But why not use the regular mount so it can project onto the 35mm sensor as in the 5D? It seems that canon is saving itself a few bucks and not passing to the consumer - can't think of any other reason...
I am sure I will be corrected if I am wrong, but the way the lens mounts to the camera does not change between your 35 mm Canon film lenses and this lens. The diffrence is that the lens is designed for the X 1.6 sensor size of many of the Canon digital cameras. I would expect that the lens would mount to your Canon film camera but it produces a diffrent size image circle which will not cover the 35 mm film or a 35 mm full frame sensor.
Making the lens/sensor full frame for the X 1.6 sensor size it was designed for.
Regarding cost, the angle of view releates most closely to a 100 mm in in the 35 mm format. Because of this I think, right or wrong that equipment makers fell these lenses should be priced closer to the 100 mm macro cost than to the 50 mm 1:2 macro cost. Any thoughts?