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I have a semi-faulty AF Nikkor 50mm ƒ/1.8 lens (early non -S or -D model, with the focus distances in a little window) that has been having issues communicating with the camera. Both my F-801 and my D7500 will show "F--" for aperture data from time to time.

This lens had an unfortunate fall when I left my F-801 on a table inside a moving train while I left briefly. When I returned the camera had fallen. At the time everything kept working fine, and for another week there were no issues. However it now started having issues communicating the data back to the camera, with my D7500 thinking it is a non-CPU lens.

I've verified the issue is not the contacts, they're clean. I checked a disassembly video and realized the part I needed to check was easy to access. I then decided to inspect the interior of the mount ring. Nothing seemed off. My suspicion something had become dislodged was disproved as there was nothing to be misplaced.

However, I couldn't, for the life of me, figure out how the aperture is even being measured, and this is the focus of my question. There are no variable/sliding potentiometers or other mechanisms that engage with the aperture ring at all. There is a rocking cam that engages with the aperture lever in the camera, but that seems to have no contact with the electronics. As far as I can tell, the whole electronics section is completely self-contained. You can check for yourself in this video at 6:10.

Can anyone more familiar with these lenses explain how this works so I can hopefully diagnose this issue better? Thank you!

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Nikon cameras measure/know the aperture setting (for non-E lenses) by the aperture lever interaction/position.

The only thing the CPU does is tell the camera the lens model, focal length, min/max apertures; plus focus distance for D type and later lenses. This is the same information you can enter manually for a non-CPU lens; which then allows the camera to control the aperture correctly (metering/etc, not distance). You can actually add a CPU to a non-CPU lens pretty easily.

I have not seen a camera report no lens attached (F--) w/ a CPU lens when the lever/spring mechanism has an issue; only if there is an electronic issue with the contacts/CPU/camera. My best guess is the CPU/contacts are out of alignment even if they appear fine. The article I linked above gives some precise measurements as to the CPU contact placement.

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  • \$\begingroup\$ Thank you! Further investigation together with your explanation leads me to think the fall had nothing to do with the failure, and my best guess is that something on that circuit was bound to go bad sooner or later. I checked all contacts seem to be matching fine with the camera, and it works just fine with other lenses. I'll attempt to find a for-parts lens and replace the circuit ring, as the optics are fine. \$\endgroup\$
    – Bruno Philipe
    Jan 22, 2022 at 23:26
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    \$\begingroup\$ A small update: After disassembling the circuit ring a bit further and probing around with a multimeter, I was able to detect a cold solder joint in a transistor. After applying a bit of solder flux and reheating it with a soldering iron, the circuit was back to life! \$\endgroup\$
    – Bruno Philipe
    Jan 29, 2022 at 16:33

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