Last weekend, I was in Seattle taking photos at the famous Pike Street market. It is a very difficult lighting with daylight, fluorescent, bright sunny shafts of light with very dark shadows. I was experimenting with the various metering systems. The Canon 7D has 4 different exposure metering modes:

  1. Evaluative metering
  2. Partial metering
  3. Spot metering
  4. Centre-weighted average metering

Pressing the shutter button half way is supposed to lock the exposure. Actually it does not!. The shutter exposure locks the exposure ONLY in the evaluative metering.

Pressing the shutter button half way does nothing when the metering is done in partial, spot or center-weighted average metering.

The only way of locking the metering when in partial, spot or center-weighted average metering is to use the AE Lock, the * button at the back.