Locking the exposure with AE lock
Use the AE lock button to store settings for consistency when taking a sequence of shots
With evaluative metering, the exposure is locked when you half-press the shutter release in One-shot AF mode. This isn’t the case when you’re shooting with one of the other metering modes; the autofocus might be locked when you half-press the shutter, but the meter stays active and exposure settings may change.
When you’re using AI Servo AF, the evaluative meter reading, like the autofocus, isn’t locked when you halfpress the shutter release. As with the partial, spot and centre-weighted average metering modes, the meter stays active and the suggested exposure may change from one shot to the next.
If you want the exposure to be consistent throughout a sequence, use your camera’s AE lock button – the one marked ‘*’. Lightly press the shutter release to activate the meter – a suggested aperture and shutter speed combination appear in the viewfinder – then press the button. The ‘*’ icon appears in the viewfinder. Keep the button pressed down to keep the setting locked in as you move the camera around. Some cameras enable you to customize this control so you don’t have to keep the button pressed down to retain the exposure settings.
Using AE lock with partial or spot metering is a quick way to set an exposure based on a small area. Remember, your camera wants to make images a midtone brightness level, so if you take a partial or spot meter reading from a bright or dark area, exposure compensation makes it bright or dark again.