Exposure Metering: Multi-Spot or Matrix

All film cameras have to live with the same design constraint: their shutter and their diaphragm are built in such a way that, for a given picture, the shutter speed and the aperture are the same for each square millimeter of the film. There is no way for the shutter of a film camera to block the light in excess in a particular zone of the scene, or to stay open longer only for the portion of the scene located in the shade. At some point the researchers of Canon were rumored to be working on an LCD based shutter, which could to exactly that, but the research never materialized.

Olympus OM-3
Olympus OM-3. Picture: Wikipedia

Since the shutter only works in an all or nothing, one-duration-fits-all mode, some zones of the film will receive more light than the optimum, and others will receive less. Films have the ability to give acceptable results when portions of the scene are a few f: stops brighter and a few stops darker than the optimum (that’s the exposure latitude of the film). As a consequence, the exposure metering systems of the cameras are calibrated to determine the correct exposure for the portion of the subject located in the mid-tones, with the expectation that the film will have enough exposure latitude to render the highlights and the shadows correctly.

Unfortunately, in some cases, the brightness range of the scene greatly exceeds the exposure latitude of the film (think of the backlit portrait at sunset with the sun in the frame); supposing the photographer can not reduce the brightness range of the scene – using a flash or a reflector to bring more light to the subject, for instance, the exposure parameters will only be optimal for a subset of the scene, the highlights or the shadows, and the rest of the picture will be burned or left in the dark.

When cameras started being equipped with Through the Lens (TTL) metering systems in the sixties, most of the manufacturers opted for Average or Center Weighed Average Metering. Those metering systems were not adapted to high contrast scenes, and the photographers had to put their experience to good use and take control manually of the exposure metering process. If they had automatic cameras, they had to use exposure lock or exposure compensation systems. The alternative – measuring the brightness of a very narrow section of the scene with a spot meter, was not easy to master for the average photographer. Elaborate exposure determination procedures such as the “zone system” were adapted to small format cameras using roll film, but their complexity put them out of reach from the majority of photographers.

The Olympus OM-4 and the Nikon FA

In 1983, two cameras manufacturers tried to address the problem of high contrast/high brightness range scenes, and they chose two very different approaches.

Olympus tried to make the principles of the zone system accessible to more photographers, and developed a multi-spot system for the new OM-3 and OM-4 cameras. With the new OMs, the photographer could make up to eight successive spot measurements, whose result were presented in the viewfinder on an analog bar scale showing each individual result and the average. The cameras also had a “shadow” and a “highlight” push button, letting the photographer compose his picture following the principles of the zone system.

The photographer Ken Norton described the process in his blog:

Olympus OM-3 and OM-4: exposure metering controls
Olympus OM-3 and OM-4: exposure metering controls. Picture: Wikipedia

“for example, I can take a spot reading of a highlight, a midtone and a shadow. Three dots will appear on the display. If I’m using a film with six stop exposure range, I’ll make sure that all three dots appear within the +/- 3 stop marks. Of course, I can bias my exposure to place a highlight or shadow anywhere I want on the scale. Digital cameras are capable of producing a histogram of an image. The multi-spot scale, with the “dots” is a poor-man’s variation of the histogram where we are able to define our bright areas and dark areas of a scene and like a histogram we are able to move these points around to place them within the acceptance range of the film.

You can read more about the subject, and see actual pictures of the viewfinder on Ken’s pages.

Nikon chose a totally different approach. In the first iteration – the Automatic Multi Pattern (AMP) of the Nikon FA – the camera was equipped with a database containing the mathematical description of thousands of potential pictures, with the exposure value to be used in each situation. The light meter was divided in five zones (a large central zone, two zones at the bottom left and right, two zones at the top, left and right also), and the electronic circuit would correlate the exposure value of each zone with other elements such as the focal length of the lens to define the characteristics of the scene, and associate it with one of the many typical pictures described in the database of the camera.

Nikon FA (the round knob controlling the exposure mode ( matrix or center weighted) is on the right side of the lens mount, at the top on this picture
Nikon FA (the round knob controlling the exposure mode ( matrix or center weighted) is on the right side of the lens mount, at the top on this picture

The FA could also be operated in a more conventional Center Weighted Metering mode, and in all honesty, the results of the AMP system were not that different from the center weighted mode. But the system required no intervention and no expertise of the user, and was rapidly made more efficient with the addition of more metering zones and the capture of more parameters (focusing distance, color of the subject, for instance). Current Nikon cameras have a color sensor dedicating to metering, with more than 1,000 metering points. Equivalent systems have been developed by Canon, Minolta and the other manufacturers, under different names: Canon’s system is “evaluative”, and Nikon’s AMP is better known now as “Matrix metering”.

