Light Metering in DSLR Cameras

Flynn Marr 27 October 2025

This is my Canon 60D DSLR
with its EF-S 17-55mm f/2.8 IS USM lens.

It occured to me the other day that I needed a refresher on how DSLR light metering works. I knew the theory vaguely but I was short on solid details. So let’s go over it.

Before we start, please understand that this subject involves advanced knowledge of optics and electronics which I don’t have so what follows here is a layman’s summary only.

Through the Lens (TTL) metering for Canon began with the Pellix camera in

1965. In the Pellix a Cadmium Sulfide (CdS) Photo Cell was inserted into the light coming in through the lens to read its strength. These cells were essentially a resistance element that changed resistance depending on the light that fell on its surface. These were used in the “F” Series cameras for several years but their use required the photographer to interpret their readings, through experience, to get satisfactory negatives. All they did was put a hand held light meter inside the camera.

Three developments made possible the more sophisticated light metering found in DSLR’s. The first was the introduction of the Silicon (Si) Photo Cell (SPC) which offered several advantages over the CdS Cell. They were small, very sensitive in low light, had a low “dark current” which meant a low drain on their battery, responded rapidly to changes in illumination, and were sensitive to a wide range of wavelengths. The SPC was basically a diode junction that allowed current to pass in proportion to the light falling on it. Canon’s first use of the SPC was in the EF camera of 1973.

The second development was the use of Integrated Circuits (IC’s) in cameras which began in 1976 with the introduction of the AE-1, the first camera in the world to make use an IC to control camera operation. Discrete transistors, resistor and capacitors were too bulky to allow for automation in the camera. However, once the miniturization possible with IC’s allowed true computing power to be built into the camera the way was open to more sophisticated light metering.

This is a representative diagram of a Silicon 16 Zone Metering sensor. The image falling on this sensor could be sampled by taking readings from individual sectors of the sensor. (The numbers are for reference here but they do not exist on the actual sensor chip.)

The single SPC sensor element was not able to measure different areas of the image and tailor the camera’s light reading to the highlights and shadows in a typical image. And so the third development was the multi zoned sensor element. In these the sensor was divided into zones that could be read individually. In the EOS 620, introduced in 1987, Canon introduced a sensor that was really six sensors on a chip that could sample points across the image. By sampling the individual sectors and applying different levels of amplification to them different metering patterns could be obtained.

As different models were introduced

over the years the number of zones on the sensor chip increased allowing for a finer texture and better definition of the Metering Modes . The Model 1D had 21 zones. The 30D had 35, the 60D 63 and the 5D Mk IV had 252.

Metering Sensors have changed over the years to extend sensitivity to a wider range of colors and to increase the pixel count in each zone into the hundreds of thousands. The metering sensors have become very much like the image sensors except with fewer pixels. The resolution of the metering sensor has become sufficient for them to be used for face and other object recognition tasks.

Canon quickly settled on four metering patterns that have been consistently used across their DSLR models for two decades. Canon’s symbols for these Metering Modes have been consistent across almost all of the Digitals SLR’s appearing in the User Manuals and on the LCD Screens and Menus of the cameras themselves.

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Evaluative Metering

For this metering mode the Canon 5D Mk IV User Manual says: “This is a general-purpose metering mode suited even for backlit subjects. The camera sets the exposure automatically to suit the scene.”

For general purpose photography this is the recommended setting. All sectors are sampled and then based on the cameras analysis of brightness, contrast, distance, and Focus Points the camera choses the best exposure parameters.

Center Weighted Average Metering

In this Mode the metering is weighted at the center and then averaged for the entire scene. Looking at the diagram of the 16 Sector Sensor above, the camera measures the output from each sector and then selectively amplifies the output of the Sectors near the center, such as Sectors 4 to6 and 8 thru 12 in our diagram to arive at a correct exposure setting.

Partial Metering

The 5D Mk IV User Manual says this mode is “Effective when the background is much brighter than the subject due to backlighting, etc. Partial metering covers approx. 6.2% of the viewfinder area at the center”. Looking at the diagram above this would be accomplished by basing exposure calculations on Sectors 5, 9, 10, 11 and 5 again.

Note that the more Sectors there are the more accurate the selective amplification can be and the more finely adjustable that 6.2% is.

Spot Metering

This is exactly what it says: it measures the light level from a “spot” in the image. In our example this would mean reading Sector 10. Obviously, the more sectors there are the more accurately the size of that spot can be set.

As the Digital SLR’s got more complex the number of Sectors went up and this meant that the computing power required was increasing as well. This was provided by Canon’s “in house” Digic processors. These went through several increasingly powerful versions from the original Digic in 2002 to the Digic X in 2020.

But physically, how was the Metering Sensor illuminated? That is an interesting piece of mechanical optical engineering as well. First, we need a diagram.

As we all know, the image comes into the camera through the lens and reflects straight up off of the reflex mirror and forms an image on the Focusing Screen. This Screen is where some of the magic happens.

The Focusing Screen is actually a sandwich of optical layers which are glued together. One of these layers has a pattern of optical wedges impressed into it which manage to divert a small portion of the incident light into a slightly different path which is shown in red on our diagram. This sends a complete copy of the image to a separate prism and lens combination above the eyepiece where it falls on the Silicon Photo Cell.

The amount of light diverted is not large because the SPC is sensitive and the output signal is amplified when received.

Another system you will see used has a semi transparent main mirror with a secondary mirror behind it which diverts a copy of the image onto the SPC which is located in the bottom of the mirror box. During exposure the secondary mirror folds up against the main mirror as it lifts up and out of the way. The degree of transmission through the main mirror is controlled by the amount of silvering deposited on it.

And that is a brief explaination of how Canon’s Digital SLR’s managed their exposure control. The only way to really take control of this subject is to take your camera out and use it. Then you have to examine the images you capture critically in light of the settings you have chosen.

Some photographers don’t like the “in camera” meters and insist on using their hand held meters instead. This is a perfectly workable choice but it does require a greater undersanding of the light and shadow encountered in a scene. And the extra piece of equipment is annoying.

I do recommend to any one intending to take the best pictures they can to study all aspects of their camera including the built in light meter and learn how to expose for maximum light and shadow detail. The only way to do this and understand it is to take pictures and then study what you get. Over time you build up a second sense about your camera and how to use it for best effect.

This website is the work of R. Flynn Marr who is solely responsible for its contents which are subject to his claim of copyright. User Manuals, Brochures and Advertising Materials of Canon and other manufacturers available on this site are subject to the copyright claims and are the property of Canon and other manufacturers and they are offered here for personal use only. Use of this website is governed by the Terms of Service set out on the page by that name.

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