Modern Classic SLRs Series :
Nikon F - Nikon F Meters & Metered Prisms Part I


Subtitle: Why have aliens attacked my camera?

"Those of you who have seen the big metering prisms for the Nikon F probably suspect that Nikon had a fine time watching photographers all over the world lug these huge, lumpy beasts all over the world in the 1960's. Those of you who have the big metering prisms are probably calling for my blood -- I use an F with the eyelevel prism, so I can't really comment on the weight of the camera, although at least one book that I've glanced through calls the F/FTn combination "the weightiest 35mm camera ever." It's not just weight, it's ... solidity ...

One more thing to consider: regardless of whether or not your prism still works, think about using a hand-held meter. In this day of auto-exposure, it's too tempting to sit back and let the camera take care of the picture for you. Decide what mood you are trying to convey with the photograph, work in a more deliberate mode (most of you using F's and F2's are almost forced to be deliberate, by the very nature of manual focus), and consider exposure to be just as much of a tool as depth of field or composition. The hand-held meter can only help your deliberation, as you decide tonal values for various areas in your picture, more flexibly than an in-camera meter will allow you to do. It's not to say that the F or the F2 can't be used for action or decisive moment photography -- with the proper technique, you can still do a creditable reportage job; let's face it, though -- most action (sports, news, nature -- animals) almost demands autofocus cameras, moving faster than the hand or eye can follow. I don't see relying on autoexposure/focus as being lazy, but one of things that I've been taught is that you cannot guarantee a great photo; there are a million ways to take bad pictures, and only a few ways to take good ones, but automation has no effect other than to remove the fairly robotic motions of determining exposure and focus from the equation. Using a manual camera is not just about control, either, as virtually all AF SLRs will allow manual overrides; it's not the satisfaction of taking pictures in spite of your equipment; it is what works for me, and I honestly can't say that my way is better or worse than yours. The best thing to do is to burn film and develop your style, experimenting with technique and/or equipment as your budget and time allow.

I might as well begin by defining the two types of Nikon F film speed/maximum aperture coupling: either Fully Manual Indexing (FMI), in which the lens's maximum aperture is manually set against the film speed in use via a dial, or Semi-Automatic Indexing (SAI), where the aperture is set by racking the aperture back and forth between maximum and minimum after mounting the lens on the camera (film speed must still be set with a dial, as with all meters). Both FMI and SAI are subsets of the more commonly-known non-Automatic Indexing (NAI or non-AI), as they both rely on the aperture ring prong at f/5.6 to transmit aperture information to the meter. Incidentally, if Aunt Ethel picks up your N90s (F90x) and inadvertently twiddles your aperture ring to max and min, you know that she cut her teeth on Nikons sometime between 1959 and 1977, not that she can't figure out how to take your lens off." - Michael Liu -

Model I Meter

Selenium. Self-Powered. FMI.
Sensitivity: ?
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

This meter was introduced with the F in 1959. It was soon superseded by the Model II, so is extremely scarce. Physically, it resembles the Model III Meter, except that the dial-side (from a top view) of this meter is round, rather than squared off (as it is on the III). All of the Model x Meters slip over the front of the prism, couple to the shutter dial through the pin protruding from the dial's surface, and couple to the aperture through the prong-and-pin system used on all non-AI meters. In concept, if not compactness (probably because the auto-diaphragm system requires aperture coupling), the system is similar to Leica's MR meter or even the Nikon SP exposure meter.

Model II Meter

Selenium. Self-Powered. FMI.
Sensitivity: ?
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

This meter had a long selenium photocell window, similar to the meter introduced for the Nikkorex F. It is also fairly rare, and will cost about twice as much as a Model III Meter in the same condition. The end of the meter close to the dial is rounded, like that of the Model I Meter. If you really want to drive yourself crazy, all of the Model x Meters came with an incident-metering attachment, which is an opaque piece of plastic that slips over the metering window and converts your on-camera meter into a directional incident meter. Then again, if you're really serious about incident metering, you've already gotten a hand-held meter ...

It is worthwhile to note that selenium meters are photovoltaic cells (they generate a voltage potential when exposed to light). There are currently three technologies in today's (and yesterday's) meters: photovoltaic (selenium), photoresistor (Cadmium Sulfide, CdS), photodiode (generally silicon). Because each one acts differently, electrically speaking, each one requires a different kind of circuit to convert a light reading into an exposure setting. Presumably, photovoltaics generating some potential which is attenuated by variable resistances, representing the shutter and aperture settings; by comparing the current across these resistances with a reference (determined by film speed), correct exposure would be reached. Photoresistors probably encode shutter and aperture settings as variable resistances in some sort of resistance bridge, with the CdS cell as one leg; correct exposure would be determined by balancing the bridge (through shutter and aperture), with the required batteries exciting the bridge. Photodiodes determine the absolute value of the light flux; they are generally made of semiconducting materials (nerd hat on), which means that the ground states are completely filled with electrons, while the valence bands are completely empty; when a photon with the right amount of energy comes by, it can knock an electron loose (out of the ground state, without enough energy to be recaptured by the valence band). Photodiodes consist of a P-N (positively and negatively doped) junction; the loose electron will tend to be attracted to the P-doped side, which temporarily will make the overall junction more conducting. (nerd hat off). Basically, the 'diode becomes less resistive to the appropriately applied voltage with more light shining on it, and exposure is probably determined in a circuit similar to a photoresistor.

Model III Meter

Selenium. Self-Powered. FMI.
Sensitivity: ASA 6-4000?
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

This is the most common of the Model x Meters, and is not too rare or expensive (although many of them have long since given up the ghost, probably due to the ring resistor). Ah ring resistor, our nemesis; you wouldn't expect much of an open potentiometer, or variable resistor, which consists of a carbon track deposited on a ceramic substrate. Of course, that's why Nikon kept on manufacturing this resistor until 1981, when the F2A was finally retired (and now has run out of spares). I have heard that some people have cleaned the resistor with good results, and there is at least one shop in the US which can replace it. The ring resistor will eventually fail, as it gradually abrades away; the process will be hastened if the resistor is dirty/gritty (think of using sandpaper as windshield-wiper blades).

| Next | Those built in metering prisms for the Nikon F
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Copyright © 1998. Michael C. Liu ®
Rearranged by: leofoo ®. Credit: Hiura Shinsaku® from Nikomat Club of Japan for feeding some useful inputs on the introductory page. The great 3D logo by Kiasu; Ted Wengelaar®, Holland for his continuous flow of input of early Nikon bodies. Stephen Gandy's Cameraquest; Marc Vorgers from Holland for his additinal images on Nikon F Apollo; Hayao Tanabe corrected my Red Dot and Early F assertions. Gray Levett, Grays of Westminster publishes an excellent monthly historical look at Nikon products, from where I learned about the high-speed F's. Made with a PowerMac, broadcast with a Redhat Linux powered server.

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