The difference between 50mm and 85mm/100mm is pretty significant. The longer the focal length, the less the relative difference becomes (i.e. 85mm is 70% longer than 50mm, 100mm is 100% longer than 50mm, but 100mm is only 18% longer than 85mm). So, the difference between 85mm and 100mm is not that much, and other than that modest difference in focal length and the equally modest difference in aperture (1/3 stop), the lenses are basically identical.


So, which do you choose? You could try setting your 70-200mm f/4L IS to 85mm and then to 100mm and see what the framing looks like in the venues you'll be shooting. If you'd rather not try that or are still unsure, go with the 85mm. The 7D gives you plenty of pixels for a slight crop if needed, but you can't always back up to get a wider framing.


I agree with Daniel that you're on the right track here. The other lens that's usually suggested for indoor sports is the 135mm f/2L - it's an excellent lens with very fast AF. But since you'll be taking pictures officially, that should mean sideline access for sporting events. The 135L is better-suited to the stands when used on a crop body. If you were shooting FF, the 135L would probably be a better choice.