3

After quite a lot of reading even, I am not being able to grasp the rationale behind the AF-L part of AE-L/AF-L button on Nikon D5300. If the camera is set to AF-C, then the concept of locking focus at a particular object/distance makes no sense. Hence, I am assuming AF-L is relevant only in case of AF-S. However, I can keep auto-focus locked by simply keeping the shutter half-pressed. Nikon doesn't provide an AF Lock (Hold) mode. Essentially, it means that the focus will remain locked until the AE-L/AF-L button is kept pressed. What is the preventing the photographer from keeping the shutter button half-pressed and then recompose rather than having a dedicated button with a fancy feature? Isn't it a redundant functionality assigned to a button which could be better used for some other purpose? Am I missing something?

4

If the camera is set to AF-C, then the concept of locking focus at a particular object/distance makes no sense.

On the contrary: if the camera is set to continuous autofocus (AF-C), it's going to constantly adjust the lens to keep focus while the shutter button is half-pressed. That's great for moving subjects, like athletes. But that's not always what you want. There are times when you're shooting in AF-C, but you'd still like to lock focus and then recompose, and that's exactly what the AF-L button does for you.

Consider, for example, that you're shooting a penalty kick at a soccer game. Maybe you'd like to take a shot where the ball and goalie are in focus, but timing is critical, so you want to pre-focus and maybe recompose as well. So you focus on the cross-bar or goalie, hold the button, and then wait for the ball to arrive at the goal plane. You could do that with single shot (AF-S) mode, of course, but switching between AF modes all the time isn't exactly convenient. AF-L is like a shortcut that temporarily modifies the normal AF behavior.

Isn't it a redundant functionality assigned to a button which could be better used for some other purpose?

Well-designed tools often offer more than one way to do things. Another example: you can configure many DSLRs to use one of the buttons on the back of the camera to activate the AF system instead of relying on the half-pressed shutter button. Also, you configure the AE-L/AF-L button for just AE-L if autofocus lock isn't useful to you.

  • In your example, (a) do you assume that while focusing on the cross-bar or goalie, my camera is set to AF-C? (b) If so, if I DON'T hold the button (I assume AF-L) after focusing, will the camera gets defocused once the ball arrives? Since I have focused on the cross-bar (static subject), shouldn't the camera stay focused even in AF-C mode? – Holmes.Sherlock Dec 31 '15 at 4:26
2

This varies slightly depending on the camera and how it is configured.

Essentially though, AF-L allows you to lock focus without locking down anything else. It obviously does not apply in MF mode but otherwise you can lock focus and then press the shutter-release halfway to lock exposure and then wait for the right moment before fully depressing the shutter.

Recomposing is a useful trick which I use often but it cannot replace AF-L completely because at wide apertures, the focus will shift enough during recomposition to miss focus slightly. Also potentially, the half-press of the shutter may inadvertently lock an incorrect exposure, unless you are in Manual mode, of course.

  • How is "recomposing" expected to replace AF-L even, either completely or partially? At wide aperture the DoF being shallow, locking focus and recomposing should result in blurry images. – Holmes.Sherlock Dec 31 '15 at 4:42
  • @Holmes.Sherlock - That is exactly what I said so I'm not sure what your question is? For cases where DoF is large, you can find something to recompose with which falls within the desired DoF. – Itai Dec 31 '15 at 6:00
2

Am I missing something?

You're missing a lot. Tying autofocus to the shutter button half press works well at times. But it can also limit the ability of the photographer to control both when and how the camera autofocuses. Allowing the user to decide when AF and AE are active and when they are not independently of each other as well as independently of the shutter button, and what button turns each on or off, allows a much greater degree of control over the AF and AE systems in a fluid way without having to stop shooting and delve through menus to change the camera's behaviour.

Why is it advantageous to separate focus lock (and/or exposure lock) from the shutter button? If, for example, the camera has been set up by choosing settings so that both focus and exposure are locked by a shutter half press, then pressing an AEL or AF-L/AF-On button can be used to override the "locked" focus distance or exposure value and allows you to update either exposure or focus without changing the other. They also allow you to maintain the same exposure or focus settings for more than one frame by holding the back button down over multiple frames. Or the camera can be adjusted so that pressing the shutter button halfway only initiates metering without turning AF on or off. In this scenario the AF-L (Nikon), or AF-On (Nikon/Canon) button controls AF completely independently of the shutter button.

By separating the two functions it allows one to turn AF on and, more importantly, off, independently of pressing the shutter button. If the camera is set up one way it can, for example, allows focus to remain locked over several frames (even if the shutter button is fully released and repressed) and then seamlessly transition to refocusing between each frame (even if the shutter button is held down in burst mode) without requiring the photographer to stop and change any settings other than pressing or releasing the AF-L button.

For more on several different scenarios where separating autofocus control from the shutter is advantageous, please see this answer to What does the AE/AF lock button do that half-pressing the shutter doesn't?

  • I don't understand: "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." If it is already locked, how can it be updated? What is that "other values"? – Holmes.Sherlock Dec 31 '15 at 4:35
  • "If both are locked..." is needed as a qualifier because some cameras allow you to set the camera up so that half pressing the shutter does not initiate AF. You could also be using AF-C in which case even if the half press initiates AF it does not lock AF once confirmed, as it would if using AF-S. – Michael C Dec 31 '15 at 4:57
  • "If it is already locked..." in cases where the AF is initiated by the shutter half press and locked once focus is achieved a press of the back button allows that locked value to be overridden and updated. – Michael C Dec 31 '15 at 4:59
  • RE: values (either focus distance or exposure). If the camera is set up so that a half press initiates both metering and AF and locks each of those once they are achieved (so that pointing the camera in a different direction doesn't change the focused distance or the exposure value), and one updates the focus distance by using the AF-On/AF-L button, then the exposure value is not changed, only the focus distance. If after recomposing one releases the shutter button and then does another half press both the focus and exposure values that were previously locked are reset and computed again. – Michael C Dec 31 '15 at 5:07
  • @Holmes.Sherlock Have you read the linked answer where various scenarios are discussed in greater detail? – Michael C Dec 31 '15 at 5:12

Not the answer you're looking for? Browse other questions tagged or ask your own question.