It's the same lens as the 55mm/f3.5 Distagon they made for the 645AF...
But with a larger aperture since 35mm sensors only use the centre of the lens so won't bring up issues of CA, vignetting and softness towards the edges at full aperture
Yowza! I'm surprised to see this "35mm sensors only use the centre of the lens" myth coming from you, Yair.
This is a common misconception. The sensor uses the entire lens
. It only uses the center of the image circle
If I may reproduce something I wrote previously on another forum:
Light rays arrive at the central region of the image circle (the area captured on a smaller format sensor) from nearly all parts (zones) of the glass, especially the front elements, and especially at longer than "normal" focal lengths. So using a smaller sensor is not like cutting cylinders through the lens elements, throwing away the outer perimeter (annulus) of all the glass, and just using "the center of the lens". In fact, the full diameter of the entrance pupil of the lens is always employed, regardless of how tight a sensor crop you take, because the entrance pupil diameter D defines the focal ratio f in conjunction with the focal length F (f = F/D). Since cropping doesn't change focal length, and doesn't change the image brightness, it cannot change the entrance pupil size either.
Or look at it this way. When you stop down the aperture of a lens, you are indeed using only the central zones of the lens. This is the reason why stopping down reduces zone-dependent abberations like spherical and longitudinal chromatic. But does stopping down come with a shrinking of the image circle - do the corners go black? No; on the contrary, corner falloff tends to improve with stopping down. This proves 2 things:
(1) The "center of the lens" doesn't just send light to the center of the image (aka the cropped sensor area); it sends light to the full image circle.
(2) The outer zones of the lens send more light to the center of the image than to the edges. Stopping down is reducing the amount of light; you cannot make something brighter by taking light away; so if the corners now seem brighter with respect to the center, it's because it is the centre which has been worst hit by the loss of the light from the outer zones. A cropped sensor will therefore "see" those outer zones at wide apertures, just like the non-cropped sensor does.
These outer zones aren't there to illuminate the outer zones of the image; they are there to improve the wide-open f-ratio mainly at the center of the image circle.
Unless the original design was already capable (mechanically & optically) of going to f1.2 but was physically limited to f3.5 due to vignetting/ CA/ soft edges...which won't be so much of a problem with the 35mm chips...
Why on earth would someone make an f1.2/f1.4 medium format lens with huge expensive elements, which (aberrations or not) would be in a sensational class of its own, and then cripple it by 2.5 stops, actually making it half a stop slower
than its competitors (cf. 55mm f2.8 lenses for Mamiya and Pentax)? Sorry, that doesn't get past Occam's Razor.
The 45mm, for example, was an f2.8 lens and it used the same basic barrel as the 55mm/f3.5...
Divide 55 by 3.5, and 45 by 2.8. You get essentially the same answer: the entrance pupils are 15.7 mm and 16.1 mm respectively.