Nikon METER COUPLING
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Nikon METER COUPLING
RIDGEIn 1977 Nikon introduced the Automatic Maximum Aperture Indexing (AI) system. The DP-3 Photomic finder for F2 was replaced by the DP-11 and DP-12 finders,the Nikkormat FT2 replaced by FT3, the Nikkormat ELW replaced with EL2 and the Nikon FM was introduced followed by the FE in 1979. Paradoxically all of these early AI cameras never get absolute information on the maximum aperture despite the name!
AI compatible lenses are supplied with a METER COUPLING RIDGE that relays the relative aperture setting of the lens in use. This is sufficient under many circumstances but see the page on full AI modification for exceptions.
The METER COUPLING RIDGE is denoted AI COUPLING RIDGE in the Nikon FA brochure but this does not make much sense as the majority of so-called AI-coupled cameras do not get information on maximum aperture.
So how does it work?
The METER COUPLING RIDGE relays the relative aperture setting. The aperture setting relative to the maximum aperture by which the light is measured.
In Nikon FA the position of the aperture ring is digitally encoded. The following table lists the 5 bit codes for the different settings of a range of lenses with various maximum apertures. In other cameras like the FM2 the aperture ring is analogically encoded by means of string, pulleys and the infamous FRE!
| A | B | C | D | E | 1:1.2 lens | 1:1.4 lens | 1:1.8 lens | 1:2 lens | 1:2.5 lens | 1:2.8 lens | 1:x lens (x > 1.8) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 1.2 | 1.4 | 1.8 | - | - | - | - |
| 1 | 0 | 0 | 0 | 0 | 1.4 | 1.6 | 2 | 2 | 2.5 | 2.8 | x |
| 1 | 1 | 0 | 0 | 0 | 1.6 | 1.8 | 2.2 | 2.2 | 2.8 | 3.2 | x + 1/3 f-stop |
| 0 | 1 | 0 | 0 | 0 | 1.8 | 2 | 2.5 | 2.5 | 3.2 | 3.5 | x + 2/3 f-stop |
| 0 | 1 | 1 | 0 | 0 | 2 | 2.2 | 2.8 | 2.8 | 3.5 | 4 | x + 1 f-stop |
| 1 | 1 | 1 | 0 | 0 | 2.2 | 2.5 | 3.2 | 3.2 | 4 | 4.5 | x + 1 1/3 f-stop |
| 1 | 0 | 1 | 0 | 0 | 2.5 | 2.8 | 3.5 | 3.5 | 4.5 | 5.0 | x + 1 2/3 f-stop |
| 0 | 0 | 1 | 0 | 0 | 2.8 | 3.2 | 4 | 4 | 5.0 | 5.6 | x + 2 f-stops |
| 0 | 0 | 1 | 1 | 0 | 3.2 | 3.5 | 4.5 | 4.5 | 5.6 | 6.3 | x + 2 1/3 f-stop |
| 1 | 0 | 1 | 1 | 0 | 3.5 | 4 | 5.0 | 5.0 | 6.3 | 7.1 | x + 2 2/3 f-stops |
| 1 | 1 | 1 | 1 | 0 | 4 | 4.5 | 5.6 | 5.6 | 7.1 | 8 | x + 3 f-stops |
| 0 | 1 | 1 | 1 | 0 | 4.5 | 5.0 | 6.3 | 6.3 | 8 | 9.0 | x + 3 1/3 f-stop |
| 0 | 1 | 0 | 1 | 0 | 5.0 | 5.6 | 7.1 | 7.1 | 9.0 | 10 | x + 3 2/3 f-stops |
| 1 | 1 | 0 | 1 | 0 | 5.6 | 6.3 | 8 | 8 | 10 | 11 | x + 4 f-stops |
| 1 | 0 | 0 | 1 | 0 | 6.3 | 7.1 | 9.0 | 9.0 | 11 | 13 | x + 4 1/3 f-stop |
| 0 | 0 | 0 | 1 | 0 | 7.1 | 8 | 10 | 10 | 13 | 14 | x + 4 2/3 f-stops |
| 0 | 0 | 0 | 1 | 1 | 8 | 9.0 | 11 | 11 | 14 | 16 | x + 5 f-stops |
| 1 | 0 | 0 | 1 | 1 | 9.0 | 10 | 13 | 13 | 16 | 18 | x + 5 1/3 f-stop |
| 1 | 1 | 0 | 1 | 1 | 10 | 11 | 14 | 14 | 18 | 20 | x + 5 2/3 f-stops |
| 0 | 1 | 0 | 1 | 1 | 11 | 13 | 16 | 16 | 20 | 22 | x + 6 f-stops |
| 0 | 1 | 1 | 1 | 1 | 13 | 14 | 18 | 18 | 22 | (25) | (x + 6 1/3 f-stop) |
| 1 | 1 | 1 | 1 | 1 | 14 | 16 | 20 | 20 | (25) | (29) | (x + 6 2/3 f-stops) |
| 1 | 0 | 1 | 1 | 1 | 16 | (18) | 22 | 22 | (29) | (32) | (x + 7 f-stops) |
| 0 | 0 | 1 | 1 | 1 | (18) | (20) | (25) | (25) | (32) | - | - |
| 0 | 0 | 1 | 0 | 1 | (20) | (22) | (29) | (29) | - | - | - |
| 1 | 0 | 1 | 0 | 1 | (22) | - | (32) | (32) | - | - | - |
0 = closed
1= open
But where is this METER COUPLING RIDGE?
