Originally posted by bull drinkwater 645 film lens = 1.26 crop factor on the D or Z
6x7 film lens = 1.62 crop factor on the D or Z
I'm not sure this makes any sense to me.
With crop factor, the lens format is not relevant, nor is the size of the image circle. The only relevance is the focal length and the reference image sizes. Obviously the minimum image circle must still be there.
So a 100mm 6x7 lens and a 100mm 645 lens (both film era) and a 100 mm DFA 645 lens all give the same field of view when mounted on the 645D or 645Z. And furthermore those same lenses on an APSC sensor work like a 100mm APSC lens. The field of view however will be narrower than the original format if the new sensor is smaller. So someone used to 135 film might say any 100mm on APSC is "longer" 1.5x than on 135, but the reality is even an 8x10 lens that is 100mm will give the same rough picture in terms of angle of view on that same APSC sensor as an apsc only 100mm would.
The lens itself may be capable of a larger image cirvle in the case of the 6x7 lens but the sensor size limits it. The crop factor is only between formats of sensors not lens formats.