I use rangefinder lenses (M and LTM mount) on full-frame digital cameras all the time; they're among my all-time favourite lenses. There won't be any problem with coverage on a full-frame sensor but you may notice some corner smearing and other issues with the wider lenses, depending on the thickness of the BMCC 6K's sensor stack. Most lenses 50mm and higher will be totally fine, it's mainly 35mm and wider that can be problematic. In most cases even 35mm is fine but sometimes you'll see field curvature artifacts. There are other degradations in performance when rangefinder lenses are used on digital cameras with thicker sensor stacks, but those are mainly visible to pixel-peepers. You probably won't notice them unless you fall into that category.
On digital, these lenses work best on cameras with a thin sensor stack, like Leica M-mount cameras. The Nikon Zf full-frame mirrorless camera does pretty well with wide-angle rangefinder lenses, but Sony cameras generally have problems with the wider ones unless you modify their sensor (Kolari does an ultrathin sensor mod for about $300 that greatly improves the performance of these lenses on Sony cameras). On a camera with a thick sensor stack you may see corner smearing and noticeable field curvature issues on wider lenses (e.g., the outer edges of the field will be in focus when they should be out of focus). The Sigma fp also shows these issues with wider rangefinder lenses.
For an extreme example, here are a couple of shots taken with the 28mm MS Optics Apoqualia on Sony A7iii full-frame camera. Note the blurry/smeared corners; the vignetting is normal and even occurs when I shoot this lens wide open on a 35mm film camera, but the corner smearing only occurs on Sony and other cameras with thick sensor stacks.
Just before sunset by
Brad Hurley, on Flickr
Riviére des Prairies, Montréal by
Brad Hurley, on Flickr
In this photo, taken with a Voigtländer Ultron 35mm/f2 rangefinder lens on Sony A7iii, you can see the effects of field curvature where the people at the edges of the frame are in focus when they should be out of focus. This doesn't bother me, but it bothers some people as it looks unnatural. The effect would be less pronounced on a Leica M camera or a Sony with a modified sensor stack.
Rain dancers, Royal St., New Orleans by
Brad Hurley, on Flickr
So performance of wider rangefinder lenses all boils down to the thickness of the sensor stack on the BMCC 6k. You won't have any problems with coverage, although note that most wide angle lenses will vignette at their widest apertures (they even vignette on Leica cameras, that's down to the lens, not the camera). But you may have some issues with corner smearing and other image degradation with wider lenses such as 28mm, 21mm, and wider depending on the thickness of the sensor stack on this camera.
Just be sure you have some neutral density filters that fit those lenses; some M-mount rangefinder lenses have unusual filter thread sizes. I've had good luck getting hard-to-find filters from filterfind.net. You will need ND filters much of the time unless you're always shooting in lowlight conditions.
Posted: Wed Oct 23, 2024 12:23 am