This is a continuation of a series of posts on the Nikon D850. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “D850”.
Yesterday I tested two copies of the Sigma 50 mm f/1.4 Art lens on the D850 and found that one, serial number ending in 123, took an AF Adjustment of 0 and the other, serial number ending in 370, needed +10. The D850 can remember the AF Adjustment values for lenses. But, at least in the case of this Sigma, it can’t tell two copies of the same model number apart. I set the +10 value into the camera with 370 mounted, then took it off and remounted 123, and the AF adjustment value for it, which had been 0, became +10.
There aren’t many amateur photographers who own two or more copies of the same lens so this little twist won’t affect them. However, some pros work from stockrooms supplied with lots of copies of a given model number, and this will affect them.
It’s worse than just the above. Here’s the skinny from nuke12 on DPR:
Every electronic F-mount lenses with EXIF has a two byte lens ID number that the camera uses to identify the lens for focus tuning. If two lenses use the same number then the camera “sees” them as the same lens. Two Nikon lenses of the same model will be IDed by the camera as the same lens. After market lenses sometimes use the same number as a Nikon lens and that can cause problems.
You can find many of the ID numbers here;
http://www.rottmerhusen.com/objektives/lensid/thirdparty.html
Thom Hogan agrees, and points out a complication:
Nikon does AF Fine Tune by lens model number. This makes for more of an issue than you might think, as the third party lens makers make up lens numbers, and Nikon sometimes then later uses them.
I looked at the EXIF data from images shot with 123 and 370. In both cases, the “Lens ID Number” field is populated with “136”.
It should be possible to make the lenses take the same adjustment by using the Sigma USB dock. According to the docs, it even supports different settings depending on distance (but not on aperture, apparently). This is probably not a solution for the “stockroom”-problem, though.
What nuke12 says is true for a-mount lenses also where some aftermarket lens models might have the same ID. If I remember correctly, Tamron licensed ID numbers but Sigma didn’t. …don’t know about others.
Both Sony a-mount and e-mount lenses typically have a 5 digit numeric ID. That’s not an issue with micro-focus adjust on e-mount but since the ID can potentially be duplicated, the lens identification from EXIF might be incorrect. Especially now that third party lenses for e-mount are becoming prevalent, I’d like someone to clarify this.