What do the various metering modes and icons mean?

Canon cameras support a number of different ways of metering light coming in through the lens. The midrange and professional models let you choose which metering mode you want, and consumer cameras generally default to evaluative in most settings with partial as an override option. Here are the various metering modes.

Evaluative metering.
Evaluative metering is the most automated metering mode. In this mode the image is divided into a number of zones - usually 3, 6, 16, 21 or 35. The camera’s computer then looks at the metering zones and applies various algorithms (computer programs, essentially) to guess a likely exposure setting. It then chooses appropriate shutter and/or aperture settings based on these calculations. Unfortunately, Canon have not published details of how these algorithms work. Nikon, incidentally, call this type of metering “matrix metering,” and sometimes people use the term “matrix” to refer to all forms of multiple-cell computerized light metering.

Evaluative metering usually works reasonably well, though the meter can often be fooled by extreme metering conditions - such as a person backlit with a bright light. A larger number of metering zones does not, however, necessarily mean improved metering. Some cameras with 6 metering zones can meter just as well or as reliably as another model with 35 - it really depends on the camera model. Evaluative metering is convenient but, since it’s so automated, doesn’t teach you much about the fundamentals of metering.

Evaluative metering is identified in midrange and pro EOS models by the [(*)] symbol.

Spot metering.
Spot meters examine a very small area (a spot) of the overall image - usually just 1% or 2% or so. They’re popular with experienced photographers who select an area that they want to appear as light grey on the final image and use that to meter from. Spot metering is an essential tool for metering in challenging light situations, but is harder to master from the point of view of the novice. Only professional and semi-professional EOS models offer spot metering. Some also offer multi-spot metering, which allows you to select multiple spots and then average out the readings.

Spot metering is identified in midrange and pro EOS models by the [ * ] symbol.

Centre-weighted averaging metering.
This mode essentially simulates the typical metering mode used in cameras sold in the 1970s. Such cameras average the total amount of light coming in across the whole image but give a bit more importance (weight) to the centre. Unfortunately Canon do not publish the weighting percentage and weighting diagrams for most of their cameras, so only experience will tell you how this mode works.

Though technically simple, this metering mode works well for images which have relatively little variation in light level across the scene. A classic example might be a landscape on a sunny day. The sky at the top will be fairly bright, but since the metering is centre-weighted the bulk of the scene should be metered correctly.

Centre-weighted averaging metering is identified in midrange and pro EOS models by the [ ] symbol.

Partial metering.
Very similar to spot metering, only a larger area of the image is used - typically 6.5%, 9.5% or 10%, depending on the model. Think of partial metering as a very fat spot. Some cameras with multiple focus points tie the area to be metered to the currently selected focus point.

Partial metering is good for giving you more control over metering results. For example, let’s say you’re trying to take a photo of something which is surrounded by darkness. Evaluative metering might be a problem as it might be thrown off by all the dark areas. With partial you can select a section of your image that you want to be medium grey and then you don’t have to worry about the meter being fooled by the stuff around it.

Partial metering is identified in midrange and pro EOS models by the [( )] symbol.