Nikon F Metering Prisms and Meters


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.

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).

Photomic Prism

Cadmium Sulfide (CdS). One mercury (PX-13) battery. FMI.
Sensitivity: ASA 10-1600, EV 2-17 (ASA 100)
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

The original Photomic prism is occasionally referred to as the "Photomic F"; I guess that this comes from calling the camera and prism combination the "F Photomic". It was originally offered in 1962; the main distinguishing figure of this lens is the dime-sized window on the prism's front, where incoming light is gathered (from a 35mm lens's angle-of-view, or about 75 degrees) onto a CdS photoresistor. Early models have a "flag" switch, or a little flap that covers the window when not in use; later models use an on/off button. To be complete, two accessories must be included: an "angle restrictor" (small pipe which screws into the ring surrounding the meter window, and changes the angle of view to that of a 135mm lens, or about 15 degrees) and an "incidence attachment" (opaque disc which also screws into the surrounding ring and turns the meter into a directional incident meter). The bottom of the prism's front has a long slot, where a slider (attached to the aperture coupling pin) roughly indicates the aperture selected. The two mercury cells are housed on the right side (i.e. same side as the ASA dial/shutter speed stack) behind a threaded door.

The top of the finder shows the ASA dial, on which you must manually set the maximum aperture of the lens opposite to the film speed in use. If the lens in question lacks the aperture coupling prong, push the pin/slider all the way to the left (i.e. in the direction of increasing f/ number) and set the ASA used next to the red dot mark that comes before f/1.2. The maximum aperture selection dial also has several different marks on it, which correspond to different exposure compensations (i.e. for backlighting, filter factors, etc.). When mounting the Photomic finder, you will want to twirl the shutter speed stack until you feel it engage the corresponding pin on the shutter speed dial. The metering display is a standard center-the-needle type. If you can't dig up mercury cells to put into the camera, you have several options:

  1. Wein zinc-air cells, which reputedly have very poor life
  2. alkalines will fit, but will require meter adjustment (for the different voltage) and more frequent changing (because of a steeper discharge curve)
  3. expensive but permanent: silver oxide SR44's can be adapted via the MR-9 adapter sold by C.R.I.S., for about $30 US per mercury cell

Photomic T Prism

Cadmium Sulfide (CdS). Two mercury (PX-13) batteries. FMI.
Sensitivity: ASA 10-1600, EV 2-17 (ASA 100)
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

There are some people who refer to the F and metering prism by combining the two: for instance, the "F Photomic T" is sometimes known as the "FT". This is sometimes confusing, as Nikon did make a "Nikkormat/Nikomat FT"; similarly, the F with Tn is occasionally referred to as the "FTn" while the F with FTn is called the "FTN". I appeal to your rational side to start calling them by "F/meter", like "F/T", "F/Tn", and "F/FTn", although I guess that that brings up a whole host of problems, like people asking when Nikon made the titanium F (did they?). Jeez, you can't win, unless you start being pedantic and typing "F Photomic T" ...

Externally, the Photomic T (introduced in 1965, following Topcon's stunning 1963 Photokina coup of through-lens metering) resembles the earlier Photomic, although, of course, without the metering window. The viewfinder's eyepiece is slightly larger than before, which required a small modification to the top deck of the body (factory-modified bodies 65xxxxx and 66xxxxx have a small red dot next to the serial number). Operation was also very similar to the Photomic head, from the FMI system to the battery compartment location. The metering switch switches on when pushed towards the prism, and causes a small button to pop out of the prism top; this small button has a red ring around it to indicate that the meter is on, and pushing it back down switches it off.

The meter itself works by diverting a small amount of light from the eyepiece, after having bounced off the mirror, going through the screen, and rebounding around the penta-roof-prism in some complicated manner; this light is focussed through two condenser lenses onto a CdS cell. The Photomic T is a whole-field averaging meter, meaning that it reads the entire view (as seen by the lens) and, based on the average level of light, makes an exposure reading. While letting every part have an equal vote works great for various sociological purposes, it makes for a lousy meter. Imagine this: you're taking a picture of (insert gorgeous model, either sex, here) walking on the beach at sunrise. You've composed it so that the sun is just peeking over the left side of the horizon, and your model is grinning and whispering words of love to you from the right side. With your trusty F/T, you capture the moment forever, based on the meter reading you got. When the pictures come back, after the model has gone, you put the chrome on the light table to see ... a bright glob on the left side of your frame and a dark, shapely blob on the right. Because your meter paid as close attention to the sun's brightness as you did to your model's face, it underexposed your once-in-a-lifetime shot; it would be better to focus more (exposure) attention away from the edges of the frame or, better yet, to have some sort of selectable spot with which to determine exposure (best of all, you'd have incident-metered the model with your hand-held meter ...). Although it took Nikon another twenty years to fit a spot meter into the camera, they did the next best thing and made the meter centerweighted the very next year with the Photomic Tn.

