Canon Rebel XTI Sports Lens?

I just got into photography (newee), I purchased a Canon Rebel XTI. I will be taking pictures of Mixed Martial Arts Fights as well as boxing, and note that I am in Media zone so I am basically right alongside the ring. I have a small budget so I need advice on perhaps the cheapest best lens I can get. Action is non stop at times, and the lighting is only good when the fighters are in the middle of the ring, but bad when against the ropes or fence. Any advice guys or gals?

Note that I cannot use a flash when the fighters are in the ring.

Well if you pick a fixed focal length you'll benefit in two ways, first you'll have a faster lens, second you need not worry about zooming in or out. With Canon's 1.6 multiplier a 50mm = 85mm lens (a f/1.4 cost about...$300). Tamron's 18-250 (28-400 w/1.6 multiplier) f/3.5 macro zoom cost about $400/$500 bucks.
I own both of these lens mentioned above, though I'd recommend a flash, Canon's 580 EX or the NEW 580 EX II. The little flash on your XTi may not be enough power for those shots across the ring, on the ropes next to you it will work. Once again I know you are on a budget (so am I) but, I got my XTi body with the Tamron lens and a few months later, the Canon 50mm f/1.4 then a couple months later the EX II speedlite.

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You'll probably want a fast lens - zoom is probably not as important. Probably your best choice for cheap and fast is the Canon 50mm f/1.8 (aka "the nifty fifty") which can be picked up new for somewhere around $75. It's a complete piece of junk build-wise but has surprisingly good optics.

Since zooming is not an issue, given the space you are working with, the Canon EF 50mm 1.8 ($80.00) will do. It is fast enough for an averagely lit room.

Then I would save my funds and get the Canon EF 70-200 2.8L IS ($1500.00)

Look for lenses with an fstop of anywhere from 1.2 - 2.8 for your interior sports shots.

With the level of light in gyms you'll need the fastest lens possible.

Look into an 85 f1.4 or a 50 f1.4.

There's also a 100mm f2 from Canon (may be discontinued).

HTH
V2K1

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