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.
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:
“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.
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.