For general information. I dismantled the optical block of the Steenbeck. After playing with a few lenses and combinations found that the small projection lens of the Steenbeck will project directly onto the camera sensor of the SI2K without the shutter prism in place between it and the film plane or any interposed optic needing to be in place between the front of lens and the SI2K sensor.
Interestingly I found that at some stage in the past, the optical block had been dropped and there was a crush deformation on one sharp corner. The retaining screw for the projection lens was also missing.
In the Steenbeck projection path, after passing through the prism shutter and the lens toward the reverse projection screen, the image is rotated upright by a prism block and reflected off two surface-coated mirrors. To focus onto this more distant target, a correction optic which can be adjusted has been interposed between the erecting prism/mirror and the screen box.
Trying to capture an aerial image from the optical path beyond this optic did not work.
The sharpness of the image is reported by the camera's focus assist as poor, at best about 25 points.
This may be related to the frame being optically enlarged for side-to-side image coverage with a top and bottom edge crop in the SI2K Super16mm format frame. With a left and right side edge crop for a full frame top to bottom, the apparent resolution might improve if this is dependent upon the sharpness of the image on the film. The film was a one light workprint struck in 1989.
The projection lens itself has no aperture control so is operated fully wide open at its rated f2.8, likely is not as sharp as it could be with a tighter aperture. There is a "goat's eye" aperture of sorts painted across the rear element of the lens to mask off the distorted inputs from the shutter prism.
The lens optical elements are permanently assembled into a cell which cannot be dismantled so there is no way to insert a fixed iris of say f4-f5.6 to sharpen the image up.
There is enough adjustment available for the projection lens to be moved relative to the film plane to achieve left-to-right edge framing or top-to-bottom edge framing. There does not appear to be any distortion. Because of my jury rigged test arrangement, edge or corner softness was hard to detect as the film image itself had areas of soft and sharp focus.
For left-to-right edge framing, the lens had to be moved forward in its holder 10mm from its normal fully inserted position. The lens is inscribed "Staeble Proj-Trigon 1:2,8/35". For left-to-right image framing, the distance from the rear element to film plane is approx 62mm. In its normal position the distance from rear element to film plane is about 52mm.
I will try an inexpensive Fujian C-mount 35mm CCTV lens with adjustable iris sometime in the future. This is unlikely to work with the Steenbeck due to the optical axis being very close to the metal platform of the Steenbeck.
However in another arrangement, this lens may be satisfactory and could be arranged closer to the film plane so that a MFT format camera frame would be fully covered by the projected image. There are MFT adaptor kits vended as a set with this lens, the 35mm Fujian found on eBay.