Suggestion for focus setting for gymnastics

I will be attending my daughters gymnastics competition this weekend and taking pictures. The last time I did this, I got some amazing shots, but also got some duds. One of my problems is that when she gets on the uneven bars, I am holding down the shutter button so the camera is taking shots quickly, but I found several times where the camera focused on the crowd behind her and left her blurry.
I am trying to decide what is better, to get the camera focused then flip the focus setting on the lens to manual and snap away, or to hold down the AE-L/AF-L button the whole time I am taking pictures. Is there a compelling reason why I would choose one over the other? I am using a 70-200 f2.8 VRII lens and am a decent distance away. Even though there is a lot of movement, it is all spinning on a bar, so she isn't changing position as far as distance goes. I don't have a monopod/tripod to put the camera on, and that lens gets heavy after a while, so having to hold the shutter release button down with one finger and the AEL/AFL button down is sort of an aggravation, but I will do it if there is a reason why I should not disable the autofocus.

Thanks for the assistance.

What is your auto-focus set at? AF-A, AF-C or AF-S?

I have the auto-focus set to AF-S. That way I can focus on her before she starts moving, press the button half way to lock it, then when she starts her routine I press it the rest of the way down to start capturing shots. I was under the impression that the focus stayed locked until the photo was taken, but then it reset itself for the next shot. It is possible that I released the button and then had to refocus again.

On AF-S mode, if I lock the focus and then keep the shutter release held down to start taking pictures quickly, does the focus readjust after each photo, even though the button is held down? At 4 photos a second, that seems like a lot of focusing.

Why no just set it on manual on the bar and then not have to hold anything down.

That is why I was asking the question. I didn't know with the D5100 if there were any compelling reasons why I would not do that. For all I know the camera disables some arbitrary setting when the lens is in manual mode which I would want to have. If there is no difference in how the camera behaves other than the focus, it would be easier to focus it, flip it to manual and then snap away when she starts.

You don't have to let it focus to begin with. Focus it manually and I think it would be faster and probably more accurate. If you do it your way it might get moved while you are in the process of clicking the switch to manual.

Ahhhh, that's your problem, AF-S! This setting is for things that do not move. You need to be at either AF-C (continuous) or AF-A (auto).

DW_ said:

Ahhhh, that's your problem, AF-S! This setting is for things that do not move. You need to be at either AF-C (continuous) or AF-A (auto).

Click to expand...

Maybe I dont understand what that means then. Although she is spinning on a bar, she is staying the same distance from my lens, so I wouldnt want the focus to be readjusting. So I thought setting it on AF-S would keep it from readjusting.
I thought that if I had it set to AF-C/AF-A, evertime the center focus point drifted off her and hit the crowd behind her, it would instantly refocus on them.
What am I overlooking?
Thank you for your help!

Couldn't you just set the shutter speed high the aperture around 8 or 11 and zone focus it? Man AF is cool but MF is a lot more fun IMO...kind of like the difference between an automatic transmission and a standard the standard being so much more fun to drive???

No, I think it's exactly the opposite. AF-C will lock onto the moving object and will adjust focus for movements and as long as you have her in the focal point, you'll stay with it. In AF-S the camera locks on and does not change it's focus and should only be used on subjects that do not move. However, unless your subject is filling the frame you will always risk losing your subject in the focus. But with digital you can let 'er rip and just toss the images that don't work for you.

Eye-level: I like having having a really shallow depth of field so that she is in focus but the crowd behind her is blurry. I thought the only way to really get that is to keep the aperture at 2.8. I am also really new at photography and am just getting past the basics. I like driving a standard transmission myself, but for me, I am just graduating up from a bicycle to a car, so an auto transmission is still advanced for me

DW: The hardest problem with AF-C and the specific event I have the most problems with (Gymnastics uneven bars) is that she is spinning around on the bars and I am standing 75-100 feet away trying to hold the focus dot of a camera with a heavy lens steady on her. Small object with quick suddens motions, but all just spinning on a series of bars the same distance from you.

I am going to get set up early and shoot a bunch of pictures while the girls are warming up and see how they look with the different focus settings.