So I have a budget around 600, and i'm currently looking at some jobs in portrait photography now. Unfortunately, however much i love my tamron 17-50, it's just not quite long enough for a good working distance portrait. I'm looking at a canon 70-200 f4L non IS, and i already know it's a great lens but i was just wondering if anyone had experience using it for portraiture? This would be on an XSi body, so 1.6 FOVCF applies. Thanks!