The Canon 7D, like most cameras in this class, has 4 different metering systems:

  1. Evaluative
  2. Partial
  3. Spot
  4. Center-weighted average

The 7D uses 63 measuring zones with a different algorithm for each metering mode. This is a color sensitive system. The metering matrix has 2 layers:

  1. The first layer is for the red/green light.
  2. The second layer is for the blue/green light.

Evaluative metering

  • The exposure is set at the time of focusing, that is if you do not remove the focus from the shutter button.
  • The exposure is set at the half press of the shutter if you do remove the focus from the shutter button.
  • Pressing the * the AE Lock button at the back will lock the exposure. Now think of the gymnastic needed to use the * button and the AF-ON button. But there’s a somewhat rescue, the 16 seconds metering timer. Pressing the * button will lock the exposure for 16 seconds.
  • The exposure is very heavily influenced by the metering zone that overlaps the focus point and the adjacent metering zones. This also includes the focusing points that almost achieved focus1. This explains why in a serie of photos, when you are in AI Servo and Evaluative, suddenly one photo has a completely different exposure from all the other photos. It’s because the focus point and/or the almost achieved focus point has moved and is in a different light.

Center-weighted average metering

  • The exposure is determined at the time of taking the photo. The Canon 7D doesn’t care about the focus point.
  • It’s possible to lock the exposure for 16 seconds in Center-weighted average metering by pressing on the * button at the back.
  • The exposure locked is the one based on the whole frame, with the 10% center of the viewfinder representing the vast majority of the exposure metering.
  • The AE lock button is good for only 1 photo. If you want to take a whole bunch of photos with the exposure locked, you will need to keep your finger on the * button at the back. What happens with the AF-ON button? You must have some very flexible right thumb.

  1. To work with the iFCL metering sensor, the EOS 7D also features a specific metering algorithm. The EOS 7D always measures focus with all AF points regardless of the selected AF mode. During the exposure reading the EOS 7D looks to see which points, in addition to the selected point, have achieved or almost achieved focus. This information lets the camera know which part of the image is the subject. It then takes metering readings from the zones corresponding to the AF points that have achieved (or almost achieved) focus and combines them with readings from all the other zones. This allows for consistent shot-to-shot exposure, even in complex situations: for example, where there are reflections from a model’s glasses. (Quote from Canon, emphasis: mine)