Nikon AI Conversion

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Note: The type of conversion described on this page only applies to cameras which can meter with AI-lenses, such as the D300, D7000, or D600. If you want to use pre-AI lenses on AF cameras without AI-metering, such as the D70 or D80, things are simpler. With the D40, D40x, and D60, no conversion is needed to mount these old lenses.

Important: this type of AI conversion by just filing down the aperture ring does not guarantee that the lens will fit an AF-body, including the digital bodies. With some pre-AI lenses, the rear parts of the lens will collide with the AF-contacts. Unfortunately, I don't have a list of lenses where this happens.

Just having had a bad experience with postal delivery of a lens (arrived damaged), I decided on a 'do it yourself' AI conversion instead of sending the lens to a service shop.

As my first 'victim' I selected a Nikkor-K 1.8/85.

remove screws First you have to detach the aperture ring. On the 1.8/85 this is simple: just unscrew all 5 screws ...
Nikkor 1.8/85 without mount ... then you can remove the mount ...
Nikkor 1.8/85 without aperture ring ... and the aperture ring comes off.
mark position of AI coupling ridge To determine the correct position of the AI coupling ridge, I user another lens with starting aperture 1.8: the Nikkor 1.8/105. I held the aperture ring of the 1.8/85 on the 105's and marked the position with a needle. An easier way is to look it up in this table.
filing off the aperture ring Then I filed down the aperture ring up to the position of the AI coupling ridge.
  The reassembly is just as easy as taking the lens apart.
Nikon ADR-scale Note: The last camera with ADR support was the F5 - if your camera is newer, you can omit this part. In particular, it's not needed for digital SLRs.

I printed the scale for direct aperture readout on a normal laser printer. At this point I grew a bit impatient and used the first glue at hand to fasten the scale to the aperture ring. Because of this, the result is neither straight nor clean. Also, the glue smudged the print somewhat.

Aperture display in Nikon F3 viewfinder A look through the finder shows the aperture setting. Normally the scale should be white on black, but I think that black on white is easier to see, especially in dim light.

While you're at it, you might also want to add the minimum aperture signal post at the correct position.

During conversion of a Nikkor-K 1.4/50 I could see first hand how much the construction of the Nikkor lenses was simplified over time. Lars Holst Hansen has described the conversion of an older 1.4/50 in great detail. The most complicated step was to get the aperture ring off, which took several steps for the old lens. For my Nikkor-K, things were much simpler: remove four screws and the mount comes off. The screw just opposite of aperture 5.6 must stay even with this version of the lens! Later the construction was simplified even further: late AI and the AI-S versions of the 1.4/50 have only three screws to fasten the mount.

Position of the AI coupling ridge

The AI coupling ridge is positioned 4 2/3 aperture stops from the starting aperture, except on lenses with a starting aperture of 1.2 to 1.8. On these fast lenses the offset is 5 stops.

Lens starting apertureAI coupling ridge at aperture
1.25.6 + 2/3
1.88 + 2/3
28 + 2/3
2.511 + 1/3
2.811 + 2/3
3.316 + 1/6
3.516 + 1/3
416 + 2/3
5.622 + 2/3

AI conversion links


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