 |
On this schematics of the F-mount,  (theta1)
is the angle from the LENS LOCKING INDENTATION to the leading edge of the METER COUPLING RIDGE (to be measured when the lens is set to its maximum aperture). This is the way Nikon uses to check the position of the METER COUPLING RIDGE. |
One can however also look at the aperture number scale or the ADR number scale.
The offset of most AI lenses is 4 2/3 f-stops. If the lens has a maximum aperture of 2.8 then the leading edge of the METER COUPLING RIDGE is positioned at 2/3 stops bellow 11.
(* * 11 * * 8 * * 5.6 * * 4 * * 2.8)
For some reason fast lenses have a slightly larger offset of 5 f-stops (giving the code 00000 in FA, when set to the maximum aperture, see above). Confirm this by looking at some images from Roland Vink's page :
| Lenses with 4 2/3 f-stops offset | Lenses with 5 f-stops offset |
|---|---|
| NIKKOR 28mm 1:2 (AI) | NIKKOR 35mm 1:1.4 (AI) |
| NIKKOR 28mm 1:2.8 (AI) | AF NIKKOR 85mm 1:1.4D |
| NIKKOR 28mm 1:3.5 (AI) | NIKKOR 50mm 1:1.8 (AI-S, N) |
| Micro-NIKKOR 105mm 1:4 (AI) | AF NIKKOR 50mm 1:1.8 |
| UV-NIKKOR 105mm 1:4.5 (AI) | AF NIKKOR 50mm 1:1.8 (N) |
| NIKKOR 800mm 1:8 (AI-S) | AF NIKKOR 85mm 1:1.8 |
To sum up:
| Lens Speed (maximum aperture) | Position of leading edge of the METER COUPLING RIDGE |
|---|---|
| 1.2 | 6.3 (= 5.6 + 1/3 f-stop) |
| 1.4 | 8 |
| 1.8 | 9.0 (= 8 + 1/3 f-stop) |
| 2 | 10 (= 8 + 2/3 f-stops) |
| 2.5 | 13 (= 11 + 1/3 f-stop) |
| 2.8 | 14 (= 11 + 2/3 f-stops) |
| 3.3 | 16 + 1/6 f-stops |
| 3.5 | 18 (= 16 + 1/3 f-stop) |
| 4 | 20 (= 16 + 2/3 f-stops) |
| 4.5 | 22 |
| 5.6 | 29 (= 22 + 2/3 f-stops) |
| 8 | 40 (= 32 + 2/3 f-stops) |
| 9.5 | 46 (= 45 + 1/6 f-stop) ** |
| 11 | 57 (= 45 + 2/3 f-stop) |
** I am doubtful as to to whether the monstrous 200-600/9.5 was ever released in a version that would allow for an AI modification, hence the MCR position could be hypothetical.
The early AI cameras does not know if a "slow" lens is stopped down "x" f-stops or a fast is stopped down "x + 1/3" f-stop for a given aperture ring setting! It has probably to do with the metering itself - fast lenses needing compensation. It is not known whether the pre-AI maximum aperture indexing mechanisms had a similar fast-lens-compensation built-in.
Go to F-mount index
Last updated 13. Mar. 2002
