Auto Exposure Lock (AE Lock) is a valuable feature with any DSLR camera. It allows for getting an accurate exposure of a very light-toned or dark-toned subject quickly, without guessing about the amount of exposure compensation that might be necessary. Here's how it works and how you can avoid the primary problem that some students in my BetterPhoto.com online photo courses have experienced.
- Metering Technique: Let's say that you're taking tightly-framed photos of a beautiful white chapel building. If you simply aim and shoot, the camera's light meter may produce underexposure: the white subject will be closer in tone to grey. The solution is to take the light meter reading from a mid-tone - in the same light as the subject - such as a grassy area in the nearby foreground. Then, lock in that exposure value with the AE-L button. Keep the button depressed while recomposing so the exposure does not change when you frame the photo to include the white chapel.
- Autofocus Problem: The above technique is very useful and likely to provide a perfect exposure. Taking the light meter reading from the mid-tone area (excluding the white building) should just about guarantee perfect results. Problem is, the distant subject may then be out of focus.
That's because some cameras combine AE Lock and Autofocus Lock ... when you use the AE-L button or when you use the shutter release button. If you lock in exposure for the grassy area, focus is also locked for that distance. Hence, the more distant subject will not be acceptably sharp.
- Separate AF and AE Lock: With some DSLRs, the AE-L button controls only exposure so the situation mentioned above will not be a problem. Cameras that do control both autofocus and AE Lock simultaneously will usually provide a menu item that will allow you to separate the two functions.
In this photo, the camera's [AE-L/AF-L] button activates and locks both exposure and autofocus. With some other cameras, the [AE-L] button also provides both of those features simultaneously. In either case, you can set a custom function so this button will lock only exposure and not affect focus.
Select the option that causes the AE-L button to provide only AE Lock and not AF Lock as well. Then point the lens at the nearby mid-tone and press the AE Lock button to take the meter reading. Keep the AE-L button depressed while recomposing. Next, point the lens at the distant subject and touch the shutter release button to activate autofocus. Maintain light pressure on the shutter button while reframing the scene for the most pleasing composition. If you used both buttons correctly, the final image should provide perfect focus for the distant subject and accurate overall exposure.
- Another Technique: The above method will work fine with any DSLR, but I recommend a simpler alternative. Use manual focus instead of autofocus. Rotate the narrow focus ring on the lens until the intended subject (e.g. the white house) looks sharp in the viewfinder. Then use mid-tone metering and AE Lock (as described above). When the camera/lens is set for manual focus, there is no need to maintain slight pressure on the shutter release button while recomposing. The camera will never try to change the point of focus.
Taking a light meter reading from nearby mid-tone rocks (in a sunlit area) and manually focusing on the more distant white boat provided optimal results for both aspects in this image. (AE Lock) Photo (c) 2010 Peter K. Burian
- With my preferred technique, you will only need to keep the AE-L button depressed after taking the light meter reading. (And some DSLR's provide full-time AE Lock: the exposure is locked with a single press of the button so you do not need to maintain pressure on it.) Manual focus plus AE Lock is more convenient and you'll get exactly the same results: sharp focus on the primary subject and accurate exposure based on the light meter reading of the mid-tone area.
NOTES: Peter Burian teaches for BetterPhoto.com's digital photography school, including these two excellent online photo workshops: Mastering the Digital Camera and Photography and Mastering the Canon EOS Digital Rebels. Also see Peter's Pro BetterPholio website: www.peterkburian.com.