Nikon’s Roots Vol. 11 – Nikon F Photomic Tn

Center-weighted light metering

In the case of external light type exposure meters, the light receiving angle is relatively wide, so deciding which part of the light receiving angle to use with what weight was not much of a problem, but with TTL metering, I started to become aware of this at that point.

In the beginning, both Topcon RE Super and Pentax SP used full-screen average metering. This is a metering method that takes in the subject light uniformly from the entire screen and does not add any particular weight. Canon Perix (1965) and Canon FTQL (1966), which debuted around the same time, adopted partial metering, which measures only a small area in the center of the screen, but this is probably due to the convenience of the metering optical system.

Full-screen average metering tends to underexpose the main subject when there is a bright area in the screen, such as in a backlit scene or a landscape with the sky, while partial metering tends to underexpose the main subject. Photometric value had some issues because of that backlighting issue.

Photometric optical system

Center-weighted metering can reduce the effects of bright areas of the subject compared to full-screen average metering, and does not have the delicate aspects of partial metering, so it is an intermediate metering method that was used until the advent of multi-segment metering in the 1980s. It became mainstream. The idea is that when taking a photo, the main subject is often located in the center of the screen, so it is best to focus on capturing brightness information from there.

To achieve this, the Nikon F Photomic Tn placed an aperture plate just in front of the CdS photodetector. In the Nikon F Photomic T’s photometric optical system, a plastic aspherical lens is placed in front of the light-receiving element, and the condensing lens refocuses the subject image on the finder screen onto the light-receiving surface, and then apertures the image. By limiting the amount of light, the effect of center-weighted metering was achieved.

This resulted in center-weighted photometry with a sensitivity distribution of 60% from the 12mm diameter area at the center of the image, and 40% from other areas. I’ve heard the story behind this: actually, I was aiming for partial metering, but due to aberrations in the condenser lens, the light rays went around outside of the targeted area, resulting in center-weighted metering.

Another effect of center-weighted metering

Why is center-weighted metering better?

As mentioned earlier, the official reason is that it can reduce the effect of bright areas in scenes with backlight or the sky, but there is actually another ulterior reason.

When using TTL metering, you don’t want to receive too much light from the periphery of the screen. A portion of the light that enters the periphery of the finder screen and is diffused by the screen enters the light-receiving element, but the intensity of the light that enters the light-receiving element changes depending on various factors. For example, the diffusion characteristics of the matte surface of the finder screen, the position of the exit pupil of the photographic lens, the aperture value, etc. As a result, even if the brightness of the photographic lens becomes one step darker, the brightness of the photodetector surface will not be exactly halved, affecting the accuracy of photometry.

With the Nikomat FT and Nikon F Photomic T, this is corrected by the interval of the scale that sets the aperture F value of the photographic lens, but for more precise light metering, it is necessary to limit the light that enters the photodetector from the periphery of the screen. There is a need to. This was another hidden purpose of center-weighted metering.

This was the starting point for the “gacha-gacha” method of the Nikomat FTn and Nikon Photomic FTn that followed.

Appearance differences from Nikon F Photomic T

The Nikon F Photomic T and Tn have almost the same appearance and are almost indistinguishable. The only difference is the power switch located at the top left when viewed from the front. Late external light Photomics have had their own power switch operated by a button on the side and a button on the top, but the Nikon F Photomic Tn has a small white button in front of the button on the top. There is also a small “N” engraved on the back of the button.

This white button is a battery check; when pressed, the light receiving element is internally disconnected and a check resistor is inserted in its place. If the ammeter needle swings to the check indicator at that time, it means the battery is OK.

The engraved “N” stands for “Narrow,” and indicates that the photometry reception angle is narrow, that is, center-weighted photometry.

Exposure meter interlocking mechanism and aperture value display window

The exposure meter interlocking mechanism of the Nikon F Photomic Tn follows the electrical interlocking method that has been used since the first-generation external light type Photomic, and uses a “fixed point type” that aligns the ammeter pointer with a fixed point. The crab claw movement of the photographic lens is caused by the movement of a variable resistor brush connected to the exposure meter circuit, and the brush ring can be used to read the set aperture value from the window above the eyepiece. is now possible. This is also inherited from the first Photomic.

Why the aperture display in this config?

The reason why such an aperture value display window was provided is because the photomic viewfinder is larger. In the case of external Nikon meters, the mechanism that transmits the movement of the crab claw to the exposure meter is relatively simple and fits in a thin space. However, in the photomic system, the mechanism that transmits the movement of the crab claw to the exposure meter is relatively simple. With the photomic type, the mechanism that guides the interlocking pin that bites into the crab claw and transmits the aperture value is a rail.

The gear train that pulls this movement, converts the direction of rotation to a right angle, and transmits it to the brush ring is placed in the space in front of the pentaprism, so the name plate that protrudes forward hides the aperture ring of the lens, making it difficult to see. To compensate for this, Nikon made it possible to check the aperture value from the rear of the Photomic viewfinder, which can be said to be Nikon’s typical thoughtful consideration.