Paul,
Apology for some time delay to respond to your question.
I finally kicked off my new feature film shoot last weekend and put XP Image speedbooster reducer / Sekor C 45mm 2.8 645 lens to test.
Camera was Alexa Classic paired with Odyssey 7Q+ recorder was in the great hands of David Le May ACS...
What I am showing you is completely ungraded footage, straight out of Alexa (ARRIRAW), debayer-ed as
Log C4 and only applied colour space transform(CST) in Resolve 20 studio.
Australian Neo Noir feature film - "Potato"
only CST applied
No grade
No reframe
Voice isolation applied (ai toggle isolation in Davinci) not final audio process.
I hope this footage will answer some of your questions about 645 Mamiya Sekor C lenses.
Needless to say, I am impressed and I see how 645 Sekor C line lens may be a great fit for CINE Ursa cameras...
Cheers,
Darko