Nikon's meter indexing mechanisms

- by Lars Holst Hansen and Rick Housh

In order to do a non-evaluative metering with full aperture and automatically stop down for the exposure a camera needs to have information only on the proposed taking aperture relative to the full aperture. The absolute values of these apertures are not necessary. Note: In the interest of clarity, it will be assumed here that all exposure adjustments will be made by aperture adjustments. Of course, aperture and shutter speeds have reciprocal effects and either may be used in real life.

Why is relative measurement sufficient? Well, let us first explain what we mean by "non-evaluative meter". We mean a meter which does not try to evaluate the subject - whether it is bright or dark, has high or low contrast etc. All a non-evaluative meter does is to assume the subject represents a standard 18% gray average reflected subject (see note). The evaluation of the variance of the subject from the assumed 18% (see note) is left to the photographer. He or she must decide whether 18% reflectance (see note) is representative of the scene or not and how much more or less adjustment in exposure is potentially needed (to illustrate, the average green grass lawn is usually very close to 18% reflectance, (see note)).

Let's say it takes X photons/m2 to expose the Y (Y=ISO speed value) film to an average 18% reflectance level (see note). Assume the meter currently measures 4X photons/m2/sec at full aperture. The photographer decides that he or she prefers to stop down two f-stops from full aperture and sets the aperture ring accordingly. The meter will now incorporate these two f-stops adjustment. Two f-stops down will allow 1/4 of the light at full opening, in this case X photons/m2/sec. A shutter speed of 1 sec will now suffice and give the "correct" exposure of X photons/m2. It will be displayed as the "correct" speed if M mode is selected or automatically chosen if A mode is selected. The important point to note here is that only relative stop down measurement is necessary. Actual f-stop values are unnecessary.

Information on the relative stop down can be obtained by the camera in one of two ways;

1: by comparing information on the maximum aperture and the taking aperture, or
2: from direct relay of the relative stop down

Manual maximum aperture indexing
It was the first generation of indexing implemented in early metered TTL prisms. With these one must manually set the lens' maximum aperture. These meters are coupled to the lens via a meter coupling pin on the meter or metered prism which engages a slot in a prong on the lens. The relative stop down is determined by "counting" the number of stops between the proposed taking aperture (set by the photographer) and the maximum aperture of the lens.

Finders and cameras relying on manual maximum aperture indexing:
Nikkormat FT
Photomic T for F
Photomic TN for F

Semi-Automatic maximum aperture indexing
The first camera with this feature was the Nikkormat FTN. One must turn the aperture ring to both its most extreme positions after changing a lens while the prong located at f/5.6 on the lens engages the pin provided on the body. The twist to the minimum aperture side releases the mechanism and the following twist to the maximum aperture side slides the body's pin, which is linked to an internal resistor band, to a position which is determined by the maximum aperture. This sets the camera's memory of the absolute maximum aperture of that lens. The camera then determines a relative amount of stop down from full aperture by "subtracting it" from the proposed taking aperture.

Finders and cameras relying on semi-automatic maximum aperture indexing:
Nikkormat FTN
Photomic FTN for F
Photomic (DP-1) for F2
Nikkormat EL
Photomic S (DP-2) for F2
Nikkormat FT2
Nikkormat ELW
Photomic SB (DP-3) for F2

Automatic maximum aperture indexing
AI in short - is a short-cut to relative indexing. Paradoxically most AI cameras do not detect the AI lens' index of absolute maximum aperture! They directly determine the relative stop down index by means of the METER COUPLING RIDGE. True AI lenses (and later but not earlier lenses converted to AI) do however provide an index of their absolute maximum aperture, separate from the meter coupling ridge, by means of their LENS SPEED INDEXING POST (sometimes called "maximum aperture indexing post"). Relative aperture stop down has an advantage with variable maximum aperture lenses as there is no need to "update" a maximum aperture. Perhaps this is one of the reasons why there are so few pre-AI variable maximum aperture zooms made.

Finders and cameras relying on automatic maximum aperture indexing (AI) for non-evaluative metering:
Nikon EL2
Nikkormat FT3
Photomic A (DP-11) for F2
Photomic AS (DP-12) for F2
Nikon FM
Nikon FE
Nikon EM
Nikon F3
Nikon FG
Nikon FM2
Nikon FE2
Nikon FA*
Nikon FG-20
Nikon F-301
Nikon F-501
Nikon F-801**
Nikon F4***
Nikon F-601**
Nikon F-601M**
Nikon F-801s**
Nikon F90**
Nikon F90X**
Nikon F70**
Nikon FM10
Nikon F5**
Nikon FE10
Nikon F100**
Nikon D1**
* camera relies on mechanical indexing of the maximum aperture by means of the LSIP system for evaluative metering.
** camera relies on electrical indexing of the maximum aperture for evaluative metering.
*** camera relies on either mechanical or electrical indexing of the maximum aperture for evaluative metering.

Electrical aperture indexing
Some cameras do not have a mechanical coupling between the meter and the lens. They rely on electrical transfer of all info, including maximum aperture and even minimum aperture. With electrical indexing the aperture ring must be set to the minimum aperture and the shooting aperture is instantaneously set by the camera itself via the STOP DOWN LEVER with no aperture ring manipulation by the photographer. This system therefore relies on a standardized stop down mechanism (movement proportional to relative f-stop change) in the lens. This is only reliably available on AI-S and later lenses. Electrical aperture indexing is in a way a draw back from direct relative indexing to maximum aperture indexing but it has the advantage that a variable maximum aperture can be precisely indexed.

Cameras relying exclusively on electrical aperture indexing:
Nikon F-401
Nikon F-401s
Nikon F-401x
Nikon F50
Nikon F60
Nikon F80
Nikon F65

No maximum aperture indexing!
If direct relay of the relative stop down is more efficient, then why bother to index the maximum aperture and taking aperture in the first place? Well it was not exactly "in the first place" as the TTL finders were predated by another system; on-the-camera non-TTL metering. The Nikon F was released with a clip-on meter (EXPOSURE METER MODEL 1) to be used with the non-metered eye level finder. It was followed by MODEL 2 that also came in a version for Nikkorex F, and finally followed by a MODEL 3. These meters work much like a handheld meter with a follow needle system. The "follower" is controlled by the position of the ISO dial, the position of the shutter speed dial and the position of the METER COUPLING PIN - again controlled by the aperture ring setting via the METER COUPLING PRONG ("rabbit ears" or "hog nose"). The ISO dial positioning is also used to compensate for any filters or other attachments like extension rings etc. When there is no need for compensation, the ISO is lined up with the default mark. If compensation is needed, the respective "factor mark" is used for alignment. The Photomic finder for Nikon F has a non-TTL meter built into the corner of the prism house. In addition to the compensation factors it has two other markings assumedly to be used with the angle limiter. The angle limiter is a little "chimney" that screws into the thread around the meter window. It limits the angle to approximately equal an 135mm lens but also limits the amount of light to reach the metering cell - it therefore needs compensation.
The roundabout way of getting the relative stop down for the full aperture metering used in the two first generations of Nikon TTL metering is a reminiscence of this non-TTL system.

Note: The "18% reflectance" assumption has been questioned. Read more ...

Thanks are much due to Richard Cochran for weeding out some bugs on the non-TTL metering and Martin Dahl of Photografica for demonstrating the clip-on meters.