Any reason to use AF-S unless you are focus-recomposing?

[New Daddy]

I've read quite a few tutorials on this, but I still can't understand the dichotomy of AF-S for static subject vs. AF-C for moving subject. Why not AF-C for a stationary subject too? If anything, I think AF-C is better even for a stationary subject because it compensates for any slight move of the subject or your hands! Unless, you are doing focus-and-recompose.

I have a camera with a touch screen, so I don't do focus-and-recompose. In my situation, I really see no reason to use AF-S even for stationary subject. Am I missing something?

[Mike Buckley]

I had never thought of the situation you're describing because I almost always focus and recompose. You're probably right that there is no reason not to use AF-C, depending on personal preferences.

I do like that my Nikon cameras beep to indicate focus has been achieved when using AF-S, but the beep doesn't happen when using other modes. It's easier for me to listen for the beep than to see whether or not the focus indicator is being displayed in the viewfinder.

Another consideration is that when using AF-S or AF-A, the shutter will not release if focus has not been achieved. Not the case when using AF-C. I've seen opposing thoughts about that aspect of the modes.

The AF-A mode automatically selects single-servo auto focus when the subject is stationary and continuous-servo auto focus once the subject moves. In some ways I can imagine that that might be the best of both worlds but I've never been in a situation in which I've been shooting a stationary subject that would unexpectedly move. So, I've never tried it. I personally wouldn't like that the shutter will release even if focus has not been achieved.

It will be interesting to see what others have to say.

[GrahamS]

Another consideration is that when using AF-S or AF-A, the shutter will not release if focus has not been achieved. Not the case when using AF-C. I've seen opposing thoughts about that aspect of the modes. 1)


The AF-A mode automatically selects single-servo auto focus when the subject is stationary and continuous-servo auto focus once the subject moves. 2)


In some ways I can imagine that that might be the best of both worlds but I've never been in a situation in which I've been shooting a stationary subject that would unexpectedly move. So, I've never tried it. I personally wouldn't like that the shutter will release even if focus has not been achieved.3)

1) This is usually subject to a custom menu setting of either "release priority" or "focus priority" depending on your camera make and model.

2)To my knowledge, this is a Canon feature called AF-Ai or "Intelligent Autofocus" and is also selectable in the AF mode settings.

3)Having the shutter refuse to release because the AF system, although set to "Servo" mode, has not acquired perfect focus "yet" can be most frustrating, especially when shooting sports such as football or rugby. In burst mode, I would rather have the first couple of frames OOF than no capture at all.

[ajohnw]

I assume this is take a shot AF against taking a movie AF. In the later case the camera needs to keep things in focus while they are moving about. Another way of describing the AF modes. Single and Continuous.

If you are taking a photo and the subject is moving in one mode the camera will try to keep it in focus. In the other it will only finally focus when you take the shot. In my experience that focus is likely to be more precise. The camera focuses as the shot is taken and will pre focus on the subject when you point the camera at it anyway. The fact that you or the subject is moving around doesn't matter. When you take the shot the camera will focus. The shutter speed needs to be fast enough to freeze the motion as much as you desire. That aspect is important also panning the camera correctly if that is needed.

Personally I would be reluctant to leave a camera on and in continuous AF for long periods as things wear out and in that mode cameras I have used hunt about. All of the cameras I have used in the other mode focus on what you point the camera at, stop and then finally focus when the shot is taken. That may happen when continuous mode is used as well. Depends on the camera. Where continuous may be of use is on subjects that are say running or more likely moving considerably faster. On the other hand S mode might track the focus adequately. Again that depends on what the camera manufacturer decides to do. You need to take some shots and find out.

John
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[Downrigger]

Like John's point about "wear out" in continuous. Maybe - maybe not, but you do have to wonder, the poor mechanisms hitch about so.

