A few days ago, I wrote this post on lens adapter tolerance, saying that my two Novoflex and one Metabones adapters were all too short by way more than you’d think manufacturing tolerance would suggest.
A reader wrote to me of a communication with Novoflex in which they said that, in their case, the adapters were too short on purpose, so that lenses with out-of-tolerance focusing settings could be sure of focusing to infinity when mounted using a Novoflex adapter.
I can see why they would do that; they certainly don’t want a lot of people calling Customer Service to complain that their lenses won’t focus to infinity.
However, there are some problems that too short adapters cause in addition to relegating the distance scale on the lens to ornamental use. Some lenses, particularly Leicas, use floating elements to compensate for errors that can occur as the lens in racked out to focus close. Those corrections only work the way the designer intended if the lens flange is the correct distance to the sensor.
Zoom lenses will not in general remain parfocal if their rear flange is not the correct distance from the sensor. That means that something focused on at one focal length won’t necessarily stay in focus as the lens is zoomed. I never trusted in parfocality anyway, so it’s no loss to me, but if you have counted on it, you won’t be able to once your lens is mounted on a too-short adapter.
One place where you really need parfocality is making movies.
I bought 3 adaptor for my Contax/Yashica lenses to Sony Nex.
All 3 are about .003″ too short. Made huge impact on the 28mm; not so much on the telephoto. Spent a week hand-machining alum shims, each one specificly matched to each lens. Now the pictures are sharp as they should be, and infinity settings are exactly right.