35mm Film Camera Light Metering Systems
The first models of light meters were referred to as extinction meters. They were handheld devices or, in some cases, integrated into cameras. They were made up that contained neutral density filters, which were arranged in an order with increasing the thickness. Each filter had a specific numeral or letter engraved on the surface. The photographer was required to look at their subject through the extinction meter and then decide which filter allowed them to see the mark. The letter or number was later compared to the chart, which showed the most appropriate combination of aperture and shutter speed for a specific speed of the film.
The extinct meter is a beautifully simple device. However, its use was hampered by an interpretation that was subjective and also variations in the sensitiveness that the eyes of humans have that differs from one individual.
Photovoltaic Selenium light meters were the next. This material converts solar power into electrical current and produces a small voltage proportional to the amount of light. Handheld and integrated Selenium Meters became commonplace. They also allowed for the development of basic automation, where mechanical systems utilized an electrical deflection in the needles of meter gauges to cause different physical modifications (to create apertures or shutter speeds).
Selenium meters cost a lot of money to construct and were free to operate, and they performed pretty well as long as they were not exposed to water. The most famous exposure meter ever is The Weston Master – was a Selenium meter.
The drawback of Selenium was the fact that it was not able to measure low light levels with precision. This issue became a problem when ever-faster film technology was developed, and CdS (Cadmium Sulphide) was introduced to replace Selenium. CDs meters function differently and take advantage of the power of photo-resistance. CDs are a type of material that provides an electric resistance flow of a current, which is inversely proportional to the amount of light. In turn, CdS based meters require batteries to supply an electrical current.
Many manufacturers included CdS meters into their cameras, and so did manufacturers of handheld systems. However, CDs eventually changed to a different type of material called Silicone. It functioned in a similar fashion to CdS but was more sensitive to illumination levels and responded quicker to changes in light levels. Silicone is the current standard material for measuring light.
After Cds were adopted, however, other factors (technological advances, as well as the demands of consumers) combined to make the use of light meters an essential and internalized component of the ever-changing camera and the design shifted towards a measurement of light along its journey to the film instead of recording an approximate measure of the lighting levels within the vicinity of the object. In this article, I am writing mostly on 35mm SLR cameras and the development of through-the-lens measuring.
The development of a battery-powered, CdS meter equipped camera is the main point of this post because the following metering methods were adopted by the manufacturers.
This is the most basic form that the camera that can utilize all the light from the whole scene to determine the exposure level. There is no weight given to any specific portion of the metered region, therefore an unusually bright spot, like it could cause the overall under-exposure. True normal metering is an uncommon thing. The majority of 35mm film cameras used the second method of meters.
Metering using a centre-weighted average
In this setup, the meter focuses up to 60-80 per cent of its sensitivity to the centre of the frame. The benefit of this technique is that tiny areas close to the edges which are extremely bright have less impact as the majority of subjects tend to be located in the middle of the frame, regardless. In reality, the centre-weighted metering was more of a consequence of the design of the feature, as light scattering from the focusing screen, coupled with the position on the cell(s), naturally led to an intensity drop towards the edges.
This type of metering operates using the same principles as the centre-weighted average. However, it deliberately ignores areas along the edge of the frame, which can affect measurement too much if they are areas that are very bright or dark. The majority of partial metering is focused on about 10 to 15 per cent of the frame. Canon was a company enthusiastic about this method at one point.
This meter can only take a small portion of the screen that is usually at the centre of the screen and typically around 1-5 per cent of the viewfinder. Spot metering is highly precise and doesn’t get affected by other parts of the frame. It is typically employed to capture high-contrast scenes. For instance, subjects that are backlit are those where a face is significantly lighter in contrast to the brilliant halo that is visible surrounding the subject. Spot metering permits the photographer to decide which part of the shot is exposed correctly and, consequently, the under or overexposure of the other regions. Spot metering is typically used as a second option with the top cameras, but it was not an everyday pattern of metering.
Matrix or Multi-zone Metering
It is a later invention wherein the camera is able to measure the intensities of light at different locations in the scene and then uses these results to identify the most suitable compromise setting for exposure. Matrix metering was first discovered using Nikon’s Nikon FA back in 1983. The camera that pioneered it didn’t do well because no one understood how the metering system operated and didn’t trust its precision. Today, the system is the foundation of Evaluative Metering, the term that Matrix or Multi-zone is becoming more widely known in digital cameras.
As we go through this list, meters become more accurate. However, the effectiveness of any system is contingent on what you’re most likely to capture. There aren’t any hard and easy rules for determining which metering method is best suited to your needs, and having a good understanding of the capabilities of any metering device is the best method to make the most of its strengths and weakness.
At one time, photographers could spot an area of light in their metering viewfinder with a centre-weighted centre and then realize that they have to adjust, but without recourse to advanced metering systems. Evaluation metering is now a necessity in the present day since photographers today want cameras to perform the thinking for them and believe in their superior capabilities.
My collection of personal film cameras includes 35mm SLRs from Contax, Fujica, Minolta, Nikon, Pentax, Topcon and Yashica, which were produced between the mid-1960s and the early 1980s. They all utilize centre-weighted average metering. I don’t know which metering method my digital camera uses, and I’m sure it functions.