I'm back on the learning curve from most of you and am learning with a D600. Mostly I've been in single servo mode, with a single focus point in the center of field and locking with the AE-L button. Then recomposing... because in continuous servo, I'm never quite sure what the focus will fix on in the last instant before the shot. I trust this makes ok sense for stationary subjects.

My question is, when firing successively on moving subjects, is OK to stay in single mode or important to go to continuous? I guess it would be OK in my set-up, to hold the AE-L button down as I follow the subject and repeatedly press the shutter release. Or not?

Also, in continuous shooting modes, does the programming automatically put the focus into continuous servo?

Thanks all. Good discussion on what one would think a basic topic.

[GrahamS]

Mark, my advice is to start using BBF (Back Button Focus) and none of these issues will be a problem. Here's a link to a thread on the subject:
Do you use back button focusing?

[DanK]

Every time I see the abbreviation BBF I think of BFF, which 30-somethings in the US use to mean "best friend forever." Which seems sort of appropriate to me, as a long-time fan of BBF.

To offer a friendly amendment to Graham's comment: I assume that what he meant is that if you use BBF, you can leave the camera set to AF-S (I shoot Canon, but I assume this means the same as AF-Servo, i.e., it keeps searching) and then just take your thumb off the button when you want AF to stop. I generally do exactly the reverse. That is, if the subject has moved enough, I just press the back AF button again. This saves battery drain, and I suppose wear and tear, when you don't need AF-S. I prefer having the control over when the camera starts trying to focus, so I don't use servo mode unless I need it.

1) This is usually subject to a custom menu setting of either "release priority" or "focus priority" depending on your camera make and model.

Yes, although on my current Canon body, there is an interesting and I think useful wrinkle. If you set the camera for focus priority and leave AF on the shutter button, the shutter will not trip if the camera fails to achieve focus. However, if you have it set for BBF, it WILL allow you to take the shot without achieving focus. I have no idea if this is true on Nikon or Sony bodies, as I have never used either.

[Downrigger]

Mark, my advice is to start using BBF (Back Button Focus) and none of these issues will be a problem. Here's a link to a thread on the subject:
Do you use back button focusing?

Thanks, Graham - I think that BBF is equivlant to my having programmed the AE-L key on the back to a focus button. Right now I have it at Single servo so that once I depress it, that focus point is held, until I depress it again. I shoot in release priority.

I am not sure what to do if I am panning a moving subject with continuous exposures, though. I wonder if I have to reprogram so my shutter release button is in continuos servo exposure mode, or whether I can just keep my back button depressed while the shutter release is held down. I guess I'll have to experiment.

[New Daddy]

My question is, when firing successively on moving subjects, is OK to stay in single mode or important to go to continuous? I guess it would be OK in my set-up, to hold the AE-L button down as I follow the subject and repeatedly press the shutter release. Or not?

Also, in continuous shooting modes, does the programming automatically put the focus into continuous servo?

1. I thought a moving subject is the on situation where AF-C is a MUST. I might be wrong.

2. I guess it depends on the camera, but mine does not automatically convert to the continuous serve in burst shooting. It locks focus on the first shot and keeps it. This in reality work to your advantage if your camera's AF is not accurate or fast. For example, the camera may miss all five shots trying to lock focus continuously all five times. But if the camera bursts all five shots with the same focus plane, one of them may be in focus if the subject happened to move into that focal plane in one of the five shots.

[New Daddy]

Mark, my advice is to start using BBF (Back Button Focus) and none of these issues will be a problem. Here's a link to a thread on the subject:
Do you use back button focusing?

Graham, I'm not really sure how BBF (i.e., decoupling AF & shutter release button + locking focus at the current focal plane) will help with moving subject. The thread you provided also has different takes. For example, one photographer says he will use AF-C without BBF in sports, unless he is worried that some other player will intervene in the scene and "steals" the AF away from his initial subject.

Do you use back button focusing?

