Sigma 17-50 f2.8 AF speed
I just got the Sigma 17-50 2.8 EX HSM delivered. I'm testing it out indoors (night time currently) and on my K5 it is huting frantically. I have only the 21,40,70 DA primes to compare against but it is noticeably slower and it gives up often when in comparision my primes do not. I'm aside from the 70, the primes are not brighter so this can't account for AF performance. Is this normal for the 17-50? Or, do I have a lemon.