Lens Question for Sports Events

[jebfsu]

First off-thank you to everyone who has given me advice thus far. I am relatively new to digital SLR photography. I currently own a Canon dSLR XT Rebel with a zoom lens EF 28-105mm 3.5-4.5 USM. I use my camera for primarily two different types of venues.
1. My daughter is an artistic roller skater and the rinks are always dark. I have found through experience (thus far) and advice, that the Tv mode at 1/125 works best. When I zoom, obviously I loose all light and the pictures are way too dark. I have been told I need a faster lens for those conditions. (I cannot use a flash.) I was thinking of purchasing the EF 50mm/1.8 lens. I have done some research and from what I've read, it seems everyone recommends having this lens. I have no problem freezing the action (although 1/250 is better), it's the lighting that's the problem. I've heard alot of great things about this inexpensive lens. Any recommendations?
2. My daughter also ice skates. Using the Tv mode at 1/250 or 1/500 I can get beautiful pictures. There is plenty of light in this venue; however, I need more zoom. Would a 70-300mm lens be good for this? Any suggestions?

I really don't want to purchase too many lenses. I have no problem with the 50mm lens, but the 70-300 is quite expensive-that's OK, as long as it's what I need. I'm not sure what I should purchase and any help you can send my way is appreciated. (Like the Walgreen's commercial with a perfect world-I like to dream that it would be nice to be able to 'borrow' lenses to try out on our camera. Then I would know which one(s) to buy.)
Thanks in advance.

[SkipD]

In my opinion, the first thing that you need to discover is the focal length(s) that are required for the tasks at hand. Then, you need to discover what aperture(s) you need to be able to shoot at your chosen shutter speed(s) while using ISO settings that give you acceptable image quality. Only after figuring all this out can you determine what lens specifications you need to get the job done. Then you can go shopping to see what lenses are available that meet your specs.

If you find, for example, that your 28-105 provides all the focal length flexibilty that you need, then you might need to merely find a faster lens (or combination of lenses).

On the other hand, if you find that the images are always too small with the 105mm setting, you can calculate how much more focal length you need. Take a shot with your lens at 105mm and make full-size - non-cropped - print (regardless of the quality) so that you can take some measurements. Assume for the moment that you want the subject to be twice as tall as in the test image. That would require twice the focal length that you used for the test shot, or 210mm. Now all you need to figure out is what maximum aperture (minimum f-stop number) you need for that 210mm lens - and what the impact on your wallet might be, of course.

Additional thought - RENT lenses to see what really works for you.