New Lens For My Nikon F; Adding Meter Prongs To an AI Lens

I got a few of those plastic "Cosina" 35-70mm f3.5-4.8 zoom lenses 'free' with some cameras. Actually both came on FM-10 bodies.

Anyway, they are not bad lenses. Clear optics sometimes are more important than lens design. Some of my old pre-AI lenses are not all that clear in the optic department (after cleaning).

So I thougt it would be nice to have one of these lense on my Nikon F. Of course the lens works fine in stop down mode. However, I find I can't turn the shutter speed knob while my right hand has the aperture preview button down. Reaching over with my left hand covers the illumination of the shutter speed readout. That problem gave me the idea of adding the aperture coupling prongs to one of these inexpensive "Cosina" lenses.

I could find very little information about adding meter prongs to Nikon AI lenses which don't already have the prongs. For example what is the pitch and diameter of the screw thread? I guess I have to figure it all out.


As many of you know there are already dimples to mark where the screws holes need to be on many lenses.

The meter prongs (so called 'Rabbit Ears') can be obtained on e-bay, but I have a foggy zoom (after cleaning) that will be the donor.

The original screw size is larger than M1.2 but smaller than M1.5. So, maybe 'good luck' trying to find an appropriate tap.

Of course the 'Cosina' lens is plastic. I'll make the threads with the screws.

The 'Cosina' lens comes apart just like most Nikkor lenses. After removing the lens mount (the three screws are in plastic so be careful putting it back), the aperture ring comes off.

Don't lose the ball bearing for the click-stop!

Since I'll be screwing into plastic, I decided to use spare screws from my stash of little screws. That way I can put the original screws back in the donor lens (which has a metal aperture ring) for safe keeping.

Drill the holes, using the dimples as a guide. I used a Dremel, but it might be safer to use the drill bit on a pin-vice and do it by hand.

Carefully create the threads in the plastic with the new screws.

Re-assemble the recipient lens.

Coupling correctly with the FTn finder of my Nikon F for open-aperture metering!

The prongs do make the "Direct Reading Aperture" scale of the AI cameras harder to read, but as mentioned earlier, I have more that one of these plastic zooms, so no big deal there. Anyway, the FM-10 (from which the lens came) doesn't show the aperture in the finder.

Here it is, correctly coupled, showing 3.5 on the maximum aperture index scale.

BTW: When doing stop-down metering, make sure the index is on the 5.6 by pushing the tab up into the housing until it clicks in place at 5.6. Everyone knew this back in 1971, but maybe some folks here weren't born yet

The variable aperture of this lens is not a problem for the FTn finder, but when doing flash photography or any other non-TTL metering, the change in aperture needs to be recognized.

Great work! Congratulations!

Elegant work!

Long ago, in my youthful experience with Nikons, I got a Vivitar 28/2.5 that had lost its coupling ear. On some of these 3rd party lenses, they used one screw to hold the prong on, which predictably was subject to rotating forces that loosened the screw and the prong was often lost. I didn't have a lot of tool resources, so I shaped a prong from a laundry detergent cap and stuck it on with foam tape. It worked, but would have been knocked off by extensive use.

I looked on thingiverse and someone has designed a 3D-printable non-AI prong / rabbit-ear: https://www.thingiverse.com/thing:6209306
Such plastic may break with hard use, but for gentle hands it could be useful.

Interesting timing on this. Yesterday I was looking into what would be needed to put a coupling prong on an AF-D lens. The screws on my prospective donor lens looked to be 1.4mm and Micro-tools sells a 1.4mm tap, https://www.micro-tools.com/products/t-1-4

I suspect that is the one!

Now let us see a photograph or two that was taken with this converted lens.