OK, let's say I'm off to an Iron Pigs game tonight...what lens do I take with me? I have a Canon, but that's not critical is it? To complicate things, part of it will be in daylight, part under the lights. I know I'd be best off with an f2.8L, but this is a hobby for me, not a business. 250mm? 135mm with a 1.4x or 2.0x converter? How low of an f-stop can I get away with?
Here's an article I just read regarding a good sports lens...
"Recommended Canon lens for sports photography
EF 300mm f/2.8 L IS USM is currently one of the best sports photography lenses on the market for Canon users. It is a fast, ring USM lens, that results in sharp images. It's also fully compatible with Canon EF 1.4x and EF 2x extenders, in case you find later on you need extra distance coverage.
Recommended Nikon lens for sports photography
For Nikon SLR users, the AF-S VR NIKKOR 300mm f/2.8G IF-ED lens is currently one of the best for serious sports photographers. It is a high speed telephoto lens that has an inbuilt Silent Wave Motor for fast autofocusing. It is also compatible with Nikon TC-17EII (1.7x), TC-14EII (1.4x) and TC-20EII (2.0x) teleconverters.
The downside to both these recommendations, is that they are expensive professional models. The EF 300mm f/2.8 L IS USM, sells for around $3900 USD at time of writing, and the AF-S VR NIKKOR 300mm f/2.8G IF-ED lens is $6500 USD. However, if you intend on doing a lot of serious sports photography, then they are well worth the money.
Yikes!! Isn't there a cheaper sports photography lens? [note: this is still part of the article, not my words

]
You could get away with a cheaper 200mm focal length telephoto lens. Especially if you also attach a teleconverter to your setup as well. A good lens for closer sports photography, is a Canon EF 70-200mm f/2.8L IS USM Telephoto Zoom Lens, which currently sells for around $1500 USD. For Nikon camera owners, there is the Nikon 70-200mm f/2.8G ED-IF AF-S VR Zoom Nikkor Lens for aproximately $1700 USD. "
To me, the "funny" part is that he thinks $1,500-1,700 for a lens is something to spend to get away cheaper.