Get Nikon Z at B&H Photo.
Observe that the Sony FE-mount version has an extra “tube” at the end; this makes up for the mirror box distance. Since the Sony A7R III has an 18.0mm flange focal distance and the Nikon D850 has a 46.0mm flange focal distance, that tube is 28mm long, or about 1.1 inches.
That means all that weight is projects 1.1 inches away from the Sony lens mount. In my view (one based on personal experience over years including repairing a Nikon lens mount at great expense), there is a significantly increased risk to the integrity of the lens mount—one good bump and a large lever arm torque could easily tweak the mount 10/20/30 microns (“huge”, but invisible).
A bent lens mount can be bent, say, 20 microns and this is invisible to the eye—but very visible in pictures. How big is the risk? A 10 micron warpage would wreck my work at wider apertures (one side blurry the other sharp e.g., a skew across the frame). I cannot take this chance. So while I might shoot the Sigma FE 85mm f/1.4 DG HSM Art on the Sony A7R III, it’s going to be under static conditions where I don’t risk the lens mount by bumping into something or the lens swinging around.
This lens mount warpage risk is true of all cameras and large/heavy lenses (e.g., any 70-200mm f/2.8 zoom on a DSLR) To quantify the concern as way of example, Leica specifically does not recommend any lens heavier than 700 grams for use with the Leica R-Adapter-M or 700-1200 grams if used via the tripod mount. The effective way is multiplied many times over if a lens is bumped, e.g., lever arm exerting torque on the mount. Almost certainly the Nikon Z mount is much stronger, and probably Sony too—but that changes only the weight/torque threshold of risk.
Sigma FE 85mm f/1.4 DG HSM Art for Sony
I have the Sigma FE 85mm f/1.4 DG HSM Art for Sony mirrorless here in my hands and all that weight feels out in front. Not only is it unbalanced and awkward, there is just no way I am about to carry that sort of thing with me while hiking.
While the Sigma DG HSM Art lenses should be outstanding on the Sony A7R III, I just do not see the larger and heavier ones being practical choices for my style of shooting. In a studio or whatever—no big deal where one hand is supporting the lens always, but out in the field—now way. I experienced this directly with the Sigma 105mm f/1.4 DG HSM Art on the Nikon D850: too large, too heavy to just let hang there—at all times I had to support the rig by holding the lens with my hand = PITA for hiking and it meant stowing it for class 2 or class 3 climbs.
Implications for Nikon Z7 and Nikon FTZ lens adapter
This all relates to the Nikon Z7 and the Nikon FTZ lens adapter as follows. The Nikon Z7 has an even shorter flange focal distance of 16.0mm, which means that the DSLR-mirrorless offset is 30mm = a ~1.2 inches. In other words, the Nikon FTZ lens adapter will add 1.2 inches of distance for an unbalanced feel with a substantially higher lever arm torque that makes me mighty nervous about the lens mount. Plus the extra bulk.
Unlike the barrel-extension of the Sigma DG HSM Art lenses for Sony mirrorless, it might be that the Nikon FTZ lens adapter itself provides some benefit in that could offer a slight amount of “give” such that the torque is partly concentrated on the leading lens mount flange, and thus diminished by the time it reaches the camera lens mount. But if the entire structure is overly rigid, that might not be so.
Below, the Sigma 85mm f/1.4 DG HSM Art for Sony E-mount has an about 1.1-inch rear end extension as compared to the Sigma 85mm f/1.4 DG HSM Art for Nikon/Canon. This creates a substantially higher lever arm torque which could damage the lens mount; any minor impact is more amplified by the lever principle.
Miguel B writes:
Regarding the FTZ adapter, I carry my camera with one of those straps that hooks to the tripod mount. I will screw the strap to the FTZ adapter which I expect would actually relieve the stress on the body mount.
DIGLLOYD: yes I think this may be of some benefit. But it does not address my primary concern: even a minor ot modest bump of the lens shade has long lever arm that travels back to the camera.