This was shot with the G 21mm on a G2, wide open at f2.8. The film was Ektar 100. Some of the darkening at the periphery is from the extreme lighting conditions. I don't have posted here (at least not anymore) another shot I took with the 21, handheld, inside the plaster model workshop in the basement of La Sagrada Familia cathedral in Barcelona. That was 1 second at f2.8, and you can barely notice any vignetting. The 16mm is much more pronounced in the falloff to the edges.
Another reason for using the G2 as opposed to a Bessa - the G2 has a top shutter speed of 1/6000, so you can pull off shots like the one I posted. Not gonna happen with a camera whose top speed is 1/1000th or 1/2000th at best, especially if you're using films faster than ISO 100.