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Nikon FOCAL LENGTH INDEXING RIDGE

NIKKOR*ED 180mm 1:2.8 (AI-S)
NIKKOR*ED 180mm 1:2.8 (AI-S) - Scan is courtesy of Howard Passman

The FOCAL LENGTH INDEXING RIDGE (FLIR) is a feature of the AI-S type MF and AF Nikon lenses - well that is the long AI-S type MF and AF lenses.

The FLIR is read by only three cameras (FA, F-501 and F4) by means of their FOCAL LENGTH INDEXING PIN and it doubles as a rear lens protector. It is therefore probably one of the least known features of Nikon lenses.

This NIKKOR*ED 180mm 1:2.8 (AI-S) clearly shows the FLIR. It is the black ridge that goes on the inside of the lens mount flanges and down to the level of the LENS LOCKING INDENTATION. It will engage with the FOCAL LENGTH INDEXING PIN of FA, F-501 and F4 indicating that the lens is above or equalling (>=) 135mm.

NIKKOR 20mm 1:2.8 (AI-S)

You will not find a FOCAL LENGTH INDEXING RIDGE on this NIKKOR 20mm 1:2.8 (AI-S) as its focal length is far bellow 135mm.
Nikon Teleconverter TC-14B 1.4X
Nikon Teleconverter TC-14B 1.4X

Even an AI-S type tele converter, as this TC-14B, has a FLIR.

Zoom-NIKKOR 35~200mm 1:3.5~4.5 (AI-S)
- scan is courtesy of Howard Passman

A zoom lens extending beyond 135mm also has the FLIR.

So what is the FOCAL LENGTH INDEXING RIDGE used for? In FA and F-501 it is used in the Dual Program mode to minimise the effect of camera movement. When a long AI-S type or AF lens or an AI-S type TC is in use a shorter shutter time is chosen. F4 does not have P mode available in connection with a lens without a chip. The function is F4 is believed to be correction of the maximum aperture (relayed by the MAXIMUM APERTURE INDEXING POST) for use in Matrix metering in connection with a tele converter. This has not been finally verified. FA also assumedly corrects lens speed when AI-S type TCs are used. F-501 does not have any form of evaluative metering, so the FOCAL LENGTH SIGNAL SWITCH probably only has two settings as opposed to that of FA (follow link bellow).

Read about the FOCAL LENGTH SIGNAL SWITCH in FA.

Go to F-mount index

Last updated 13. Mar. 2002