Using Auto Exposure (AE) Lock
Auto Exposure Lock (sometimes called AEL) is a feature on all EOS cameras. It’s an excellent method of gaining added control over exposure, without losing the speed and convenience of automation.
What AE Lock does is simple: It “freezes” the camera’s exposure settings (shutter speed, aperture, ISO and white balance), so that if the camera is moved from one area to another, the auto exposure system won’t change aperture/shutter speed values. There are many situations where this may be useful:
AEL = Auto Exposure Lock. Whatever the exposure (basically the f/stop and shutter speed combo) is when you press the button will stick even if you move the camera. This is useful when you're trying to meter off something that isn't centered in the image (point the camera at the subject first, hit the AEL button, and then reposition).
FEL = Flash Exposure Lock. Similar to AEL, except that it locks the power output of the flash
All EOS models (to date) automatically lock exposure when you’re using Evaluative metering and One-Shot AF mode. Press the shutter button half-way down, and the exposure settings will be locked in-place with no further effort on the photographer’s part. If you keep partial pressure on the shutter button, you’ll see as you move the camera side-to-side that the shutter speed/aperture numbers don’t change. Pull your finger off the button, and the camera immediately begins to update exposure settings as the camera is moved.
However, this only happens when you combine One-Shot AF mode with the Evaluative metering. Switch to AI Servo AF, and/or use any other type of metering, and exposure always continually updates itself as you move the camera.
All EOS camera models, film and digital, have a button on the back of the camera that’s marked with an asterisk or star icon.
This is the AE Lock button. Pressing it when you’re in any “creative zone” auto exposure mode — P, Av, Tv, or A-DEP — will immediately lock exposure in-place, and you’ll get an asterisk icon in the viewfinder to advise you of this.
The procedure is pretty simple:
The asterisk will appear in the finder, so you know exposure won’t shift as you move the camera to re-compose the shot. You do need to keep pressing the shutter button half-way to keep the meter active (and locked); if you were to pull your finger totally off the shutter button, the camera would turn the meter off in about 4 to 6 seconds, and at that point you’d lose the reading you just locked-in.
As mentioned above, it’s not necessary to do this if you’re in One-Shot AF with Evaluative metering; just pressing the shutter button half-way will freeze exposure. But all other AF or metering modes require you to press the AE Lock button if you want to lock exposure.
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