There are many ways to set your camera up for great pictures of kids playing sports…none of them are any better or more “right” than others…it’s just a question of whatever you’re most comfortable with and whatever you feel gives you the best chance for the quality you want.
My typical style is to put the camera in “Aperture priority” mode (on the Nikon, it’s the setting with the “A” on the little digital readout screen on top of the camera), and let the camera determine shutter speed (I choose ISO manually). What ISO number I choose depends on what lens I’m using, how bright the scene is, and what kind of shot I’m going for. Generally speaking, if I want to get a good freeze on action, I aim for around 1/1000th a second. Then I experiment with ISO, and take a few test shots. If there is motion blur in the subject, I bump up ISO or open up the aperture. If the shutter speed is unnecessarily high, I bring down ISO or close down aperture a stop.
You can just as easily use “shutter priority” mode, which allows you to set shutter speed on your own, and the camera will adjust aperture to properly expose the shot. It’s just my personal preference, but I prefer to have control over aperture all the time…especially if I want to isolate the subject with a soft blurry background, or, on some lenses, make sure I don’t shoot at the widest aperture (lowest f-stop), which can sometimes degrade aspects of the image (i.e. objects can appear unnaturally soft, vignetting at the corners, flare, etc…).
Having a faster lens (a lens with a lower f-stop) can be advantageous when shooting action sports. The extra stop(s) of light you get with a faster lens will allow the photographer to shoot at a lower ISO, ideally creating images with less noise. For example, instead of shooting at ISO 1600, 1/1000 f/4, you can open the aperture to f/2.8 and still shoot at 1/1000 at ISO 800…or if your lens opens up to f/2, you can shoot the same shutter speed at ISO 400. That’s a nice advantage to have in your pocket.
That said, using faster lenses can be a relatively expensive option, and there is usually no other significant difference in performance between comparable focal length lenses…the image quality is essentially the same (everything else being equal). If we are comparing, for example, the Nikon 70-200mm f/4 and the Nikon 70-200mm f/2.8, the f/2.8 version is heavier due to requiring more glass to handle the faster aperture, but you won’t get a sharper picture from it compared to the f/4 version. The image quality between both lenses is equally exceptional. The 2.8 costs over a thousand dollars more than its f/4 brother, basically because of the extra f-stop. If you only shoot landscapes, I can’t think of a situation where you really need to shoot 2.8…maybe if you’re messing around with depth of field…but that’s an awful expensive option just for shallow DOF. However…for action, that extra f-stop can be very handy.
The shot below was taken at f/2, ISO 1600, 1/640. The gym was poorly lit (lighted?) by those old, cheap, pale white, flickering fluorescent bulbs hanging from a high ceiling. If I were to have taken this shot at f/4, I would have had to bump ISO up two more stops, to ISO 6400, to remain at a shutter speed of 1/640. Regardless of how well your camera handles high ISOs, it would no doubt have introduced more unwanted noise into the image.
But, generally speaking, lenses perform at their best when closed down a few stops from their widest aperture. So, shooting at f/4 might produce a slightly higher quality image (assuming depth of field isn’t an issue) than shooting wide open at f/2. These are the things I consider when choosing a specific aperture/ISO number.
One thing I’ve found to be very important when taking pictures of kids playing sports: make sure you get down to their level of the action. If you take pictures while standing up, the viewer will be looking down on everything in the photo (you are creating a perspective we are used to seeing all of the time). If you get down to the child’s perspective (or even lower), it creates a more dramatic and lively point of view. Additionally, if you lay down while shooting, you can better stabilize the camera than if you’re kneeling or standing.