35mm Film Camera Light Metering

The extinction meter was a beautifully simple solution, but in use it suffered from subjective interpretation, and variations in the sensitivity of the human eye, which differs from person to person.

Photovoltaic Selenium light meters came next. This material converts solar energy into electrical current, and generates a tiny voltage proportional to the intensity of light exposure. Both hand held, and built-in Selenium Meters became commonplace. They even allowed the development of simple automation, where mechanical systems exploited electrical deflection of a meter needle pointer to induce other physical changes (to set apertures or shutter speeds).

Selenium meters were inexpensive to make, and cost nothing to run, and did a pretty good job provided that they were not exposed to moisture. The most iconic exposure meter of all time – the Weston Master – was a Selenium meter.

The shortcoming of Selenium was that it is incapable of measuring lower light levels accurately. This became an issue as ever-faster films were developed, and so CdS (Cadmium Sulphide) replaced Selenium. CdS meters work differently, and exploit the phenomenon of photo-resistance. CdS is a material with an electrical resistance to the passage of a current that changes proportionately to the intensity of light exposure. Accordingly, CdS based meters require a battery to provide a current.

Most manufactures incorporated CdS meters in their cameras, as did the makers of hand-held systems, but CdS eventually gave way to another material; Silicone. This worked in the same way as CdS, but was even more sensitive to lower light levels, and reacted faster to changes in illumination levels. Silicone is today’s standard light measuring material.

Once Cds had been adopted, other forces (technological advancements, and consumer needs) conspired to make light meters an integral, and internalised component of the evolving camera, and design moved towards the exposure meter measuring light on its path to the film, rather than capturing an approximate measure of lighting levels in the vicinity of the subject. Here I am thinking and writing primarily about 35mm SLR cameras, and the development of through-the-lens metering.

The advancement of battery operated, CdS meter equipped camera brings us to the point of this article, since the following typical metering systems were adopted by manufactures.

Average metering

This is the simplest form, where the camera will use all the light coming from the entire scene to determine the exposure setting. No weight is given to any particular portion of the metered area, so an anomalous bright spot, for example, can result in overall under exposure. True average metering is a very rare thing. The vast majority of 35mm SLR film cameras employed the second form of metering.

Centre-weighted average metering

In this system, the meter concentrates between 60 to 80% of its sensitivity towards the central part of the viewfinder. The advantage of this method is that small areas at the edges of the viewfinder that vary greatly in brightness have less influence, and most subjects are generally in the central section of the frame anyway. In truth, centre-weighted metering was more of a consequence that design feature, since light scatter from the focusing screen coupled with the positioning of the meter cell(s) naturally caused an intensity fall off at the edges.

Partial metering

This type of metering works on the same principle as centre-weighted average, but intentionally ignores areas on the edges of the frame, which could otherwise influence the metering unduly if there are either very bright or dark. Partial metering typically concentrates on around 10-15% of the entire frame. Canon was a manufacturer quite keen on this system at one time.

Spot metering

Here the meter will only measure a very small area, typically at the centre of the view screen, and usually between 1-5% of the viewfinder area. Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot high contrast scenes. For example, backlit subjects, where perhaps a face is much darker than the bright halo of sunlight around the subject. Spot metering enables the photographer to select which element of a shot is correctly exposed, and the consequential under or over exposure of other, less important areas. Spot metering was usually a second option on high-end cameras, and not an every day metering pattern.

Matrix or Multi-zone metering

This is a much later development, where the camera measures the light intensity at several points in the scene, and then combines the results to find the best compromise exposure setting. Matrix metering was first seen on the Nikon FA, back in 1983. This pioneering camera didn’t sell well, because nobody understood how its metering system worked, and so didn’t trust its accuracy. Yet today, this system is the basis of evaluative metering, the name by which Matrix or Multi-zone has become more commonly known in digital cameras.

Obviously, as we move down this list, the metering systems become potentially more accurate, but the desirability of any system really depends on what you mostly want to photograph. There are no hard and fast rules to say which type of metering is most appropriate, and familiarity with the performance of any metering system is really the best way to exploit its particular strengths and weaknesses.

Once upon a time, the photographer might notice a bright spot in their centre-weighted metering viewfinder, and realise they needed to compensate, without recourse to more sophisticated metering systems. Evaluative metering has found its place in the modern age because photographers now want cameras to do the thinking for them, and they trust their superior powers.