Autoexposure lock is a way to control your camera's light meter.
To understand use auto exposure lock, you need to understand how cameras expose images and how the process differs from the human eye. Look at a piece of paper, like a page in a book or black-on-white business card, in the sunlight. You should be able to see both the crisp black ink, as well as the white paper. In fact, your eyes, if you look closely enough, can see fine variations in the tone of the ink and discolorations or imperfections in the paper. The ability to see both details in white and in black at the same time is called dynamic range. Cameras cannot do this. At some point, either all of the blacks or all of the whites will merge together--cameras cannot see both at the same time. Exposure is the process of choosing what brightness levels to keep inside the camera's dynamic range and what levels to allow to fade to white or black. Auto exposure lock allows you to easily modify what a camera chooses to expose.
Instructions
1. Set your camera for either spot or center-weighted metering in accordance with the manufacturer's instructions. These modes allow you to force the camera to choose a limited area from which to set exposure, which gives you more creative control.
2. Use the program, aperture-priority or shutter-priority shooting mode on your camera. For example, on Nikon cameras, these modes are indicated by choosing "P", "A" or "S" exposure mode, and on Canons, they are indicated as "P", "Av" or "Tv". In manual ("M") mode, the camera does not make exposure decisions for you, eliminating the need for AE-Lock.
3. Look through the viewfinder, and put the object which you want the camera to expose correctly in the center of the frame under the camera's focusing or exposure brackets. Although the exact shape of the bracket or indicator will vary from camera, putting it in the dead center of the viewfinder should work fine.
4. Depress the shutter button down halfway to take a meter reading. While doing this, press the autoexposure lock button, which may be labeled "AE-Lock", "AE-L" or something similar. Depending on your camera's settings, you may or may not need to hold the "AE-Lock" button down, but you can release the shutter button once the autoexposure lock has been engaged. The AE-Lock will stay engaged until the meter shuts down.
5. Recompose the picture in your viewfinder to reflect your desired composition. This can include changing zoom settings or simply reorganizing the arrangement of elements in the frame. One example of this is to use the rule of thirds. To do this, you would use autoexposure lock to set correct exposure on your subject while it is under the camera's center-weighted metering, then recompose the image with your subject off-center.
6. Press the shutter button down all of the way to take your picture. If the autoexposure lock remains engaged after taking your picture and you do not want it to, simply press the "AE-L" or "AE-Lock" button to release it.
Tags: your camera, autoexposure lock, button down, shutter button, AE-Lock button, center-weighted metering, dynamic range