A key to understanding these lenses, and how to use them for actual shooting, is to learn the actual image circle projected and its behaviour when focus varies. The image circle at infinity focus is approx. 23mm for the Rodenstock TV-Heligon 50/0.75, and at least 26mm for the Oude Delft Rayxar 50/0.75. The circle is smallest at infinity focus and will enlarge as the lens is focused closer. Concomitantly, field curvature, spherical aberration, and coma increases and eventually will be manifested also in the near centre of the frame.
In the typical X-ray application, two superfast lenses with apertures in the f/0.7-f/1.1 range were used either facing front to front or back to back. The usual close-focus loss of lens 'speed' was avoided by the design, thus very weak X-ray images could be viewed on a TV monitor and/or recorded onto film. as shown below
From forum.mflenses.com
The TV-Heligon obviously was intended to transport the image onto TV. The initial record was through a corresponding XR-Heligon lens. The rôle of the Rayxar apparently is similar to that of the XR-Heligon, if I understood the [Italian] description at
http://www.marcocavina.com/articoli_fotografici/Rodenstock_De_Oude_f_0,75/00_pag.htm . For our photographic applications, either system could be put to good use.
Another Norwegian, Espen Susort, describes his modification of the Rodenstock for Sony A7 cameras here:
https://espensusort.no/2021/03/rodenstock-50mm-f-0-75-4/. I just became aware of his posts when my own adapters had reached the first prototype stage. Evidently, the additional 2mm of register distance of the Sony cameras created additional issues in the quest for infinity focus. His comments and the published test image for infinity focus clearly indicates he couldn't quite reach true infinity focus with his setup. He claimed best performance from 1.5 to 10m which is not consistent with my own observations. The lens indeed is at its sharpest for infinity focus, as it should be, and the performance graciously declines when the lens is focused closer. This behaviour is shared between the Rodenstock TV-Heligon and Oude Delft Rayxar lenses, and reflects the design criteria for their intended application as a component of a tandem relay system for X-ray machines.
Thus, the quality section of the projected image circle might be quite small, but with judicious framing of the subjects, we can make interesting images with most or all of what the lens projects. The aberrations increase very rapidly outside the core area, as evidenced below where I show the peripheral part of a Z9 frame done with the TV-Heligon. This was a blue-hour capture done well after the sun had set and thus the street lamps provided intense point-light sources.
Also note that even with the lens focused to a subject approx. 500m away, the depth of field still is inadequate to include much of the middle ground subjects. Consider the vintage of the lens and the effective speed of f/0.75, the lens really is amazingly sharp in the central parts of the frame. The designers needed 9 high-refractive elements in 6 groups to achieve such a performance, which obviously came at a considerable expense in its time.
