Effective AI
modification
requires
completely
removing the
aperture ring.
modification
requires
completely
removing the
aperture ring.
Precision
AI Modification
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 . After
refining my techniques over a number of years and learning lots more, I developed
an efficient and precise system for doing these modifications. A number of people asked
me to share that system, and so this page was first created in 2000. Since then I have
successfully modified hundreds of lenses for people in the U.S. and abroad. I've even
done lenses for customers in Japan, as ironic as that seems, it being the home of Nikon.
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 . After
refining my techniques over a number of years and learning lots more, I developed
an efficient and precise system for doing these modifications. A number of people asked
me to share that system, and so this page was first created in 2000. Since then I have
successfully modified hundreds of lenses for people in the U.S. and abroad. I've even
done lenses for customers in Japan, as ironic as that seems, it being the home of Nikon.
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
the back of the lens. The location of the ridge conveys aperture information to the camera.
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
An old Nikkor 50/1.4 before and after
A I modification.
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.
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 originally designed
for use by a mechanical auto exposure system Nikon introduced. In a
number of more contemporary cameras that pin is used to indicate
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.
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 the precision crossfeed of a vertical milling machine 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.
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.
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 originally designed
for use by a mechanical auto exposure system Nikon introduced. In a
number of more contemporary cameras that pin is used to indicate
Meter-coupling prongs (from left): Old triangle, "crab
claw," AI-modified "crab claw," Nikon AI "hognose."
minimum aperture in connection with automated exposure
schemes.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.
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
precisely mark the positions for the AI ridge and the EE coupling node. Nikon describes
the position of the indexing pin as 37 degrees 30 minutes on f/1.4 and
f/1.2, but 35 degrees 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 the precision crossfeed of a vertical milling machine 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.
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.
An end mill is used to precisely machine the AI ridge
andEE coupling tab.
andEE coupling tab.
New holes are drilledand tapped for theAI-style coupling
prong.
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 or minimum
aperture indicator 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.
Another service I do not provide is "chipping." This is a procedure in which a computer chip is implanted into an older lens to allow it to communicate electronically with newer bodies. The way this is usually done is a replacement computer chip is obtained, either as a repair part or from a junker lens. The chip has to be from a lens with a similar f-stop range as the original lens because its job is to trick the camera into thinking it has a current lens on board without actually providing the full electronic functionality of a modern autofocus lens. I've never been comfortable with this modification, and I am apparently not alone, as I don't know of any reliable professional repairman who offers this service.
Another service I do not provide is "chipping." This is a procedure in which a computer chip is implanted into an older lens to allow it to communicate electronically with newer bodies. The way this is usually done is a replacement computer chip is obtained, either as a repair part or from a junker lens. The chip has to be from a lens with a similar f-stop range as the original lens because its job is to trick the camera into thinking it has a current lens on board without actually providing the full electronic functionality of a modern autofocus lens. I've never been comfortable with this modification, and I am apparently not alone, as I don't know of any reliable professional repairman who offers this service.
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.
© 2000-2008 by William Sampson,
all rights reserved.