I am in the similar situation like Theodoros with a Contax 645 system and the efforts to mount on it what ever lens is possible (in my case in order to use with a full frame back).
I considered a conversion or adaption of the Pentax 645 25f4 lens (must be the first version D-FA where the lens shade does not reduce the usable image circle with should be full 56mmx41,5mm) for Contax 645 since quite a time for the same reason that this is the only lens for a 645 mirror reflex system which would achieve a field of view in 135 of a 16mm lens (using a 54mm x 40,5mm digital back). The flange distance of Pentax 645 is 70,87 mm, Contax 645 is 64 mm, so 6,87 mm space for the adaption.
I did not know that there is a mechanical lever to set the aperture, and due to the fact that it anyhow would require to close the lens to f11 to get an acceptable level of corner sharpness, I considered to use the lens fixed stopped down to f11 (by the common method to first attach it to a Pentax 645 camera, set the aperture to f11, press the DOF button and unmount the lens).
From the few full resolution pictures available online by this lens and the 44mmx33mm Pentax 645D/Z camera which already show the corners becoming soft and user reports about rather strong CA and purple fringing towards the corners, I have doubts that the lens corner performance at full 54mmx40,5mm (= 6,25 larger radius of image circle compared to the Pentax 645D/Z sensor) would justify the purchase of the lens and the conversion.
(I never saw a high resolution scan from a picture by the lens on a Pentax 645 film camera that would give an idea of the IQ on full frame 645.)
So my plan ... still ... is to find a Pentax 645 25f4 D-FA at a second hand dealer, go there with my FPS + IQ180 + Pentax 645 - Canon EF adapter and do some test shots. Then we would know the full frame performance of the lens. I don't expect the corner quality of the Canon TSE 24 or even the Rodenstock 23 but it should be hopefully reasonable.)
Or maybe there is an owner/user of that lens in a range of a few hundred kilometer around Munich, then we could meet and do that test together.
A year ago I also contacted the Chinese guy Steel Chen (
www.jasring.com) who does Contax 645 AF conversions similar to Conorus or Metabones and asked him if he could technically imagine a full auto aperture and auto focus conversion of Pentax 645 lenses to Contax 645 and here is his answer: "I am not familiar with P645 lens, but looks like an AF adaptor shall be possible for P645 D FA lenses. Definitely and easily, but auto-aperture is a little bit trouble, need to be done with an add-in motor in the adapter to drive the aperture open/shrink. Anyway, success possibility of this mission is 70%."
So maybe after confirming the lens performance on a FPS (which has to be proven first) we should put our knowledge and efforts together to make such conversion reality :-)
(As far as I know, the new 28-45 mm lens only covers the 44mmx33mm sensor, so not usable on Contax 645 with full frame sensor.)
Christoph