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Precision AI Modification
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By William Sampson
I started modifying non-AI Nikon lenses soon after getting my wife a Nikon EM. I had a number of great older Nikkor lenses, that I used daily on Nikon F and Nikkormat cameras as a newspaper reporter and photographer. But they wouldn’t work on my wife’s camera nor on the FM and F3 bodies that were becoming increasingly common in the newsroom then. After studying AI lenses and taking advantage of some metalworking and machining experience, I made my first attempt on an old Nikkor 105/2.5. The modification was so successful, that soon I was modifying both Nikkors and some third-party lenses. Now, after refining my techniques over a number of years and learning lots more, I have developed what I think is an efficient and precise system for doing these modifications. A number of people have asked me to share that system, hence this page.
What is AI modification?
Older Nikon lenses used an external prong to couple the lens to the camera’s exposure meter. The prong worked well enough but it required a second maneuver when changing lenses to let the camera meter know what the maximum aperture of the lens was. On later model F series cameras you simply rotated the aperture dial back and forth; earlier cameras required adjusting the meter’s ASA dial.
Those secondary actions were made unnecessary when Nikon introduced cameras with AI (aperture indexing) features. Instead of coupling to the prong on the outside of the lens, AI cameras mate to a raised ridge on the back of the lens. The location of the ridge conveys aperture information to the camera.
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An old Nikkor 50/1.4 before and after AI modification.
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AI lenses also feature Aperture Direct Readout (ADR) numbers, a second row of f-stop numbers that can be seen in the viewfinder by many AI-compatible cameras. To illuminate those numbers a new “hog nose” shaped prong replaced the “crab craw” prong that had been ubiquitous to Nikon lenses. The holes in the hog-nose prong let light shine on the ADR numbers so they could be better seen in the viewfinder. AI lenses also have an EE coupling pin on the aperture ring for use by a mechanical auto exposure system Nikon introduced.
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Meter-coupling prongs (from left): Old triangle, "crab claw," AI-modified "crab claw," Nikon AI "hog nose."
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When the AI system was first introduced in 1977, Nikon offered a service of “converting” older lenses to AI. The Nikon conversion consisted of replacing the old aperture ring and prong with an AI-style ring and prong. In some cases the Nikon conversion sacrificed one f-stop, because the conversion rings would not go to as small of an f-stop, for example, f/22 instead of the f/32 on the older lens. Any older lens with a smaller f/stop than the new ring, would lose that feature in the conversion. Although Nikon no longer officially offers this service, some of the conversion kits are still available in the hands of repair shops. But for the bulk of non-AI lenses in circulation, the most common option is to mechanically modify the aperture ring so that it is compatible with AI cameras. That’s what I do.
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The modification process
The first step to modifying a lens is to remove the aperture ring. This is not exactly the same for all Nikkor lenses, so it requires some experience or the aid of repair manuals. Some people have tried to modify lenses without removing the ring, but this is highly likely to damage the lens by resulting in metal filings getting inside.
After removing the ring, I use a gauge I’ve developed to mark the positions for the AI ridge and the EE coupling node. Nikon describes the position of the indexing pin as 37°30’ on f/1.4 and f/1.2, but 35° on slower lenses. This is equivalent to positioning the ridge’s leading edge 4 2/3 stops from the widest aperture on most lenses and adding an extra 1/3-stop compensation for the faster lenses.
Next, I mount the aperture ring in a special holding fixture I machined and prepare the ring for milling. Some people simply file down the aperture ring to create the AI ridge, but I find that using an end mill and precision crossfeed ensures a more precise result.
Once the aperture ring is milled, I turn my attention to the old meter-coupling prong. The best alternative is to replace the old prong with a genuine AI prong, but often one is not available. In that case, I use a very small milling cutter to machine the old prong to look like an AI prong. This step is far from cosmetic, because the holes and extended center slot allow light to illuminate the ADR numbers. You can see from the picture above the two unmodified prongs on the left, my modified AI prong, and on the far right a true AI prong.
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Effective AI modification requires completely removing the aperture ring.
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It is not uncommon to see some AI modifications where the old prong has been left in its original location and ground down along with the edge of the aperture ring. Sometimes the grinding goes right into the screws! It’s much better to replace the prong with an AI prong or one modified to look like an AI prong. However, this also requires flipping the prong around and drilling and tapping new holes so the prong’s foot faces toward the front of the lens instead of toward the body of the camera.
Once all the metalwork is done to the aperture ring, it’s time to clean up any sharp edges left by the milling and apply some black touchup paint to the bare metal. When it is dry, the aperture ring is ready to reassemble on the lens. The last step in the process is to add an ADR strip in the form of a label that has numbers that closely match the type style and spacing of original Nikon ADR numbers.
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An end mill is used to precisely machine the AI ridge and EE coupling tab.
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New holes are drilled and tapped for the AI-style coupling prong.
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Touch-up paint is applied to all the bare machined metal surfaces
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After reassembly, an ADR label is applied to allow viewing the f/stop numbers in the viewfinder of cameras so equipped. Compare the finished modified lens to the lens before modification.
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What I don’t do
My AI modification procedure has the same result as Nikon’s own AI conversions: You get a lens that correctly meters with both pre-AI and AI-compatible camera bodies, couples with EE servo applications, and has ADR numbers so you can see the aperture number in the viewfinders of cameras that offer this feature. In a slight improvement over the official Nikon procedure, my modification maintains the full use of all original f-stops, no matter the range.
However, neither my modification nor the official Nikon conversions give the full functionality of real AI lenses. There is a small protrusion on the back of true AI lenses that transmits the lens speed to a select few cameras that have a lever inside to read this. It is used primarily in AMP and Matrix metering with FA and F4 bodies. I have seen some repairmen add a screw to serve as a lens speed indicator post, but at present, I am not doing that modification.
Variations
The height of the aperture ring from the lens mount is not consistent in all lenses. In most cases, there is enough material to do an AI modification simply by milling as described above. However, sometimes there is not enough material to properly mate with the camera’s AI indexing tab. In such cases, material must be added to the aperture ring. I use screws and epoxy to add an AI ridge in these cases. In some third-party lenses without a raised ridge, I have fabricated a metal tab that pairs with the old meter-coupling prong. Click here to see a gallery of AI modifications or some of my more unusual modifications.
More information?
I work on cameras and lenses primarily as a hobby. When people ask me to do AI modifications for them, I do them as my time permits and generally charge $35 plus any shipping. I also offer limited other services in addition to AI modifications. If you have any questions about this site or the services I offer, please e-mail me, and I would be happy to respond.
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©2000-2002 by William Sampson, all rights reserved.
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