I've had a few questions in after my recent posts on focus, so I thought I'd try to answer them here. Here is a comment emailed through the other day which I think pretty much sums them all up....
" I've been researching focus modes and focus points. i'm officially more confused than ever. i'm a Nikon user and have the options of af-a (auto), af-c (continuous), and af-s (single) and the the points options of single, matrix, and auto. i assume that with children i should keep it on af-c and matrix. I'm completely confused about choosing a focus point. I understand holding the shutter button half way, obtaining focus, and recomposing. from what i understand, you can only do that in af-s mode and not af-c mode. but i could be wrong on that. i've been trying to understand the back button technique, and i still don't...arg! do you press the af-on and release then press the shutter? i'm completely confusing myself!"
I'm going to try and explain this a bit further, but if you haven't already done so, please read the posts on Focus Modes and Focus Techniques again, then come back here for a bit more explanation!
Although you have Auto Modes too, you will either be using AF-C / Al Servo, which is for subjects that are moving (or likely to move) or One Shot / AF-S, which is for still subjects. (I'm not going to cover Auto because there is no need - it will do it all for you). I'll go through these both individually, and try to use the terminology used by both Nikon and Canon - but please, please bear in mind that there are a lot of different models out there that have different features and capabilities, plus have different names for these (just to confuse matters further) so I'm going to be general, and you'll need to look at your specific camera model. Also, remember I don't use Nikon, I use Canon, so I might not be 100% there with the terms but you should get the general idea.
I'm also going to run through some background so everyone is clear on what Back Button Focus (BBF) is. Normally, the camera focuses when the shutter release is half-depressed. Once focus is achieved, you then push the shutter all the way down to take the picture. Back button focusing is when you set your camera so that the shutter button doesn't control the focus, but another button on the back of your camera does. (The back button is really just doing the same job as half depressing your shutter) Remember, you will need to manually set your camera to use this back button for focus first. Which button this is will be depends on your camera model.
AF-C / Al Servo
I like to use AF-C/Al Servo with kids as they tend to move about so much. If you are using this with your shutter locking focus (the normal way) keep your finger half depressed on the shutter until you are ready to take a shot, then push the shutter down fully. (The camera will keep focus on the moving subject until you press the shutter fully) To use AF-C / Al Servo with Back Button Focus (BBF) you press and hold down the back button and use your shutter to take the picture. If you keep holding down the back button you can fire off as many shots as you like with the shutter. As long as you keep your finger on the back button, the camera will keep focus and is the reason BBF works great with Al Servo / AF-C.
Bear in mind that whether you use the shutter or BBF to focus, you won't hear a beep to tell you that focus is locked in the same way that you would with One Shot / AF-S.
You can't use the focus-recompose method with AF-C (Al Servo for Canon). If you try to recompose in this mode, the camera will think that the subject has moved and you will lose focus. You can choose your focal point, but Al Servo is most reliable when using your central focal point so shoot a little wider, and simply crop to your chosen composition in editing. I've gone over how to do this with Canons in the previous posts, but for those with Nikons, Single Point AF is the one single empty box icon with no hashes (see below). Both are the same idea. With Nikons, you also have the additional option for using Dynamic Area AF - this is the icon with one small empty box in the middle, surrounded by four hashes. With this, the camera will track focus and choose a focal point for you, and is a very good option!
BBF & One Shot / AF-S
If you little darlings are sitting relatively still, you can change to AF-S/One-Shot. You could still use Al Servo / AF-C, but if you are taking a few shots of a still subject it's better to use One Shot as it will be slightly sharper. To use BBF in this mode, you press down the back button to lock focus, release, then press the shutter to take your shots. You do not need to press the back button for each shot you take, provided the subject hasn't moved, or the distance between the camera and your subject hasn't changed. I tend to use the back button to focus every couple of shots even if I haven't moved just to be on the safe side, but perhaps with practice I will get more confident and not feel the need! You will be using Single Point AF, and you can use the central focal point and use focus-recompose (i.e press the back button for focus, release, recompose, then press shutter) or you can toggle your focus points.
Let's lastly look at changing your focal points and how you would put this to practical use. Here is a photo I took recently and I've given a rough overlay showing the focus points available. (sorry, I whipped this up in under a minute as you can see - the points aren't even in line!) I can choose to use the central one, or I can toggle to any of the other points that fit my composition better, and save me having to recompose the shot. In this particular instance, I toggled until I got the focal point with the arrow (which was over his eye) As you can imagine if your toddler is running about this is nigh on impossible, in which case, use focus-recompose.
Lastly, here is a rundown on when I would use what.....
Sleeping babies or children - One Shot with Single Point AF - Choose your focus point
Toddlers / Kids Playing Sports / On swings etc - Al Servo with Single Point AF - Use central point, or choose focus point but beware not as reliable. Do not Focus-Recompose.
Older Children / Adults - One Shot with Single Point AF mode - Choose your focus point or use Focus-Recompose.
Sleeping babies or children - AF-S with Single Point AF & choose your focus point
Toddlers / Kids Playing Sports / On swings etc- AF-C with Dynamic Area AF mode for shooting with high aperture numbers, or Single Point AF if shooting with low aperture numbers. Do not focus then recompose.
Older Children / Adults - AF-S, Single Point AF mode - choose your focus point or use focus recompose.
The most important thing to get your head around is the focus modes of your particular camera. If you don't yet fully understand these - get to grips with them first, before trying back button focus. Perhaps get confident using one of the modes, with one of the focusing techniques before moving onto the next one. Once you get in nailed, and you know which mode to use when, you will start to find that your "hit" rate of in-focus shots starts to increase!
I'm reading all this again before I post and not sure if I am doing a terribly good job at explaining, but I'm afraid it may be the best I can do! I hope this has cleared up a couple of points for some of you. If anyone has any questions, please leave them in the comments section below so that I can reply where everyone can see the answer - if you have a query, it's likely others have the same one too!