Sometimes, it can be quite uncanny just how accurately our camera meters are at getting the right exposure. The technology is highly sophisticated, advanced sensors, complex calculations and large image databases combine to give stunning results. But not always. Like any machine, the best and most accurate computer is the one with the grey matter operating it. To truly master our cameras, it is important to understand metering, how it works and what modes we should use.
How Metering Works
A camera’s exposure meter may be advanced, but compared to our eye/brain combination it is pretty crude. If you have ever wondered why, when you take pictures of snow or brightly lit beaches, they often come out underexposed – here's why.
When you meter a subject, the exposure meter has to have some sort of reference point to compare it to. This reference point is, in crude terms, a card which is 18% grey. This card represents an average day at noon with equal amounts of red, green and blue light. When our meters read the light, they refer to a digit representation of this card and make their adjustments accordingly.
Of course this kind of light is actually very rare and so the metering systems compares its readings to a database of known scenes within its computer. This, for the most part, is accurate but sometimes can be confused.
In the snow example, the camera thinks the snow is 18% grey and under exposed, hence they grey looking images you often get. So with a basic understanding of how metering works, lets look at typical metering modes, namely, matrix, center weighted and spot.
A knowledge of metering preserves definition in the ice] by Jason Row Photography, on Flickr
Understanding Matrix Metering
Those of you with a liking for the works of Ansel Adams, may recognize matrix metering immediately. It is, in effect, Adams' zone system whereby the image is broken into multiple regions each of which it metered separately. An average of these metered zones ins then made to give us the recommended exposure. It is by and large the most accurate metering mode for general use, it is however not infallible.
If for example you wanted to take a backlit portrait of someone on a bright sunny day, you may find that for example 8 of the 9 metering zone are bright and one, the subject's face, is somewhat darker. When averaged out, the scene will expose mainly for the brightly lit background, making the face seem somewhat darker. For this and similar reasons, we have our next metering mode, center weighted.
This scene was metered using matrix mode by Jason Row Photography, on Flickr
As its name suggest, this mode gives more “weight” to the center of the frame. It still meters the outer parts of the frame but when averaging out the exposure, it gives a ratio of around 75% to the center and 25% to the outer regions. This makes it ideal for subjects such as the aforementioned backlit portrait, where we are looking for a smaller, central region to be correctly exposed.
Center weighted preserved detail in the penguin's feathers by Jason Row Photography, on Flickr
This mode is perhaps the most difficult to master and yet the most powerful to use. The metering area is around 6-10% of the central part of the frame with no weighting to the outer parts of the frame. This means it is ideal for subjects that may be small in the frame and or heavily backlit, for example a bird in flight.
However it is important that you meter exactly for the subject. If your metering point is a little off, in the bird in flight example maybe a little in front of the bird, your reading may be wildly wrong. For experienced photographers spot metering can be a great way to measure difficult lighting conditions, using a similar “zone” system similar to matrix metering but working out the calculations yourself to get optimum exposure.
Spot metered on the yellow building by Jason Row Photography, on Flickr
Understanding metering opens many doors to creativity in our photography. When we combine our knowledge of metering, with that of exposure compensation, understanding histograms and an understanding of light itself, we can become master’s of that light.
There are some more advance topics about metering that we will cover in a future article. This will include the difference between incident and reflected light metering, flash metering and white balance metering. All subjects that can further improve your understanding of light itself.