One of the things I’ve noticed with my Lumix G1/Leica M-mount lens combination is that the lens can “overfocus” at infinity. What that means is that I can’t simply rotate the lens focus ring on my Voigtlander 40mm Nokton to infinity and expect sharp images. What happens is that the lens over focuses or, in other words, focuses beyond infinity. This isn’t a problem for me as I can simply back off focus by about 1.5mm. That equates to setting focus to the leftmost loop of the infinity symbol on the lens.
For others, some have encountered the opposite where they can’t achieve focus with manual lenses. Obaksan discusses this phenomenon in his blog post in my view …: G1 adapters and infinity focus (or much ado about not much). As he points out, there are probably two main factors to this focus issue, lens tolerance and camera tolerance. I’d actually like to add a third which is mount tolerance which can include camera, lens, and in the case of the G1, adapters. Since it doesn’t take much to throw focus off on precision optics, a combination of small variations through the mount chain can add up to enough variance to over- or under-focus at infinity.
From my experience with the G1 to date, I agree with Obaksan and believe the errors start with the camera itself. As he said, today’s electronic cameras with auto focus have “no need for any mechanical optical precision as in the days of Leica coupled range finder mechanisms.” The technology takes care of any image plane distance issues by simply adjusting focus for us. We don’t care if the lens shifts a bit, as long as the camera selects proper focus that’s fine.
For manual lenses where humans need to be making focus decisions, slight manufacturing differences in the image plane, mount position and thickness, etc. does matter especially if one scale focuses. Admittedly, over-focus is far better than under-focus because one can simply not focus entirely to infinity to achieve focus. In an under-focus situation, focus can’t be achieve without, as he said, stopping down the lens to say, f8.
