Headshots at Roche Diagnostics in Rotkreuz

I have never taken headshots before so here is my first attempt.

Here some photography geek talk: I borrowed a medium format camera (Contax 645) with a digital back so decided to take it for a spin. I photographed two friends from Roche Diagnostics. My battery died during the first day so some are taken with Nikon D800 and some with Contax 645. Can you tell the difference?