There are geeks and then there are photo geeks. In the old days, geeks worked in carnivals and were oddly entertaining folks who swallowed swords, hammered spikes into their nostrils, and decapitated chickens and snakes without benefit of a cleaver. The photo geek, by contrast, is not nearly so riveting. In fact, photo geeks are downright dull. They photograph things like test charts and brick walls, and talk about spherical aberrations and aperture diffraction rather than composition, light, and shadow. In general, I tend to avoid partaking in the nerdier aspects of photo geekiness. But ever since I began sharing M-mount lenses between the Leica M8 and the Panasonic DMC-G1, I've been consciously aware that they perform quite differently on the two cameras. Because of the way I use these lenses, I'm not actually bothered by this — but readers of this blog feel otherwise, and they asked me to write specifically about the differences between these cameras when using M-mount lenses. This article discusses one such lens — a 1991 Leica v5 50mm f/2 Summicron — and the performance differences one sees when mounting it on a Panasonic DMC-G1 vs. a Leica M8.