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The Sony A6000 has an AEL button, while Nikon SLRs have an AE/AF lock button. What do these do that half-pressing the shutter to lock focus and exposure, and then recomposing, doesn't? I'm used to the latter method, and never used a camera that has a dedicated button.

I read that you have to keep both the AE/AF lock button pressed and the shutter half-pressed while recomposing. Isn't this harder than just keeping the shutter button half-pressed?

I'm aware that you can customize the AE/AF lock button to lock only focus, or only exposure, or both. This leads to two follow-up questions:

  1. Can't you customize the half-press shutter to lock only AE or only AF?
  2. If you don't care about this level of customization, does the AE / AF lock button do anything that half-pressing the shutter doesn't?
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Most advanced cameras allow you to separate exposure and/or focus lock from a half press of the shutter button to allow each photographer to choose how and when focus and exposure are locked for a given composition. Even what happens by default in the camera's "factory" settings will often vary based on what shooting modes in terms of exposure and focus are selected.

Why is it advantageous to separate exposure lock and focus lock from the shutter button? If both focus and exposure are locked by a shutter half press, then pressing an AEL or AF-L/AF-On button allows you to update either exposure or focus without changing the other value. They also allow you to maintain the same exposure or focus settings for more than one frame.

  • Suppose you want to take a series of shots, such as a panorama, with different composition but the same exposure settings. Press the button on your camera that locks exposure and focus and hold it down while you take the series. I saves you the trouble of metering and then switching to M exposure mode to preserve the exposure settings for more than one shot.

  • If you are using a form of continuous focus and find the need to lock focus. With most advanced cameras there are several ways to do this. You can usually set the camera to either allow a half press to start focus (in AF-C/AI Servo AF the camera will focus continuously without locking as long as the shutter is half pressed) and a press and hold of the AF-L or AF-On will lock the focus OR you can set the camera so that a half press does not start focus and pressing the AF-L/AF-On button is required to focus. In the second case you would always focus by pressing the AF-L/AF-On button. If you are using AF-C/AI Servo AF, as long as the button is pressed the camera would continuously focus. If you wanted to lock the focus at a particular distance you would simply release the AF-L/AF-On button while the camera is focused to that distance.

  • You can hold down the AF-L/AF-On button to take multiple shots at the same focus setting. Just as with exposure, there are times you may want to maintain the same focus distance for a series of shots. Holding down the AF-L/AF-On button allows you to do this if you are using an AF-S/One Shot focus mode. Otherwise your camera would refocus between shots. Another advantage of this method is that it may increase your burst rate because the camera isn't refocusing or confirming focus between each frame.

  • If you are using a form of "One Shot" or AF-S focus. In this scenario if the shutter half press tells the camera to find focus and then lock it as well as begin metering, you can set the AF-L/AF-On button to refocus after you have recomposed. By continuing to hold down the shutter at half press the exposure settings will remain locked while you refocus.

  • If you are shooting in a mode that locks focus and exposure at the shutter half press, pressing the AE-L button will allow you to update the metering without altering the focus. This is useful for focus and recompose or when the lighting conditions of a scene may be changing rapidly. The AE lock button allows you to update the metering without changing the focus.

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Digital cameras are highly configurable these days, so neither buttons has a fixed role. The main point is that there are two or sometimes three button which so that you can decouple the individual steps. The camera needs a button to take the shot and that was the original purpose of the shutter-release.

There are two things the camera can also do:

  • Auto-Exposure (except in Manual mode) which can be done on the half-press. If it is, then you can meter and recompose. Of course, that means your focus is locked in AF-S mode too, so you need another button if you want to separate these actions.

  • Auto-Focus (except for MF) which can also be done on the half-press which may lock exposure too. Again, without a separate button, there are only two actions, one on the full-press, the other on the half-press.

To control all three actions separately, all you need is an extra button but is restrictive. If you want to be able to meter and focus in easily any other, it is much easier with a button for each. That is why high-end cameras have separate AE-L and AF-L buttons (sometimes AF-On which usually means AF is not done on the half-press).

One critical thing about these separate buttons is that they can be kept pressed even after the shutter is released which is useful for repeating shots with AE or AF fixed.

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On Nikons, pressing the shutter button halfway does not lock exposure, and if your focus mode is in any continuous mode, it won't lock focus either.

If you're taking a panorama for example, and you only use the shutter button, each shot will very likely have a different exposure as the camera meters each scene separately. If you use AE-L, the exposure locks so it is consistent throughout the panorama.

You can set the shutter button half press to lock exposure, but seeing as there is a dedicated button for it, and you are arguably more likely to want the exposure to be accurately metered more often than locked, it's default is off.

Many cameras let you set the AE-L button to activate AF, which is termed back button focusing. You can also often set them to toggle so that one press locks exposure until a second press.

  • 1
    in some modes Exp is locked with focus. At least on canon. not sure if that is common behaviour – Michael Nielsen Jun 8 '14 at 8:48
  • Let's assume I'm in AF-S mode (single-shot focus) rather than AF-C (continuous focus). Let me see if I understood you both correctly: Are you saying that the default behavior of half-press shutter is to lock focus but not exposure, and that the default behavior of the AEL button is to lock both exposure and focus, and that's exactly why it helps to have both? – Vaddadi Kartick Jun 8 '14 at 14:44
  • 1
    Yes, precisely. Personally I set my AE-L button to just lock exposure not focus. – ElendilTheTall Jun 8 '14 at 16:18
  • @VaddadiKartick Only if you are using certain Nikon cameras that behave the way this answer describes. With a Sony α6000, YMMV. – Michael C Apr 24 at 7:55

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