Nikon LENS SPEED INDEXING POST

NIKKOR 35mm 1:2 (AI-S), showing the theta3, see text

The LENS SPEED (or MAXIMUM APERTURE) INDEXING POST position encodes the maximum aperture of a true AI-coupled lens. In order to decipher the code one can measure the angle relative to a set point like the LENS LOCKING INDENTATION. This can be done on the lenses directly or on scans. The angle is officially known as "theta3" (, see bellow)

The information can be used for a full AI modification.

The picture on the left is a simple flatbed scan.

See a table of the s

Nikon Teleconverter TC-200 2X

A so-called AI coupled teleconverter (as the shown TC-200)  paradoxically has not got a LENS SPEED INDEXING POST.

It looks just like an AI-modified lens.

NikonTeleconverter TC-14B 1.4X

An AI-S type TC (here the TC-14B) has a variable spring loaded mechanism that will relay the maximum aperture of the lens in use. 

Micro-NIKKOR 55mm 1:2.8 (AI-S), scan supplied by Howard Passman

The LENS SPEED INDEXING POST is not always a POST! On the Micro-NIKKOR 55mm 1:2.8 (AI-S) it is merely a part of a larger protruding area (see image on the left)