TSchrief wrote in post #15252058
You can't possible be referring to the Sigma 70-200 f/2.8 OS HSM. It is sharper at 2.8 & 200 than the Canon 70-200 you do recommend. It is cheaper than any of the Canon 70-200 2.8s. The AF is excellent. It has IS. It also comes with tripod collar and a hood.
I am. I didn't say anything about the sharpness comparison though, but since you mentioned it, it looks to me from TDP crops that they are even at 70mm but better with the 70-200/2.8 non IS at 135mm and 200mm.
http://www.the-digital-picture.com …omp=0&FLIComp=4&APIComp=0
My point was concerning the AI servo AF accuracy of the Sigma 70-200 OS versus any of the Canon L's. This is not based on personal experience with the Sigma for me but from owner comments here on POTN though. Seeing as how you own it, how is the AF accuracy compared to say your 100-400 when shooting moving subjects in AI servo? I am a big fan of my Sigma glass in general, but in AI servo, they fall slightly behind my USM glass in servo accuracy.
TSchrief wrote in post #15251758
If 2.8 is not enough, 1.8 isn't going to help much. HAVING to go from f/2.8 to f/1.8 to get the shot will only take you from "I can't shoot." to "I got a lousy, very noisy shot." The 1.3 stop difference will never turn "can't" into "WOW". Think of going from ISO 12,800 to 5,000. You just went from "unusable" to "horrible". That is not worth the loss of convenience of a prime lens in your situation. Get the zoom.
Sure you probably aren't going to go from unusable 12K to SI cover quality, but for many people shooting kids sports like the OP, often in terrible shooting conditions, that 1-1/3rd stop will give a significantly better image at ISO 5000 than 12K. The IQ of the 18mp crop bodies really tanks hard at 12K ISO. I consider ISO 3200 to be the top of the line if I can help it. My 85/1.8 lets me shoot at 3200 F2.0 when my daughter is in indoor swimming lessons. If I were using a 2.8 zoom, I would have to shoot at ISO 6400. Nothing against the zoom, but I think both zooms and primes have there place when shooting indoor sports. This is less of a concern with a newer body like the 1Dx or 5D3 with the improved ISO performance, but on crop, I think it is still needed.