Instruction: Generate an answer to the question by utilizing the information contained in the document.

Input:
Question: Is this lens good for sports photos? Is it also good for sports photography in low light situations, like stadium lighting?
Document: With 17-40mm lens, I can take well-focused images shot after shot. Even under challenging light, I can hit fairly high percentage of photos without flash. At 200mm, situations get considerably trickier. Under very good lighting, I can take one great looking photo after another. Under somewhat limited light, however, I found myself with rather high percentage of out-of-focus photos. Increasing the ISO speed helped a good deal, but even that has a limit.
I was able to get pretty good handheld shots above 100 mm. with low shutter speeds such as 1/30.2) Remember that bokeh (background blur) will be a problem in some situations considering the minimum aperture value of f/4. You will not be able to abstract your subject effectively from a complicated background in some shooting situations or focal distances. Additionally, minimum aperture value renders this lens slow for action photography especially in low light situations.
I use it to do dog photography and it is great outdoors. Indoors for action shots in low light, I would go with the 2.8 over the f/4 to get those really crisp photos in low light conditions. Otherwise great all around zoom lens at a good price. Image stabilization has really come in handy too.
I also have to then switch to Time Priority and set to 250 to 320 to stop movement in sports such as football. Football game photos under stadium lights then become very dark and must be corrected latter with my editing program (which you will need). The photos with that kind of ISO then turn out high noise and hence will also need corrected. Over all I love this lens for day time sports.
When taking sports photos in good full sun this lens does great. I have a Canon Rebel and usually keep my ISO at 100 in that kind of light with Aperture Priority Set and shutter speeds usually hit 500-2000/sec normal. Plenty fast to stop all action except maybe a hit baseball. When the sun starts going down it becomes a different matter all together. I find myself quicky turning up my ISO to compensate.
This is a nice lens that I regulaly use for wedding photography. F4 will work in most situations, but not all situations. The image stabilization will provide several stops of benefits. It is a good lens for backpacking and such as well.
Not as fast as the 2.8 version, but WOW! at a lesser cost, and less weight too, and still delivers very great photos, excellent IQ. The IS and steady f4 aperture allows shooting quality images in dusk / dawn situations. Extremely very good lens, with sturdy and well-built feel to the construction, feels and operates solidly. Well worth the purchase.
This is my first L-Series lens and I've taken a few sample shots with it and I already love it. I rented this lens for about 6 weeks a while back for use with my kids outdoor soccer season. The lens was great. It is not going to do to much for you in low light situations, but it is outstanding in outdoor and areas where there is plenty of light.
I spent a lot of time doing research trying to decide which zoom lens to get and decided to spend the extra money for this one and the results have been well worth it, very fast,accurate auto-focus even on low light situations, IS works very well, Image quality is superb. This has been regarded as one of Canon's best lenses. I think you would have a hard time finding anyone who doesn't love this lens. I'm using it with my 30D and have consistantly gotten excellent results.
Therefore, if you are not strong, it will tire your hands and arms after a while.The picture quality is worth suffering a little. To me the f2 f4 debate is solved in part  with a good Canon 580 EX II external flash, I took pictures of a social event with my 4 external flash guns and and they came out fantastic. However, low lighting auto focus is very difficult with this lens without bumping ISO.
Answer: This lens is great for bright daylight sport photography.  Although it is a constant aperture F4 lens, it may not be fast enough (i.e. collect enough light to keep your shutter speed high) to be able to stop sports motion.  Also the image stabilizer will not help you to stop motion, it is only designed to prevent camera shake which it does well.  Keep in mind the other comparable lens is the significantly more expensive EF 70-200 2.8L but it is 1 stop faster (i.e. doubles your shutter speed or you can lower your ISO).  So it is possible to take good sports photography with this lens but you'll need to crank up your ISO but 1 stop more than you normally would with a F2.8 lens.  This is especially true with the newer camera sensors where you can crank up the ISO without much of a noise penalty.