Full Nikon AI modification should include a LENS SPEED INDEXING POST

- by Lars Holst Hansen

When one AI-modifies a pre-AI lens it is common to only provide the lens with an AI or METER COUPLING RIDGE (by either milling away a part of the aperture ring or by fitting a new aperture ring). The LENS SPEED INDEXING POST is usually ignored. This is in itself paradoxical since AI stands for Automatic (Maximum Aperture) Indexing (see original flyer @ www.aiconversions.com).

Most people won't mind lacking the LENS SPEED INDEXING POST but it is infact used by a range of cameras;  EM (1979), FG (1982), FA (1983), FG-20 (1984), F-301 (1985), F-501 (1986) and F4 (1988).

It is used for three major things:

This functionality is of course lost if the AI'd lens is not given a LENS SPEED INDEXING POST.
The advanced auto flash of EM and FG-20 is diverted to normal auto flash, and the evaluative metering of FA and F4 is diverted to center weighted. P mode in FG, FA, F-301 and F-501 will still work but the camera will assume a maximum aperture of 2.8.

In order to fit a MAIP one needs to know where it should be positioned.

The exact LENS SPEED INDEXING POST position corresponding to the maximum aperture can be measured if one has an appropriate lens or a scan of a lens.

You can also add a FOCAL LENGTH INDEXING RIDGE.The focal lenght indexing system is independent of the lens type signal system, and will enable dual program with FA and F-501.

For an illustrated report from my first AI modification attempt see this
For details on how the maximum aperture is relayed to the FA see the page on LENS SPEED INDEXING mechanism
For a table of lens scans including links to actual scans see Lens Scans
External links: MIR provides detailed information on EM, FG, FA, FG-20 and AMP (Automatic Multi Pattern). Ken Rockwell provides Nikon's brochure on AMP.

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