Photomic Tn Prism

Cadmium Sulfide (CdS). Two mercury (PX-13) batteries. FMI.
Sensitivity: ASA 20-1600, EV 2-17 (ASA 100)
Aperture Coupling Range: (f/1.2-f/4.5) - f/22

The Photomic Tn cosmetically differs from the T in one respect: behind the top-deck "on" indicator, Nippon Kogaku engraved a capital letter "N". Functionally, the meter includes a battery check (the small white lever in front of the "on" indicator) and is now 60/40 centerweighted (in the center 12mm circle); it is otherwise the same as the Photomic T. If you really want to get snooty, you can mention that your F/Tn has aspheric lenses (two condensers in the Tn provide the centerweighted pattern). The maximum lens aperture is finally set via a lift-and-turn outer ring on the shutter stack, rather than moving the central dial of the stack (as on all previous meters).

So what's the big deal with centerweighting, anyway? Besides, it's just a 60/40 split, which seems barely more than averaging over the entire finder. Let's look at it more closely: the 35mm frame is a patch 24 x 36mm, or 864 sq. mm. Let's say that each sq. mm area has a light-reading "vote" of 1 unit in the Photomic T finder for a certain uniformly lit 18% grey scene, so that the total "reading" is 864 units. When we meter the same scene with the Photomic Tn, we get the same 864 units, but 60% of them (518.4 units) are distributed in the center 12mm circle, which has an area of 113.1 sq. mm, while the remaining 40% (345.6 units) are distributed among the outer area of 750.9 sq. mm. Thus, each sq. mm in the central circle has a "vote" of 3.06 units, while each sq. mm outside has a "vote" of 0.46 units. The effective sensitivity is increased threefold in the middle while the outside's sensitivity is cut in half, which is a greater effect than hearing "60/40" would have you believe.

Photomic FTn Prism

Cadmium Sulfide (CdS). Two mercury (PX-13) batteries. SAI.
Sensitivity: ASA 6-6400, EV 2-17 (ASA 100)
Aperture Coupling Range: (f/1.2-f/5.6) - f/32

Nikon brought semi-automatic aperture indexing (SAI) from the Nikkormat FTN into the professional "F" with the FTn finder. In general, Nikon has been conservative (slow) when introducing new features into its professional line, preferring to test reliability and acceptance in its advanced amateur lines first (e.g. electronic shutter control, autoexposure, autofocus). After setting the lens to f/5.6 (or lower, e.g. f/4), mount the lens on the camera and cycle back and forth between the minimum and maximum apertures. Based on the ASA that you set atop the prism, some spring-loaded gizmo in the finder automatically communicates the maximum lens aperture to the metering circuit. Externally, this meter differs from the Photomic Tn by the addition of a finder-release lever, a smaller front aperture slot/slider (which now only indicates the maximum aperture of the lens, rather than the actual aperture in use), and the battery compartment moves from the side to the bottom of the finder.

The metering pattern and circuit are otherwise the same as the Photomic Tn. The viewfinder now includes the extra bonus of having the shutter speed visible, adding to the visual clutter for the mind to sort out from the picture. It will index to f/32 (previous meters could only go to f/22) and will read the "T" shutter setting (which appears as 4 sec. in the finder, as the "B" setting appears as 2 sec.). The additional finder release lever activates a pair of claws which cinch the meter on to the body via the "Nikon" bodyplate. Older F's (serial numbers before 6900000) may need some filing to get the nameplate to accept the FTn properly. The "on" button is now on the side of the finder, and battery check is accomplished by depressing the "off" button while the meter is off (which will cause the meter needle to move slightly right of the center notch if the battery is still good).

Note that, in general, different lenses with different screens will give different meter readings, even if they are all at the same f/stop; this is because all of the TTL Photomics measure light coming off the meter and through the screen. However, screen types A, B, E, F, J, K, L, P, and R all require no adjustment with any lenses used.


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