I have a Canon XTi and looking for a zoom for indoor Hocky Games, Outdoor soccer ect. I'm looking at the Canon 70-200 2.8, or the 70-200 4.0 is.
What is the pros and cons of the 2.8 vs the 4.0 IS[{POST_SNAPBACK}][/a]
For starters, one full stop. This means you'll be shooting at a higher ISO to achieve a shorter shutter speed, compared to using the f2.8 version. How this affects you is subjective. You may or may not like the increase in noise in the resulting images. Only testing and pixel peeping will give you that answer.
Size is a factor. The f2.8L IS is 3.4" x 7.8" (diameter x length). The f4L IS is 3.0 x 6.8 in. Although this isn't a huge difference in size, you need to determine if you camera bag/case is large enough.
Another factor is weight. The f2.8L IS weighs in at 1.47kg. The f4L IS weighs in at .76kg. That's about half the weight of the f2.8.
Another factor is filter size. The f2.8L IS uses a 77mm diameter thread. The f4L IS specs 67mm. Generally, you want the same filter size on all your lenses. It makes buying filters less costly.
Lastly is optics. Take a look at [a href=\"http://www.usa.canon.com/consumer/controller?act=ModelDetailAct&fcategoryid=150&modelid=14260]the MTF curves[/url] and decide if they're good enough for you. In most cases, users don't care --they just want the lens in that focal length with those features within their price range.