Source of pincushion distortion with 6x12 rollfilm back?

[Micah Marty]

I'm looking at an architectural project suitable for a 6x12 back on a view camer a but I'm concerned about pincushion distortion.

I've owned both the Calumet Series I 6x12 holder and the Horseman 6x12 (bought i n 1999, new). The Calumet was badly designed, with terrible film curl from a wea k pressure plate, and I quickly returned it (I have not tried the redesigned MkI I Calumet 6x12's). The Horseman worked well (albeit with slightly uneven frame s pacing) but I sometimes could see pincushion distortion a centimeter or so in fr om each end of the frame. I wish I could be more scientific about my observation s, but I didn't get a chance to do side-by-side comparisons between the 6x12 bac k and 4x5 sheets before I sold the Horseman back (and the two lenses I used it) with as part of a package deal.

Because I used the Horseman 6x12 rollfilm back with only two lenses and didn't d o a full test, I was unable to tell whether the pincushion distortion was caused by film curl in the film back or by the lenses I was using on the Cambo Wide (4 7XL and 75SA).

I've seen unacceptable pincushion distortion even when shooting sheet film (8x10 ) with expensive modern LF lenses, so I would have assumed that the problem was with the lenses if no one else questioned the film flatness of 120 rollfilm. But on the Perez/Thalmann/McDonald medium-format lens-testing site I saw this note: "It appears that film flatness is a serious issue with some medium format camer as. It may be that 120 film "bows" in the middle of the pressure plate for some of the cameras tested. It shows up most frequently when shooting 6x9 [and pres umably larger] format cameras and film backs and earlier Rolleis." (copyright Ch ris Perez et al)

Granted, the "bowing" they refer to may have nothing to do with pincushion disto rtion toward the edges, so the above note may be irrelevant to my concern. But r ecently a friend who uses the Horseman 6x12 back with Schneider 65 and 90 lenses has shown me photos which also exhibit some pincushion distortion on vertical l ines running perpendicular to the film length (e.g., if you photograph a horizon tal box with the 6x12 used horizontally, the ends of the box will bow in). Could my friend's two lenses and my 75SA (I never shot straight stuff with the 47XL) all be exhibiting the same kind of distortion? Or might the problem be in the 6x 12 back? Would the film curl even be in the appropriate direction to cause pincu shion distortion? (It seemed to me that it would cause barrel distortion, but I' m not smart enough to know how the light rays strike different portions of the f rame.)

Thus my main question: Has anyone WHO SHOOTS ARCHITECTURE with the Horseman 6x12 back noticed this pincushioning (I specify architecture because it would not be noticeable in landscape shooting). Anyone have photos they could e-mail or post showing the distortion or lack of same?

And are the pricier Linhof and Sinar 6x12 backs designed differently (than the H orsemans) to better avoid film curl?

Thanks in advance....

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[Walter Glover]

Micah,

I currently use the Sinar Zoom 2 and the Linhof Techno Rollex, in the past I have used the Horseperson as well. I am a critical shooter of architecture and have never seen the distortion of straight lines with any of these backs - and that's with lenses down to 35mm Apo-Grandagon.

What you will find, however, with very short lenses and the Sinar, is that the black edge (rebate) may be curved. This is the product of bowing of the adjustable aperture masks. It is hardly noticeable with normal or long lenses where light falls onto the film almost perpendicular but with very short lenses the light path is so oblique that it slants in under the mask.

Walter