect is kind of "pincushion" with bent verticals. It doesn't seem to happen with a 65mm. Does this mean that I will have to try a "Singer" RFB, or can someone suggest a fix? It does not happen when shooting the same subject "portrait", so it has got to be the film - hasn't it?
Forgive my stunned incomprehension.

Are you shooting 6x6, 6x7, or 6x9? Yes I know, y'r lens will more than cover 6x9.

Did you say that the pincushion distortion happens with the camera held base down but not with the camera held on end? How do the verticals look on the ground glass? Does that change with camera orientation?

Where in the frame is the distortion worse? Ends or center?

Cheers, back to scratching head in amazement,

Dan
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Not MPP



Joined: 26 May 2002
Posts: 18
Location: I.O.W. England

PostPosted: Thu Aug 01, 2002 8:15 pm    Post subject: Reply with quote

Hi,
Many thanks for the suggestions. The lens went back to Rodenstock, and they kindly sent me another - same results! I then tried it on my Sinar with an MPP (British) RFH, - no curvature. I had a trawl through this site yesterday, and found mention of rollers in the Singer backs. It then occurred to me that I had bought a spare RFH "shell" on Ebay, that someone had fitted rollers to. I had shelved it for spares, but when I tried it, sure enough, I got vertical verticals!

What a relief!
Thanks again.
Quote:

On 2002-07-31 12:23, Les wrote:
First off lets define the problem first. If the pincoushion effect is sharp, then I'd bet money it's the lens.

if the effect is that the image is soft on the edges in an area in the shape of a pincoushion then it's probably the back.

If the same back on the same camera gives different reslts with different lenses then I would start looking at the the lenses.

film curl happens at different rates depending on where you are in the roll.

I'd shoot two rolls of transparency film of the same subject. On roll one shoot the apo first for 4 frames, then te 65 angulon for the last 4. On the second roll reverse the order.

If the apo is consistanly showing pincoushion and the 65 is not. Then the next step would either to send the lens back or contact Rodenstock. To be absolutely sure I'd go to the extent of trying to some how shoot sheet film. Grandted I wouldn't expect it to cover 4x5 but you would see the entire image area and could judge the lens's capabilities. Now finding a 4x5 camera that can focus a 35mm lens will be a project in itself.

Now if the effect is intermittent some on the 65 some on the 35 then it's the back.
Try to borrow a Maymia back from an RB67 and test that
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Not MPP



Joined: 26 May 2002
Posts: 18
Location: I.O.W. England

PostPosted: Thu Aug 01, 2002 8:22 pm    Post subject: Reply with quote


[/quote]Forgive my stunned incomprehension.

Are you shooting 6x6, 6x7, or 6x9? Yes I know, y'r lens will more than cover 6x9.

Did you say that the pincushion distortion happens with the camera held base down but not with the camera held on end? How do the verticals look on the ground glass? Does that change with camera orientation?

Where in the frame is the distortion worse? Ends or center?

Cheers, back to scratching head in amazement,

Dan
[/quote]

Hi,
As you will see above, I have solved the problem, but to answer your questions, it is a 2.75" x 3.25 ( why don't they put fractions on British keyboards any more?)
With these extreme wides, the corners of the screen are so dark, even wide oppen, you can only really see the centre clearly. The distorsion had the effect of verticals being curved (as per centre of the film).
I have used the camera on its first paying job today, and it is so much easier to use than a darn great monorail!

Thanks again.
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