1. Tables: The very first support tools for photographers consisted of exposure tables that have evolved since 1844, almost 20 years after the invention of photography. The tables contained suggestions for exposure durations based on weather and light conditions, time of the year and time of the day, type of subject and film speed. As there were no powerful lenses used at that time, the exposures suggested by the tables typically ranged from several minutes in bright daylight to a good hour in cloudy conditions.

2. Calculators: In 1888, the first exposure calculator – called Actinograph – was constructed by the scientists Hurter and Driffield based on their scientific research. The Actinograph and subsequent calculators were based on the exposure tables, but combined various slide rules and rotating scales. The calculators were the first approach to ease camera operation for photographers.

3. Actinometers: Actinometers were the first instruments not relying on pure experience but actually performing some active light metering. In an actinometer, a small test exposure was made on light sensitive paper. The usual procedure was for the paper to be exposed until its tint matches that of a comparison surface. The time to darken the test paper was then translated into the input values for the shutter speed and aperture value. Actinometers typically had the shape of a pocket watch and became popular during the 1890s and remained so through to the 1920s. Compared with the tables and calculators, the actinometer was independent from environmental characteristics and therefore profited from more versatile applications.

4. Extinction Meters: Another type of active metering tool was the extinction meter. This instrument did not perform a test exposure on photosensitive paper like the actinometer and presented some quicker readings to the operator. Through the viewfinder of the extinction meter, a photographer looked at a row of numbers, arranged behind a celluloid surface that is gradually increasing its opacity. Depending on the particular design, the highest or lowest number just barely visible was determining which light situation was given. This number could then be translated into the required camera exposure settings. Other versions of extinction meters had a pattern visible through the eyepiece, and a control dial varied the amount of light allowed into the device until the pattern could only just be seen. The position on the control then indicated the exposure value. These tools typically had a telescope shape and were rotable to adjust for pre-defined camera settings such as film sensitivity. Extinction meters have been very popular in the 1920s.

1. Selenium Photocells: The first photoelectric exposure meters consisted of a single selenium photocell, a type of semiconductor that became commercially available in the early 1930s. A selenium cell consisted of a steel support plate that had one side covered with selenium, and an ultra-thin layer of gold applied above the selenium layer. Light illuminating the top surface of the selenium cell generated an electric current between the gold layer and the steel plate as soon as these were connected. The more light illuminated the photocell, the more current was generated. That current was tiny but it was displayed by a needle moving over a scale, and a calculator was then used to combine the reading with the film speed to give the result of shutter speed and aperture. An advanced version of this system was called the match-needle meter as described in the previous chapter.

The first selenium light meters were packed into separate handheld devices. The clip-on-camera implementation of the light meter offered a certain degree of integration and was the beginning of exposure automation in photography. The original Canonflex SLR camera (1959) is an example of a camera system with a clip-on exposure meter that was directly coupled with the speed dials. It was not long before cameras around the 1960s had the metering system integrated into the camera body. The Canonflex RM (1962) already had a selenium photocell built into the front of its case. A window at the top of the camera displayed the needle that pointed at the correct aperture that was required depending on the shutter speed that was selected. In operation it was similar to the clip-on meters on the original Canonflex, but the reduced size was another major step in the evolution of exposure metering. With the rapid developments in exposure metering automation, most manufacturers stopped the production of handheld meters until 1970.

Selenium had several favorable properties as photocell material. As selenium cells induce their own current during exposure to light, no batteries had to be used. Furthermore, selenium cells were sensitive to wavelengths similarly to camera film, as opposed to the human eye. Therefore, these types of sensors reacted to light more like camera film and provided quite accurate results. On the other hand, selenium cells would eventually lose capacity over time. With the devices directly integrated in the camera bodies, they were much harder to replace. In addition, selenium photocells often required a significant surface area to generate enough current to drive the meter's needle, often measuring around 40 mm × 40 mm. Around 1960, another material has already been found to eliminate these disadvantages – cells made of cadmium sulfide.

2. Cadmium Sulfide Photocells: Unlike a selenium cell which created current during exposure to light, cadmium sulfide (CdS) photocells were resistors that varied depending on the intensity of light. More precisely, the resistance dropped as the illumination increased. A battery was used to provide an electrical current, and a cadmium sulfide cell varied that current depending on the exposure. Again, a needle was used to display the current on a scale. Compared with selenium cells, one advantage of cadmium sulfide cells was their high durability. The system relied on a current produced by a battery which could easily be replaced once depleted. Also, CdS cells were more sensitive and offered a more accurate reading in low-light conditions.

Their strongly reduced size made them even more suitable for an integration inside the camera body. In particular, the small CdS photocells were the first metering systems that have been installed behind the photo-taking lens so that they would receive the exact intensity of light available on the focusing screen. This technology was called through-the-lens (TTL) metering and brought exposure metering to a whole new level.

Being innovative and small, CdS cells also had some drawbacks. One minor drawback was that they reacted to light similarly to the human eye, which was not the same way most camera films reacted to light. Another major disadvantage was that CdS cells suffered from a temporary night-blindness – an effect where the cell was unable to quickly adapt from a bright scene to a dark one. However, in the early 1970s, camera manufacturers again upgraded their metering systems with a revolutionary new material – silicon.

3. Silicon Photocells (SPC): Silicon photocells combined all the best properties of both selenium and cadmium sulfide cells: Just like cadmium sulfide cells they are small, they offer good performance in low-light conditions, and they are driven by batteries. Like selenium, they rapidly adapt to intensity changes and they react to a wider range of wavelengths. The Canon EF SLR camera (1973) used a silicon meter instead of a CdS meter. Since then, this new material has increasingly gained popularity and widely replaced its predecessors.

4. CPU-Controlled Auto-Exposure Metering: Up to the mid 1970s, electronic circuits were still very limited inside SLR cameras, and almost all of the camera's functions had been carried out by mechanical parts. The Canon AE-1, introduced in April 1976, was the first camera in the world to incorporate a central processing unit (CPU) to control its core operations. These included the auto-exposure system, aperture calculation, shutter control, and others. A silicon cell located inside the viewfinder was used as the light-sensitive element.

This general principle of TTL exposure metering and signal processing by a CPU has remained unchanged right up to the last generations of DSLR cameras. All the advancements in exposure metering technology since the 1970s have primarily concerned on the metering sensors, which have been divided into multiple zones and optimized for color recognition.

Through-the-lens (TTL) metering is an approach for in-camera light metering that was introduced in the early 1960s, and has since been constantly improved and refined. TTL metering is still state of the art for all modern DSLR cameras. The concept is to have a metering sensor built somewhere into the camera body where it can use the light that has passed through the main photographic lens. The actual measurement is typically performed on the preview image which is projected on the focusing screen. Depending on the camera model, the metering system can also use light-sensitive elements inside the autofocus detector which is located in the lower part of the camera body. Regardless of the exact position of metering sensors, the basic principle for TTL light metering is always the same. TTL metering is considered to be the most precise type of exposure metering as it takes into account the focal length as well as individual lens transmission characteristics.

The Canon F-1 SLR, introduced in 1970, had a CdS cell attached to the side of the focusing screen. A beam splitter built into the focusing screen directed a small fraction of light from the focusing screen towards the metering sensor. The beam splitter was designed so that it would only redirect light from the central 11% of the focusing screen surface towards the metering sensor.

The Canon EOS series of cameras, introduced in 1987, have their metering sensor attached to one face of the pentaprism directly above the eyepiece lenses. In this position, the sensor receives light that is emitted by the preview image.