[Dave Humphries]

Speaking for myself;

a) I almost always use AF-C (continuous)
b It shouldn't "wear out" as long as I don't keep my finger on whatever button is focusing for extended periods of time (e.g. shutter half pressed or "BBF" button held down)
c) For moving subjects this (AF-C) works well (and most of my subjects move, or might in the blink of an eye)
d) I tend pick my composition then select the nearest (single) focus point and place it on the subject
e) Therefore I rarely focus and recompose
f) If I do, I may either switch to AF-S, or use "BBF" and take my finger off that button once focus is achieved
g) On my D7100, I actually program the Fn2 button as my "BBF" (so actually it is round the front!)
h) If both camera and subject are stationery; e.g. tripod shooting still life, I might manually focus or use AF-S
If nothing else, this will remove the possibility of the small focus hunting movements the system may use in AF-C to determine if it is optimal being caught during exposure - although these ought to be very small offsets (and yes, I guess it will reduce wear and tear on focus motor)

Cheers,

[GrahamS]

Graham, I'm not really sure how BBF (i.e., decoupling AF & shutter release button + locking focus at the current focal plane) will help with moving subject. The thread you provided also has different takes. For example, one photographer says he will use AF-C without BBF in sports, unless he is worried that some other player will intervene in the scene and "steals" the AF away from his initial subject.

Do you use back button focusing?

Rick, I use BBF exclusively. On my Nikon, I assign AF-On to the AE-L/AF-L button, but it could be assigned to other buttons if preferred. The auto-focus mode is then set to AF-C, permanently. To set focus on a particular subject, I position the AF point over the area I want to focus on and momentarily press and release the AF-On button. I am now focused on my chosen subject, for e.g. the eye of a portrait subject, and I am able to re-compose without the focus changing.

If I want the focus to track a moving subject, such as a running ball player or a bird in flight, I just hold the AF-On button down. Because the AF is set to AF-C, the focus will track the subject until I release the button.

I don't have to change any settings to use both single shot AF and continuous AF. If I set the AF point to "3D Auto" or to "3D Single point" the system works just as it would previously if I held the shutter button half pressed, as long as I hold the AF-On button down. Note that when the AF actuation is assigned away from the shutter button, "Servo" or "Continuous" AF only operates as long as the alternative button to which AF has been assigned, is held depressed. This applies to both Nikon and Canon cameras.

With the Canon 7D camera, I assign AF-On to the * button which is more prominent, although most people assign it to the AF-On button. I set the AF mode to Ai-Servo and everything works exactly the same as described above. I am now so accustomed to using BBF that I find it difficult to use my wife's Nikon D3100, which does not have the facility. I would never go back to using shutter button AF. At first, it is confusing and takes perseverance, but it's worth it in the end.

[ajohnw]

Speaking for myself;

a) I almost always use AF-C (continuous)
b It shouldn't "wear out" as long as I don't keep my finger on whatever button is focusing for extended periods of time (e.g. shutter half pressed or "BBF" button held down)
c) For moving subjects this (AF-C) works well (and most of my subjects move, or might in the blink of an eye)
d) I tend pick my composition then select the nearest (single) focus point and place it on the subject
e) Therefore I rarely focus and recompose
f) If I do, I may either switch to AF-S, or use "BBF" and take my finger off that button once focus is achieved
g) On my D7100, I actually program the Fn2 button as my "BBF" (so actually it is round the front!)
h) If both camera and subject are stationery; e.g. tripod shooting still life, I might manually focus or use AF-S
If nothing else, this will remove the possibility of the small focus hunting movements the system may use in AF-C to determine if it is optimal being caught during exposure - although these ought to be very small offsets (and yes, I guess it will reduce wear and tear on focus motor)

Cheers,

I've used one where it hunts around continuously where ever the camera is pointed, finger on finger off. Largely for video use. No mention of what camera in the post so mentioned in case his does the same. I still feel it's use depends on the camera anyway, If a camera choose to track in it's S mode as it's called here there is no difference anyway. If they don't track there would be a rather blurred image via the view finder or screen.

John
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