Here's a quick overview of lenses that will hopefully give you an idea of what you are looking for when purchasing a camera/lens combo. If you have a camera with a fixed lens, I still suggest you read through it, although you can skip the sections on interchangeable lenses and extenders as they won't pertain to your fixed lens.
The focal length of a lens is the distance between lens itself and the sensor on the camera. It is noted in millimeters (mm) and determines how much magnification you get on your photos. The longer the distance (higher mm), the more magnification you will get (and conversely, the less angle of view you will capture). As a point of reference, 50mm is considered what most people see naturally with their eyes. For most sports you'll want a lens with a longer focal length so that you can capture what is happening across the field.
One thing to note is that if you have a crop-frame sensor, the focal length will be affected. For instance a 50mm lens with a crop frame camera that has a 1.6 crop factor will mean that it's like having an 80mm lens. So for every lens just take the focal length and multiply it by the factor.
Types of Lenses
Lenses used for most photography (including sports) are usually categorized into two categories: whether they are prime or zoom lenses, and then by their focal length (wide-angle, standard, telephoto, etc.). There are also additional categories for macro lenses (good for focusing very close) and fisheye (which give a rounded look), but neither of these are really as useful for sports so I won't cover those here.
Prime versus Zoom: A prime lens has one focal length. It cannot change the magnification/angle of view. A zoom lens can change both and will be listed as having a range of focal lengths (ex. 70-300mm). Zoom in this case does not necessarily mean that the magnification is large as you can have a zoom lens that has a very short focal length (there is a 10-20mm zoom lens for instance).
Focal Length: The three main types of lenses are wide-angle (generally no more than about 35mm), standard (around 50mm), and telephoto (anything larger than a standard). Telephoto lenses that reach over 300mm are often referred to as super telephoto lenses.
If you have a camera with an interchangeable lens, the most important thing to know is you need to make sure the lens you are buying works with your camera. Lenses are designed both for a specific brand of camera and then for a type of camera. For instance, Canon lenses come in mirrorless, crop-frame, and full-frame (although crop-frame cameras can use lenses designed for full-frame cameras, but not vice versa).
Lenses don't have to be from the camera manufacturer. Many third party companies make lenses that are compatible with different brands and types of cameras. Also, there are adapters that allow you to use lenses meant for another brand, however I don't recommend these as they will often cause issues with the auto-focus (usually making it sluggish at best) which is exactly the opposite of what you want to shoot sports.
Finally, there are often different qualities of lenses. For instance, I shoot with a Canon camera and started with their lower end 70-300mm telephoto zoom. I then moved up to their luxury line (denoted by L in the name) and now shoot with a 70-200mm telephoto zoom. Here is an example of two similar photos but shot with the two different lenses. The one on the left is using the lower quality lens and you can see that it isn't quite as sharp (especially if you look at it full size). The color is also better on the second photo. Higher quality lenses also often have better/faster auto-focusing and are probably better weatherproofed.
Aperture is how wide the blades that cover the inside of the lens open. Lenses have various maximum apertures (aka how wide the blades open) and on some zoom lenses the maximum aperture can vary based on the focal length you have chosen. Apertures are written as f-stops where the lower the number the larger the aperture. So a lens with a maximum aperture of f/2.8 would be able to open up larger (and let in more light) than one with a maximum aperture of f/4.0. Most lenses have a minimum aperture around f/22.
Apertures are generally thought of in "full stops" (f/1.4, f/2.0, f/2.8, f/4.0, etc.) where the next number up is double the previous f-stop. Each full stop going upward halves the amount of light that enters the lens from the previous full stop.
Why is this important? There are two advantages to having a lens with a large aperture (low f-stop). The first is that it will let in a lot of light, which if you are shooting games at night or in dimly lit gymnasiums is very important. The second is that the larger the aperture, the more the background can be blurred out, which helps put focus on the subject you are shooting (as long as they are relatively far away from the background). This is called bokeh and, when shooting against the background of a crowd, can really make your picture pop. See the photo below for a great example.
Don't worry if you don't understand this now. I'll post a whole piece on aperture later (and even more when I discuss exposure). Just know that if you can afford a lens with a larger maximum aperture (lower f/* number) you'll be better off for shooting sports.
To be honest, if you're going to just use your lens for shooting sports (or maybe wildlife), then you don't really need image stabilization. The general rule of thumb is that if your shutter speed is about the same as the focal length you are shooting at (or your shutter speed is higher) then you don't need image stabilization. Since you'll be shooting at least 1/500 sec, you'd only need image stabilization if you've got a ridiculously large lens (500mm). And since stabilization can be expensive (the lens I have costs $800 more to add it), it's something you can live without if it breaks the bank.
These are specific to the manufacturer and even to the specific lens. The best thing I can say is to do some research and get the lens with the best drive you can find since you'll want something that captures focus quickly and does a good job of keeping focus.
Extenders (aka Teleconverters)
Extenders (which are not the same as extension tubes) allow you to get more magnification out of your lens at all focal lengths. For instance if you have a 70-300mm lens and a 1.4x extender, you'll be shooting at 126-420mm (you multiply your focal length by the extender amount). While this is sometimes nice, it will cost you a full stop of aperture for a 1.4x extender and 2 stops for a 2x extender. This will obviously cost you the ability to shoot in darker situations. If you usually shoot under great light, then they might be worth it. But if you're shooting at night it might really affect the noise level of your photos. Also, be very careful that the extender works with your lens as they often only work with some of the higher end lenses. Finally, don't forget if you have a crop-frame camera that also changes the focal length so a 1.6 crop factor camera with a 1.4x extender on a 70-200mm lens will top out at the equivalent of 448mm (200 x 1.4 x 1.6) on a full-frame camera without an extender. But of course the minimum will be 157mm (70 x 1.4 x 1.6) which means getting shots up close will be difficult.
It's all about trade offs!
Last updated 6 February 2017