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