I am planning to do a lot of low-light event photography soon, and I want to know if my 18-200mm kit lens that comes together with my 50D can deliver high quality photos when paired with Speedlite 580EX and using f8 (since I want to get sharp faces for all attendees)? I still do not have budget to get good lens (f <= 2.8) now.

Please advise. Thanks.