I assembled the first prototype of the Mamiya RZ67 helicoid adapter for my Contax 645 camera. The helicoid is a MF / large diameter version of similar smaller 135 size helicoids offered at eBay by certain Chinese sellers. This one I found at a local shop in Guangzhou. It has a focus range of about 15mm which is enough to focus from about 10m to infinity with the RZ tele lenses. It comes with screw mounts on both sides.
I machined a front ring to mount the RZ67 bayonet and a rear ring to mount the C645 bayonet. The front ring is mounted in a turnable collar to provide horizontal and vertical camera position. And the collar connects to a front rail to support the lens weight.
I plan to use the set-up with the RZ67 250f4.5 Apo, the RZ67 350f5.6 Apo, the RZ67 500f6 Apo and the 500 with 1.4x Extender.
First tests show several issues:
1. The unsupported camera + back weight tilts the helicoid slightly which leads to unsharpness of upper and lower section of the picture if the center is in focus.
2. It is generally very difficult to achieve perfect sharpness with such long focal lengths lenses on a 5um MF back.
3. Also the whole construction might not yet be stiff enough to damp the mirror vibration at moment of exposure.
4. The lenses are quite prone to internal reflections of out of view angled light sources, so I need hoods.
5. After mounting the lens onto the RZ67 bayonet, adjusting the aperture and turning the aperture lever to close the aperture, sometimes the leaf shutter completely closes. I have to figure out how to prevent that.
6. I should add a second movable rail to support the camera in order to avoid the tilt in the helicoid but that makes the whole construction much more complicated and it would not be longer possible to change from horizontal to vertical.
So the RZ67-C645 helicoid mount is up to now more at an experimental stage, I have to see when I find some time to work on improvements.
