As you've found out, a zoom lens for low-light photography can be quite expensive.
Instead of a zoom lens, the alternative, as others have hinted, would be to go with a
prime lens. Again, a "prime lens" is a lens that does not zoom. You have to physically get closer or farther to your subject if you wanted to "zoom". That's why some people say you "zoom with your feet" when using a prime lens.
For low-light & sports photography, prime lenses are cheaper than zoom lenses. There's also an added benefit to prime lenses. Oftentimes, prime lenses have larger maximum apertures than zoom lenses. A larger aperture allows more light into the camera...this part is crucial for low-light & sports photography. Since gymnastics takes place indoors, it is relatively lower light.
There are 2 things to look for in a prime lens: the focal length and the aperture.
In laymen's terms, the
focal length determines whether the lens is "wide angle", "normal view", or "telephoto". The focal length determines how much of the scene you want to capture. Focal length is expressed in millimeters (mm). The smaller the focal length, the wider the angle of view (ie. capture more of the scene). The longer the focal length, the narrower the angle of view (ie. zoomed in, or capture just a small part of the scene).
If you got a 20mm lens, for example, you could take photos of the entire gymnastics scene, but your daughters would be pretty tiny. If you got a 100mm lens, you view of your daughters might be too close or too "zoomed in", depending on where you sit.
That's why previous posters asked you where you planned to sit for the gymnastics meets. If you're gonna be pretty far away up in the stands, perhaps you could look into an 85mm or a 100mm lens. If you're gonna be close to the floor & close to your daughters, perhaps a 50mm lens might work.
How else could you determine what focal length to buy for a prime lens? Well, you stated that you haven't bought your dSLR yet. When you do, your dSLR will come with a standard "kit lens". Usually, this kit lens is a zoom lens that goes from 18-55mm. Try out the kit lens at your girls' gymnastics meets. You'll quickly see that 18mm is very wide and probably captures too much of the scene.
However, zoom in to 55mm and see whether this is enough "zoom" to capture your daughters on the apparatuses. If it is, then perhaps a 50mm prime lens is the way to go.
The other type of kit lens would be the 18-135mm lens. With this lens, you can zoom even more to capture closer views of your daughters. Using this lens, try zooming in to approximately 85mm. And then try zooming in to 100mm. Are either of these focal lengths close enough (but not too close)?
Using this method, you can then determine the focal length of your prime lens purchase.
The second thing to look for in a lens is the
maximum aperture. Think of the aperture as the "hole" in the lens that light goes through to get into the camera. The larger the aperture (the larger the "hole"), the more light goes into the camera. This is good for low-light and sports photography. The smaller the aperture (the smaller the "hole"), the less light goes into the camera. This is not so ideal for low-light / sports photography.
In photography, aperture is expressed as f-stops or f-numbers. It sounds backwards, but a smaller f-number = a larger aperture. Conversely, a larger f-number = a smaller aperture.
So the prime lens you're looking for will have small f-numbers (ie. larger apertures). The kit lenses I mentioned above have apertures from f/3.5 to f/5.6. At first glance, these seem like small numbers, believe it or not you can go even smaller than these.
The 70-200mm lens that others mentioned above has an f-number of f/2.8. This means that this lens lets in more light than the kit lens. Actually, the 70-200mm f/2.8 lens is a very popular lens used by professional sports photographers (and other professional photographers, too). At f/2.8, this lens lets in approximately 2x-4x
more light into the camera than the kit lens!
I mentioned earlier that prime lenses can have larger apertures than zoom lenses. A 100mm f/2 lens, for example, will let in 2x more light than the f/2.8 lens above. Or, if you went with a 50mm f/1.4 lens, it will let in 4x more light than the f/2.8 lens.
What's the advantage of letting more light into the camera? (or, what's the advantage of larger apertures?) When you let more light into the camera using a larger aperture, this allows you to use faster shutter speeds to "freeze" the action. You'll learn more about this when you learn the basics of photography, specifically, when you learn about the "
exposure triangle" (aperture, shutter speed, and ISO -- and how these 3 interact with each other).
The fact that larger aperture lenses allow you to use faster shutter speeds is the reason sports photographers specifically look for these types of lenses. That's also why large-aperture lenses are also called "
fast lenses". You may have seen this term used earlier in this thread.
So those are the 2 things to look for in a low-light prime lens. What are your options? If you chose Canon, here are some fast prime lenses:
- Canon 50mm f/1.8, $119 - your best value! (link to B&H)
- Canon 50mm f/1.4, $379 (link to B&H)
- Canon 85mm f/1.8, $399 (link to B&H)
- Canon 100mm f/2, $474 (link to B&H)
Note the larger apertures (smaller f-numbers) of these lenses. All these apertures are larger than f/2.8 (ie. all the f-numbers are smaller than f/2.8). That's what you're looking for.
I only know about Canon stuff. But other brands will have similar lens offerings. If you decide to go with Nikon, Sony, Pentax, etc, then at least you'll know what to look for in a fast prime lens.
Hope my ramblings aren't too confusing.