Modern digital cameras all use elaborate variants of matrix or evaluative metering as their default exposure mode, and give good results in a huge majority of cases. They use so many parameters that it’s sometimes very difficult to understand how the camera chose a particular exposure value; in doubt, photographers can visualize the picture they’ve just taken on high definition displays, and use histograms to analyze the exposure of their pictures. Olympus’ Multi-Spot system happened to be too complex for the huge majority of photographers, and left no legacy.

More about Exposure and Metering

Cambridge in Colour. A good (and free) on-line tutorial about digital photography.
A good book about exposure: “Exposure“, by Chris Weston.

Destin- The beach (Nikon FA- Nikkor 24mm AF - Kodak Ultramax) Sept.09
Destin- The beach (Nikon FA- Nikkor 24mm AF - Kodak Ultramax) Sept.09

50 years of lens mount evolution (Part III of VI)

Automatic Exposure

The first SLR cameras with TTL metering were semi-automatic – the needle of a galvanometer indicated how far the shutter speed/aperture combinaison was from the ideal exposure, but it was up to the user to turn the aperture ring or the shutter speed knob to adjust the exposure parameters following the directions provided by the needle.

The next step was obviously to design a camera which would set automatically the aperture value or the shutter speed corresponding to the ideal exposure.

Olympus OM Mount
Olympus OM Mount: it remained unchanged from 1971 until the end of the production of the OM-4 in 2003. Note the design and the wonderful finish of this mount

Automatic exposure was first adopted by point and shoot cameras, and by a few amateur oriented reflex cameras.

Before Integrated Circuits capable of controlling a focal plane shutter became available, the simplest way to provide auto-exposure was to keep the conventional mechanical shutter, and to use a servo-motor to link mechanically the match needle of the exposure galvanometer to a series of rods controlling the aperture of the diaphragm. It was easier to implement such a system on a fixed lens camera, which explains why at first auto-exposure SLRs (such as the Mamyia Auto-Lux 35 or the Canonex) did not have removable lenses.

The engineers developing conventional SLRs had a few technical hurdles to pass before they could produce auto exposure SLRs capable of attracting a wider audience of seasoned photographers. The difficulties were cleared at the beginning of the 70s, and two types of automatic SLRs started hitting the market between 1971 and 1973.

Aperture Priority Automatic Exposure

The development of integrated circuits made Aperture Priority (pre-selecting the aperture and letting the camera set its shutter speed automatically) relatively easy to implement. The mechanically controled shutter had to be replaced by an electronically controlled model, but no change was needed on the lens mount -at least for Minolta and Nikon. Aperture priority cameras did not need a delicate linkage between the body and the lens, and could be used even with specialized diaphragm-less lenses (mirror lenses, for instance). Pentax had to adopt full aperture metering – they added a few cams to the 42mm screw mount for the introduction of the first electronic SLR, the Spotmatic ES. Nikon and Minolta launched their own aperture priority SLRs soon after (1971-1972). Olympus followed with the OM-2 in 1975.

Shutter Speed Priority Automatic Exposure

On the contrary, Shutter Speed Priority (pre-selecting the shutter speed and letting the camera set the lens aperture automatically) did not require any change on the shutter mechanism used on previous semi-automatic cameras, but precision mechanics was needed to ensure that the diaphragm would close at the exact value determined by the exposure system of the body.
The automatic diaphragms used in pre-selection lenses – see: 50 years of evolution of SLR lens mounts (Part I of VI) had so far been working in an all or nothing mode: full aperture, or closed down as far as the lens could go, namely to the aperture pre-selected by the user. With Shutter priority, the brute force approach did not work anymore: a lever on the mount (camera side) had to transmit to the lens the exact aperture value determined by the automatic exposure system, and all the lenses of the system had to react identically to the movement of the exposure lever of the camera.

The lenses and the mounts had to be modified again, either by the addition of aperture transmission pins (that’s what Canon did in 1971 with the new FD mount), or by the transformation of the all or nothing stop down command of the diaphragm into something much more linear, where the movement of the stop down lever was proportional to the value of the aperture to be used. Nikon chose the latter route with the AI-S mount introduced progressively after 1979, in preparation of the launch of future cameras offering Shutter Speed Priority and Programmed Exposure modes.

The first Shutter Priority SLR of one of the “big four” was were introduced by Canon (the EF model in 1973) following the tracks of Konica, which had been producing its Autoreflex T series since 1968.

More about it
A history of the Nikon F mount by Denton Images