As noted in earlier sections, Canon sets exposure to the AF point that you select. This approach is fine as long as the area of critical exposure falls within the point where you want sharpest focus. However, that's often not the case. And when you need to decouple the exposure from the point of sharpest focus, Autoexposure Lock (AE Lock) is the technique to use.
AE Lock allows me to get exposures that are biased to the right of the histogram yet maintain detail in the highlights. There's a bit more latitude in RAW capture because you can recover varying amounts of highlight detail during image conversion. If you shoot JPEGs, using AE Lock is a great way to ensure that the image retains detail in the brightest highlights.
For example, I use AE Lock in combination with Evaluative metering and manual AF point selection when exposing to maintain fine detail in the highlights of a bride's wedding dress, to retain highlight detail in macro and nature shots (although, if you composite multiple exposures, bracketing may be a better technique), and to maintain detail in skin highlights in portraits.
However, there are some considerations for using AE Lock. First, it doesn't work in Full Auto or, of course, Manual modes. And in Tv and Av modes, you have to use Evaluative metering, manual AF point selection, and autofocusing rather than manual focusing (via the switch on the side of the lens). And if One Shot AF or AI Focus AF is set and AI Servo AF isn't active, pressing the Shutter button halfway down automatically sets AE Lock.
Before you set the AE Lock, ensure that the camera is set to Av or Tv mode, Evaluative metering, and manual AF point selection, with the lens focusing switch set to AF.
To set AE Lock, follow these steps:
1. Press the Shutter button halfway down to focus on the part of the scene that you want to meter. For example, if you want to ensure that detail is retained in the highlight area on a subject's face, focus on a bright highlight.
2. Continue to hold the Shutter button halfway down as you press and hold the AE Lock button on the back-right side of the camera. This button has an asterisk above it. An asterisk then appears in the viewfinder to indicate that the exposure is locked.
3. Once the asterisk is visible in the viewfinder, release the Shutter button, move the camera to recompose the shot, press the Shutter button halfway down to focus, and then take the picture. As long as you continue to hold the AE Lock button, you can take additional images by using the locked exposure. When you release the AE Lock button, the exposure is reset when you focus the lens again.
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