Now that we have reviewed how exposure works it’s time to talk about the camera’s built-in light meter. The camera’s light meter is what makes auto-exposure modes possible. While the concept of a light meter is pretty straight forward, how it can be used and manipulated by the photographer is a whole ’nother story.
First off it helps if we take our idea of a built-in camera light meter apart. When we refer to the camera’s light meter we are actually lumping two separate components under the umbrella of one name. The first component is the actual meter itself and the second is the computer that processes the meter’s light reading.
The computerized part of the metering system allows the photographer to manipulate the readings taken by the actual meter sensor. This manipulation is a very useful skill and it will be covered in the next article in the series. For this article we are only going to discuss the actual meter sensor and how we can affect the way it is exposed to light.
Handheld Light Meter
A light meter does only one thing: it measures and then quantifies the amount of light that strikes its sensor. That’s all there is to it. Light of X intensity will always give us a reading of Y. The trick for pre-1960’s photographers was to convert the reading from a handheld light meter into a setting for their cameras. Today our built-in meters speak to us in stops of aperture openings and shutter speeds. The translation from meter-speak to photo-ese is now built in.
Now I know that I sound like a broken record, but for all of the computer programming found inside of today’s cameras they can only be called smart, not intelligent. The cameras are really good at doing what their vast programming tells them to do, but they are really bad at making judgments and decisions. In order to get the best performance from our hardware we occasionally need to make some settings for ourselves. This is especially true when it comes to how the camera meters light.
The way our camera meters read light can be altered to suit our shooting situation. In general there are three settings common to all DSLR cameras: Multi (aka Evaluative, Matrix), Center Weighted, and Center Only (sometimes called spot metering).
The Multi-Meter mode is the most highly programmed metering mode. As such each manufacturer puts their unique spin on it. Yet in essence they are all the same. Multi-Metering evaluates the entire scene and compares the brightness and darkness of different areas. Comparing those metered values to a built-in table of common exposure challenges and scenes a meter reading is output. The exposure calculation is strongly biased for the area immediately around the focus target that registers as being in focus.
Multi-Metering is deeply tied into the onboard program of the camera. Part of the meter’s evaluation is in an assumption of the scene type that the camera is pointed at. For example if the meter senses lots of bright values in the top half of the frame and darker values in the bottom half it might assume that the scene is a landscape (bright sky, darker ground). To capture a landscape the camera will try to expose more for the ground allowing the brighter sky to become over exposed.
Now lets’ take a look at a similar example: Bright top half of frame and darker bottom half. The system makes a similar assumption as with the landscape above. The camera returns a meter reading that is biased toward the darker lower part of the scene. Oops, it wasn’t a landscape it was a stage performance. Exposure failure results.
There are a number of ways to work around the stage lighting example above but the easiest is to simply change metering mode. If we change the camera meter from Multi-Metering to Center Weighted we can overcome the computer’s biases which are based on pre-programmed scenes.
Center Weighted Metering can vary by manufacturer but those variations are minor. In general with Center Weighted Metering the camera meter once again measures the light across the entire scene but gives more weight to the center portion of the scene – a lot more weight. In most systems the center portion of the scene will account for 80% of the meter value.
Center Weighted Metering Pattern
A camera set for Center Weighted Metering used at our stage performance example wouldn’t have been fooled by the much darker foreground in the scene. Center Weighted is also a good choice for outdoor scenes with high contrast differences between the subject and the environment – such as sporting events. Imagine a soccer player brightly lit in the foreground and the darker bleachers across the field in the background.
One complaint about using Center Weighted Metering is that there is a tendency to ‘bulls eye’ the subject in all compositions. In other words everything always centered in the frame and the composition can look static. As a way to overcome this some manufacturers will allow the weighted metering zone to be centered on whatever focus target has been selected by the photographer. Your camera’s instruction book will tell you if your model has this neat trick up its sleeve.
Our last metering type is called Center-Only Metering. Your camera may call it ‘Spot Metering’*. This type of metering uses the narrowest part of the scene to measure the light, something around 2.5% of the entire scene. None of the area outside of this narrow target area will be measured or used to calculate an exposure.
Center-Only Metering Pattern
Center-Only Metering is quite precise given the extremely narrow area of the scene measured. Often a photographer will select Center-Only metering in a highly complicated environment. For example a portrait setup under a tree with dappled areas of bright sunlight. Other photographers will use the meter to take several readings from different but important areas of the scene and then calculate their own exposure settings.
Applying Center-Only Metering to the original stage performance scene mentioned earlier: Center-Only could be the best setting if the performer is in a spotlight on a stage with an equally dark background and foreground.
Again, manufacturer variations exist. For example Canon offers two different Center-Only options on some camera models: a 9% “Partial” setting and a 2% “Spot” setting. The percentage of Center-Only coverage can vary between brands and even between models from the same brand.
Summary: The behavior of the camera’s built-in meter is an important component in overall exposure control. Today’s cameras have three common metering types built-in:
• Multi-Metering – the most automated and the easiest to use. Accurate for general photography
• Center Weighted Metering – Removes the camera computer bias from the exposure calculation. Good to use in higher contrast environments. Still generally easy to use.
• Center-Only Metering – Meters light from a very narrow part of the scene. Especially useful in complicated lighting because it restricts the meter reading only to the area defined.
Setting the meter type is easy to do on DSLR cameras. Usually the setting is made with a dedicated switch or through a top level menu. Please review your instructions so that you know the icons used in your displays to indicate the metering type chosen. And like all of the settings discussed to date, metering types cannot be changed by the photographer in the Easy (Green Zone) or Icon modes.
Metering Pattern Icons for Nikon and Canon
*Though camera manufacturers use the name ‘Spot Metering’ to describe their center-only metering type it’s an inaccurate label. A true spot meter measures light in a very narrow range of 1 degree or less. This is an incredibly tight angle of view and it is a standard that photographers who shot transparency film and those who used the Zone System relied upon for accuracy. No camera with a through the lens meter could ever claim to have a true spot meter because the degrees of scene coverage would change with each lens mounted and at every zoom length.