Modern Classic SLRs Series :
Nikon F - Nikon F Metering Prisms and Meters


Nikon F standard model.jpg (21k)

Nikon F Photomic Prism.jpg (21k)

Nikon F, 1959

With Standard Non-Metered Eye Level Finder

Nikon F Photomic
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 Nikon F Photomic: Introduced in 1962, discontinued in 1966.

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)
  1. 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

    Update: From:
    Subject: Nikon F Photomic Meter
    Date: Sat, 11 Dec 2004 05:24:54 +0000

    Dear Sirs,

    I have used your Nikon pages for reference many times-they are wonderful! I
    would like to contribute to your effort in a small way by correcting your
    description of the operation of this meter.

    The following text on your site re. this meter is incorrect:

    " 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.)."

    Actually, to use the original Photomic meter as a reflected light meter, you must: Turn the ASA dial until the film ASA value is opposite the black arrow, or if using the telephoto attachment, opposite the yellow arrow. Starting from these arrows, the ASA dial may be turned clockwise until the film ASA value is opposite the appropriate filter factor for the filters in use, if any, or until it has been moved the appropriate number of stops for backlighting or other corrections you wish to make. When using a lens with a meter coupling prong, turn the aperture ring and/or shutter speed until the meter needle is centered. For lenses without a meter coupling prong, manually move the aperture slider on the front of the meter until the needle is centered, read the aperture value from the small window on the back of the meter, and set the lens to the indicated aperture.

    To use the Photomic as an incident light meter: Install the opaque disc attachment, stand near the subject, point the meter generally toward the light source, and adjust aperture and/or shutter speed as described above.

    Best Regards,
    Jeff Schneider

    Nikon F Photomic T Prism.jpg (19k)

    Nikon F Photomic T

    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
    Nikon F Photomic T, introduced in 1965, discontinued in 1966.

    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.

Nikon F Photomic Tn Prism.jpg (21k)

Nikon F Photomic Tn

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
Nikon F Photomic Tn, introduced in 1967, discontinued in 1968.

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.

Nikon F Photomic FTn Prism.jpg (22k)

Nikon F Photomic FTn

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 F Photomic FTn, introduced in 1968, discontinued in 1974.

(MORE: Click here to go to details of the FTN Finder Instruction Manual Section)

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.

* Note: Since the FTN finder is the last metered prism and it is one of the most popular, we will go through the FTN finder in detail and outline its operational procedures. Together, you may also download a copy of the instruction manual as well in the mext section for future reference.

Relative: Special/Variations of the Nikon F

| Next | The Nikon FTn Photomic Finder
| Back | Index Page for Finders/Prisms

Main Reference map in HTML & PDF:
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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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