Let's not talk about lenses yet. 300mm like you already have is plenty of "reach" for most kids sports, though there is still room for improvement. More about a lens upgrade later... first let's talk technique, setup and camera.
I'm going to let you in on a little secret. Most sports shooters DON'T use manual exposure (M) most of the time. The reason is that shooting sports very often means subjects in varying lighting conditions and no time to be adjusting manual exposure settings. Often sports shooters like to use TV mode (shutter priority) mode so they can select faster shutter speeds, letting the camera choose an appropriate lens aperture. You still need to confirm that the range of apertures necessary is available... checking that you have sufficiently large aperture to accommodate the darkest area you'll be shooting and that the brightest doesn't call for an aperture smaller than your lens can provide (I would bias toward the larger aperture, since very small apertures tend to mean too slow shutter seed and/or problems with diffraction). Adjust your ISO to control that range of apertures. The camera will tell you if you're exceeding the range. If you have insufficient aperture available for a correct exposure in the darkest area you will see the largest lens aperture flashing in the viewfinder. As long as it's solidly lit, you're okay.
Get in the habit of testing your exposures occasionally when you are setting up to shoot sports. Point the camera toward the darkest areas to see if there is risk of under-exposure... then point it toward the brightest area to see if over-exposure might result. Adjust your shutter speed and ISO as needed. For sports you usually want to use a fairly fast shutter speed... in order to freeze the action. At a distance you might get away with 1/200 or 1/250... but closer subjects and faster movement (such as the swing of a bat or a foot kicking a ball) will require much faster.... 1/500, 1/800, 1/1000 or even faster. The direction of movement also matters.... subjects coming towards your or heading away from you are more easily frozen than subjects moving tangentially to you.
For the above shot of fast moving cars I used 1/1250, although because they were coming toward me I probably could have used a little slower shutter speed, if necessary (if light hadn't been as good and I was trying to keep from using too high an ISO).
Sometime experiment with slower shutter speeds and "panning" techniques, depending upon the subject matter. This usually works best with vehicles but can sometimes be used for special effects where you blur part of the image to give a greater impression of movement.
Here I used 1/100 shutter speed while panning with the fast moving car, in order to blur down the background and make the subject stand out. Using this technique takes some practice and even then not all images will be successful (we only share the good ones where the technique worked 😛)
As you can see, "dragging the shutter" (1/30) also can work with slower subjects such as a trotting horse & rider. I combined that with a flash technique (2nd curtain sync) to partially freeze the subject.
Now let's talk lenses. If your 75-300mm is the Canon EF 75-300mm f/4-5.6 III that was often sold in kit with the Rebel T7 camera.... well, to be honest, that's a very "budget" lens that leaves a few things to be desired. When it comes to sports, the biggest concern is that the lens uses a micro motor to focus. An STM (stepper motor) or USM (ultrasonic) lens would be faster focusing... acquiring focus more quickly and tracking movign subjects better. For example, the Canon EF 70-300mm f/4-5.6 IS USM (first version or II) would be a lot better lens for it's autofocus capabilities. It's also got better image quality (especially at 300mm) and Image Stabilization, but the better AF performance would be its biggest advantage.
Another good lens, but with not quite as much reach and not quite as quick focus, is the Canon EF-S 55-250mm f/4-5.6 IS STM. .
It's not just the lens though. Your camera also plays an important role in auto focus performance. To get the best possible AF for sports out of a Rebel T7 you will need to use only the center AF point, must use AI Servo focus mode and might want to learn to do "Back Button Focus" (BBF). You'll need to do your part too, keeping that AF point right on the subject, where you want the camera and lens to focus (starting and stopping AF with the button, if using BBF). Also you might try setting your camera to it's fastest frame rate, though the T7's maximum 3 frames per second is not very fast. Rattle off short bursts of 2 or 3 shots to try to catch the peak moment in sports. Study whatever sport you are shooting to learn when those peak moments occur.
If you need more "reach", the Canon EF 100-400mm IS USM (either version) would be a good choice. However, these are relatively large, heavy, expensive lenses. They may seem unbalanced on a small, light camera like the T7.
As long as you have good light, an f/5.6 lens like the EF 70-300mm or EF 100-400mm would be fine. But in more challenging lighting conditions an f/4 or f/2.8 lens would be better. This gets more expensive and trickier, though. There are f/4 70-200mm lenses, and there is a 300mm f/4 available. But there is not a zoom that gives both f/4 and reaches 300mm. There is a 200-400mm f/4, but now you're looking at a very big, very expensive. There also are f/2.8 70-200mm lenses as well as f/2.8 300mm and 400mm. But, again, those are big, heavy and expensive.
You probably also got an EF-S 18-55mm lens with your camera (I have never seen a T7 offered without one.) Are you using it? If not, why not? Another response suggested an EF-S 17-55mm, which is certainly an upgrade. But if you don't use an 18-55mm much, you aren't likely to need or use a 17-55mm.
Hope this helps!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2), EOS M5, some other cameras, various lenses & accessories
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