Chris R writes:
Here’s another observation, in the not so distant future, regardless of which system you use, there will be an adapter for just about every system so that any third party lens, or indeed, camera system will be universally adaptable to any lens so it won’t be as much of an issue which system you settle for.
DIGLLOYD: sort of, but not so much at a practical matter, for many reasons.
I do go to the trouble of adapting lenses in special cases, such as shooting Zeiss DSLR lenses in Multi-Shot High-Res mode, or F-mount lenses on the Nikon Z7 or Canon EF lenses on the Canon EOS R, but it’s far from ideal from a handling perspective (and no EXIF either).
Flange focal distance
First, the flange focal distance governs whether a lens adapter can be inserted between a lens and the camera. For example, the 16m flange focal distance of the Nikon Z7 lets (in priniciple) all other mirrorless and DSLR and rangefinder lenses mount via a lens adapter.
That’s because the flange focal distance of other camera present camera systems range are 18mm or greater, thus allowing at least a 2mm gap for an adapter to be inserted between lens and camera. While 2mm is iffy for support/stability reasons, it already exists for Nikon DSLR lenses (46.5mm FFO) to Canon DSLR (44.0mm FFO). Thi
The foregoing is why just about any lens can be mounted on the Nikon Z7, but Nikon NIKKOR Z lenses cannot be adapted to any other system, at least not without inserting additional optics (yuck) or dubious into-camera-throat designs.
Most lenses these days lack an aperture ring, so a lens adapter has to, at the least, provide electronic translation from the camera to the lens for aperture control. And when it comes to autofocus support, good luck with many adapters—poor AF performance.
Camera brand X does not support lens correction of random Camera brand y lenses—so distortion correction and chromatic correction and vignetting correction are all off while shooting. This is sometimes OK, but sometimes a serious problem in that framing becomes difficult for a lens with significant distortion. Worse, most raw convertes including ACR do not provide any selectable lens profile support for Lens Y on Camera X, recognizing the len properly only when shot natively.
Many lenses are too heavy and too awkward to be practical and increase the risk of damage to both lens and camera flange (bumps, sheer weight). There are also two additional mounting surfaces which have significant risk of having planarity deviations versus a single mounting interface of a native lens.
Optics and sensor cover glass
The variations in sensor cover glass* thickness can be small to large, but high performance lenses can be very sensitive off-axis to differences. Thus performance of a lens designed for 0.8mm thick Leica M sensors is most often degraded massively on mirrorless—no exceptions so far—see MTF on Mirrorless Cameras of the Zeiss ZM 35mm f/1.4 Distagon for the huge losses (although it can offer stunning performance by f/8).
* Total effective filter stack thickness includes the sensor cover glass and its index of refraction, plus additional layers such as an IR-blocking layer.