Recently I was asked for help identifying a mysterious spring mounted tab on a Nikon lens mount that was protruding on one body, present but not protruding on a second body and missing entirely on third body. So what does this mystery tab do exactly?
The small spring-loaded tab is a mechanical AF drive coupling for old AF Nikkor and non Nikon mount lenses that do not have a built-in AF motor. On the left side we can see the AF drive coupling protruding with the body AF/MF switch in AF mode and on the right side we can see the coupling retracted with the AF/MF switch in MF mode.
Unlike modern lenses with built-in AF motors the focus ring on older AF lenses should not be turned manually without first disengaging the AF drive coupling via the AF/MF switch on the body. With the AF drive coupling retracted you can safely manually focus an early generation AF lens without any worries about damaging the lens or AF linkage.
If the AF drive linkage is missing from the mount, not just retracted in the mount, the body will not able to drive the autofocus on older F-Mount AF lenses Nikon or non Nikon (you can of course use the lens in manual focus). This means here are lots of early generation autofocus F-mount lenses, Nikon or independent manufacturer lenses on the used market that will work great on a current Nikon body.
This is the female or lens side AF drive coupling on an old AF Nikkor 50mm f/1.8D lens. In addition to old Nikkor AF lenses I also own a few older Sigma lenses without built-in AF motors that have the same type of female mechanical AF coupling. These older AF and AF-D marked lenses cannot be used on some of the consumer type Nikon bodies like the D60, D3000,D3100, D5000, D5100 since they do not have this AF coupling for the older type AF lenses. Newer AF-I and AF-S Nikon mount lenses all have built-in AF motors and do not need a mechanical AF drive coupling.
The backward compatibility of the Nikon F-mount on most Nikon DSLR bodies means you can pick up older AF or MF lenses for a steal and still enjoy auto-aperture, AF and great image quality even on a modern body like the D800.
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