Today’s Question: Could you explain more this statement that was in a prior “Ask Tim Grey”:
“back button autofocus enables you to separate the metering functions of your camera from the focus functions”
Am I right that only the shutter button meters the scene? So if I want to meter a spot on the upper left of the scene but want to focus on low center, how would I do that?
Tim’s Quick Answer: The “back button” focus option available on many cameras obviously enables you to separate the focus and exposure metering features. If you also want to lock the exposure, you’ll either want to employ the auto exposure (AE) lock feature, or employ the Manual exposure mode.
More Detail: In most cases exposure metering on a camera is triggered by the shutter release button. You can generally press the shutter button halfway to initiate the automatic exposure, and then press the button the rest of the way to actually take the photo. But in your example you want to be able to re-frame the scene after establishing exposure settings based on metering in a specific area of that scene.
Obviously you could employ the Manual exposure mode to lock in specific exposure settings. Then it doesn’t matter how you reframe the scene, because the exposure won’t change based on the changes in lighting as you change the framing.
If you want to use one of the exposure modes that enable automatic exposure, you’ll need to actually lock that exposure. Otherwise when you reframe the scene you’ll also cause the metering to update and the exposure settings to change.
The specific options available and method of employing auto exposure lock will differ from one camera to the next. But the basic idea is very straightforward. You can point your camera to the area you want to meter on and press the shutter release button halfway to actually establish exposure settings based on that metering. Then press the “AE Lock” (or similar) button to lock the exposure based on those settings. You can then reframe the scene without the exposure settings changing.
In some cases you simply press and release the AE Lock button to lock the exposure for the next photograph. In other cases you need to press and hold that button. And in some cases you can change the behavior through menu options on your camera. But the point is that many cameras do indeed feature an AE Lock feature.
Making use of “back button” focus along with AE Lock enables you to exercise considerable control while still taking advantage of the automatic features of your camera. You can set focus in one area, establish exposure based on another area, and then frame up the scene without altering focus or exposure settings.