Originally Posted by ElCid52
That means 'sneaker zoom' is going to be a problem, so a zoom will allow a lot more flexibility on framing your shots than a prime.
Originally Posted by ElCid52
Depends on the gym. An f/4 lens will very likely not work, an f/2.8 might, might not. It would do ok at a Pro or Division I/II college game, which are lit for television coverage. A typical high school gym is not well lit.
If you're not set on buying the lens immediately (i.e. if you can go to another game with your current gear), do your research directly. Set the 55-250 to 85mm and see what the framing is like from your seat. Try 135mm. Try the 70-200mm range. That's the 'focal length research' we've mentioned. You can also do 'aperture research' in a couple of different ways. First you need to determine the maximum ISO that you're comfortable using (in terms of image noise) on your XSi - let's say that's ISO 400 (it might be something else for you - I've never shot with an XSi). You can set the17-55mm to f/2.8 (Av mode)and ISO 400 and determine the best metered shutter speed you can get in the lighting of the gym. You can also use the 55-250mm lens - at >150mm it's f/5.6, so if you push the ISO to 1600 (and your comfort level is 400, two stops lower), then you can ignore the noise and see if you can stop the action as you want (ISO 1600 at f/5.6 is equivalent to ISO 400 at f/2.8).
You'll probably need a shutter speed of 1/500 s to completely stop player motion, but 1/250 s might be sufficient. Some motion in a shot can be ok (arms, legs, the ball) to show action, and 1/125 will likely be enough to stop body/head motion. Also, keep in mind that with the 70-200mm f/2.8, you'll likely need a shutter speed of around 1/320 s to eliminate camera shake at the long end of the zoom range.