Nikon’s Roots Vol. 10 – Nikon F Photomic T

Photomic TTL

The Nikon F Photomic T was released in September 1965, two months after the release of the Nikomat FT/FS. Since its younger brother, a TTL camera, had been released, the Nikon F naturally had to evolve into a camera with a built-in TTL exposure meter, and this is when the advantages of the Photomic format were fully utilized.

Up until the external light type Photomic, the necessity of having a built-in exposure meter in an interchangeable viewfinder was not that important. As mentioned in the Nikon F Photomic section, it is assumed that the interchangeable viewfinder was made in order to introduce an exposure meter needle into the viewfinder field of view, but it would have been sufficient to simply change the light receiving element of the external exposure meter Nikon Meter, which used a selenium photocell, to CdS and provide a battery case in an appropriate space.

Now, in the age of TTL, the Photomic format finally showed its true potential. Various manufacturers have devised various methods for the light receiving section of TTL metering, but the most common method is to measure the brightness of the subject image formed on the viewfinder screen. This method makes it possible to measure the light that passes through the photographic lens without affecting the viewfinder optical path or the photographic optical path. However, this requires that the light receiving element be located adjacent to the viewfinder optical system. Since the Photomic format is essentially a combination of a pentaprism section and an exposure meter, the TTL metering system could be completed on the interchangeable viewfinder side without any modifications to the camera body. Even users of previous Nikon F cameras and external light Photomic cameras could get the latest TTL metering by simply purchasing a new interchangeable viewfinder.

TTL light metering optical system

As the metering system of SLR cameras evolved from ambient light to TTL, various metering optical systems were devised. TTL metering determines the exposure by receiving light that passes through the lens with a light receiving element, but originally the light that passed through the lens was directed to the SLR viewfinder when not shooting, and was directed to the image plane (the film plane at the time) at the moment of shooting.

Neither of these optical paths should be disturbed for metering. Therefore, attempts were made to split the optical path by cutting a slit in the main mirror of the SLR and attaching CdS to the back of it, or by making parts of the main mirror or the condenser lens of the viewfinder into half mirrors, and directing part of the subject light to the light receiving element. Examples of this category include the mirror meter of the Topcon RE Super and the cut condenser method of the Canon FT.

However, this method of splitting the optical path results in a loss of light both in the viewfinder and in the light metering, which can cause parts of the viewfinder to become dark and can cause a slit pattern to appear in the viewfinder image when the aperture is stopped down.

What were alternatives?

As an alternative, the Pentax SP placed two CdS light-receiving elements on either side of the eyepiece, on the exit surface of the pentaprism, to receive the subject light diffused by the viewfinder screen. This diffused light is not used as viewfinder light and is essentially “discarded” light, so it is not lost. As a result, it has been adopted by many cameras over time, and although there have been improvements such as consolidating the two light-receiving elements into one placed above the eyepiece, it has remained the standard for TTL metering up to recent digital SLR cameras.

Nikon also adopted this method as its TTL metering optical system.

Differences in photometric optics from Nikkomat FT

Although the Nikomat FT and the Nikon F Photomic T both use the same system of metering the diffuse light from the viewfinder screen, there are differences in their configuration. Both have a condensing lens placed in front of the light receiving element to deal with light entering the eyepiece in reverse, but whereas the Nikomat FT places the CdS light receiving element directly facing the exit surface of the pentaprism with a Fresnel lens in between, the Nikon F Photomic T reflects the light from the viewfinder screen upwards with a right-angle prism attached to the exit surface of the pentaprism and receives it with a CdS light receiving element placed facing downwards. The condensing lens placed just before the CdS uses a plastic aspherical lens.

This TTL metering optical system was later inherited by the Photomic on the Nikon F2.

Exposure meter linkage mechanism

All Nikkor Auto interchangeable lenses for Nikon F are equipped with a “crab claw” for aperture interlocking, so naturally TTL full aperture metering is used. In the days of the external exposure meter Nikon Meter before the Photomic, the mechanical interlocking method (combined with electrical interlocking in some models) combined the movement of the shutter dial and the movement of the aperture ring to move the dial on the same axis as the ammeter pointer, but from the Nikon F Photomic onwards, this was changed to a purely electrical interlocking method.

The rotation angle of the shutter dial, after adding information about the film speed, rotates a large ring shaped like a tiara on the roof of the pentaprism, inside which is attached a resistor made of polyester film coated with carbon.

Aperture Setting

On the other hand, the aperture setting is a crab claw movement that is picked up by a pin under the front of the Photomic Finder and transmitted to a brush ring that is coaxial with the resistor ring. The brush (contact) on this brush ring slides over the resistor, and together they form a variable resistor. In other words, the relative positions of the “resistor ring that moves according to the shutter speed and film sensitivity” and the “brush ring that moves according to the aperture value” are introduced into the light meter circuit in the form of resistance values.

In the case of the Nikomat FT, the same function is achieved using the shutter dial and aperture linkage dial around the lens mount, but since the position and direction of rotation of the shutter dial is different on the Nikon F, its movement and the movement of the aperture ring are pulled by gears and concentrated in a ring on the pentaprism, making the mechanism more complex.

This allows for TTL maximum aperture metering, but just like the Nikkor FT, it was necessary to adjust the film sensitivity setting on the shutter dial and correct the maximum aperture every time the lens was changed.