Auto Exposure Lock (sometimes called AEL) is a feature on all Canon EOS camera models, as well as some PowerShot cameras. It’s an excellent method of gaining added control over exposure, without losing the speed and convenience of automation. In fact, since most photographers today use some form of auto exposure the majority of the time, an understanding of how AE Lock operates can add a new dimension to their photography.
Photographers experimenting with the EOS 5D Mark II Movie Mode may find it necessary to master the use of AE Lock, as this function offers something close to manual control over the exposure during filming (you can read about Vincent Laforet's experience with AE Lock on the EOS 5D Mark II during the production of his short video Reverie here.)
What is AE Lock?
What AE Lock does is simple: It “freezes” the camera’s exposure settings, 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. A photographer shooting a portrait, for example, might want to place the subject off-center. Taking a meter reading off the subject, locking it (along with focus), and then moving the camera to re-compose the subject means that exposure won’t shift if the background is lighter or darker than the subject itself. Another example might be a shooter taking a sequence of images, panning the camera from one area to another (following a moving subject, for example). If there are differences in the background or lighting, it’s possible that exposures will vary from one shot to the next. With AE Lock active, exposures would be consistent from shot to shot.
Evaluative metering and One-shot AF mode:
Many EOS shooters already use Auto Exposure Lock and don’t even realize it. 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.
The AE Lock button:
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.
So the procedure is pretty simple: Aim the camera at the part of the subject or scene that’s most important to meter accurately. Press the shutter button half-way to start metering, and then press the rear AE Lock button. 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.
Using AE Lock for successive pictures:
Normally, if you lock exposure with the AE Lock button and then take a single shot, the locked exposure will be cleared within a second or two after you pull your finger off the shutter button. If you want to take a series of shots using the same locked exposure reading (such as a sequence of portraits, for instance), the simplest way is to just keep pressing the AE Lock button. You can also hold the reading in place by maintaining half-pressure on the shutter button after you take each picture, but sometimes this can be tricky, and if you accidentally lift your finger completely off the button you may lose the reading and have to repeat the procedure to set it again.
Clearing a locked exposure:
Once you’ve pressed the AE Lock button, the camera holds the meter reading in place. If you change your mind and want to totally eliminate AE Lock, don’t press the AE Lock button again. That simply takes a new meter reading and locks it in.
Instead, either pull your finger off the shutter button and wait 4 to 6 seconds, and wait for the finder display to go blank. Then, press the shutter button back down again. If you’re in a hurry and don’t want to wait a few seconds for the reading to clear, press the AF point select button (right next to the AE Lock button, on the back of the camera), and then immediately let it go and press the shutter button again. You’re back to live metering that’s not locked in place.
What if you’re using “back-button AF”?
Custom Function options differ from model to model, and not every EOS cameras has these options. But in general, EOS digital SLRs that offer back-button AF will provide some choices similar to these:
“AE Lock / AF” (or similar)
The rear AE Lock button will activate Autofocus when pressed. Pressing the shutter button half-way will lock exposure at that time, and exposure will not shift if the camera is moved to a lighter or darker area. In other words, AE Lock is moved to the shutter button.
“AE / AF lock, no AE Lock” (or similar)
No AE Lock possible at all. Exposure will adjust continuously as the camera is moved if you’re in the P, Tv, Av, or A-DEP modes. AE Lock button is strictly to activate AF. Only real way to lock exposure would be to switch to Manual exposure mode.
EOS models with a separate AF-ON button in the back allow back-button AF while preserving AE Lock functionality by pressing the separate AE Lock button (marked with the asterisk). This includes cameras like the EOS 50D, 60D, 7D, and EOS 5D Mark II. These models have a separate option as well: a Custom Function that allows you to reverse the roles of the AE Lock and AF-ON buttons. This is mainly for photographers who like to use back-button AF, and find it easier to reach the AE Lock button with their right thumb than the more-distant AF-ON button. It's also useful for photographers who may have gotten used to older EOS cameras without an AF-ON button, in order to keep the control layouts as consistent as possible.
Extending the time for AE Lock with EOS-1D and EOS-1Ds models:
One benefit of the top-of-the-line pro EOS models is the incredible flexibility they offer the advanced photographer. One example is how they can make extended use of AE Lock a lot easier.
As mentioned before, out of the box, EOS cameras will clear a locked-in meter reading if you remove your finger from the shutter button after 4 to 6 seconds. If you take a picture, it’s a little tougher: unless you keep pressing the rear AE Lock button, if you pull your finger from the shutter button, the reading will disappear within two seconds. Sometimes, this can make extensive use of AE Lock sort of challenging.
One answer is to simply make a note of the locked-in shutter speed and aperture, and put the camera into the Manual mode and apply the same settings. Once you’re in Manual mode, settings won’t change unless the photographer changes them.
But the top-of-the-line EOS-1D and EOS-1Ds models offer another way to work with AE Lock. With these models, you can extend the meter-on time, up to a full hour if you need to.
Here’s how it works:
- Go to Custom Function IV-12 (IV-13 with the EOS-1D Mark IV body): Timer length for timer
“6 sec. timer” is the length of time the meter stays active if you press the shutter button but don’t take a picture. Navigate to “Register” and press the SET button. Now, change the default 6 seconds to any available time you want, up to 1 hour, by turning the rear Quick Control Dial. “Timer after release” is how long the meter stays active after a shot is taken. By default, it’s a brief two seconds. Change this in the same way as the 6-second timer (any available value you like is fine, up to 1 hour).
- Use the Quick Control Dial to highlight “APPLY”, and press the SET button.
With these modified times set, be sure the word “Enable” is highlighted in blue, and press SET one more time. You can always jump back to the factory-default settings by highlighting the word “Disable” and pressing SET.
Auto Exposure Lock is a surprisingly useful tool that really expands what the photographer can do to apply exposure control, without losing the convenience and working speed of automatic exposure. In many cases, using it requires little more than identifying an element in a scene that you’d like to concentrate upon, aiming the camera at it and pressing the rear AE Lock button, and then re-composing to get the framing you want. It can lead to more consistent exposures when you’re shooting a sequence of shots, and is ideal for shooting with a primary subject off-center. It’s a great thing to get familiar with, so that you’re ready to use it when an opportunity presents itself.
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