Why I Don't Like Auto-Focus / Auto-Exposure Lock Buttons
When automatic functions were added to cameras, photography became something most anyone could do. But sometimes you can find yourself fighting against those automatic features when they don't do what you want them to. Sometimes though, popular solutions to regain control are completely unnecessary.
You're probably familiar with how these sorts of things work. Take auto focus for example. You want to focus on something that isn't where the focus sensor is. You point the focus sensor at your subject and halfway depress the shutter release to activate the electronics. Everything seems great. But now you swing the camera over to recompose, press the shutter release again, and find that the darned autofocus does what it was designed to do. It refocuses again, and now the background is sharp as a tack but the subject is thrown out of focus. You have to press the shutter release to take the photo, but when you do, that also engages the autofocus.
Depending on the focus mode, you can prevent this problem by keeping the shutter release half pressed while you recompose. Nikon calls this "Single-servo" as opposed to "Continuous-servo" focus mode. But having to keep your finger partially holding down the shutter while you adjust a tripod-mounted camera is awkward at best.
An alternative is to use the auto focus lock button. As its name implies, with the AF Lock engaged, the focus won't change until you take the picture. You can safely let go of the shutter release without fear that pressing it again later will engage the autofocus since it's now locked. But now you have to keep the AF Lock button pressed to maintain the lock. You've merely changed which button you have to keep your finger on while you recompose. At least some cameras do have a custom setting to have the camera keep the lock until you either take the picture or press the AF Lock again so you can let go completely.
Both auto focus lock and auto exposure lock controls function pretty much the same. Press the button and it temporarily stops the corresponding automatic feature from doing its thing. The functions are so similar in fact that they are combined into a single button on many cameras.
The real problem, for me at least, is that these controls are completely unnecessary. I can stop a camera from auto-focusing simply by switching it to manual focus. I can likewise stop the auto exposure from kicking in by switching to manual metering. I don't need more controls on a camera when the standard ones will already do the trick.
If you're a frequent reader here at Earthbound Light you probably already know that I shoot with manual metering much of the time anyway. When set to manual, it doesn't matter that the meter reading will change as I move the camera to recompose. The meter reading helps me choose an optimal aperture and shutter speed, but it doesn't force me or the camera to do anything. I do use autofocus much of the time, but I also know how to switch to manual focus. There's simply no need for another button.
If you want a button you can keep your finger on to stop the camera from doing its automatic best to do things for you, you already have one. It's called the shutter release button. If you want a control that lets you safely take your finger away completely until you're ready to actually shoot, you've already got one of those too. Just switch the camera focus or metering mode to manual.
Sometimes I think new shooters spend too much time learning to use these sorts of extraneous controls rather than the ones they need to master to take control of their photography.