My 100-400 is not focusing consistently specialy when the light is not optimal. I have done the MA three times now and I am not happy with the results. There does not seem to be a consistent behaviour if the light is not optimal. If I shoot a subject in sunlight, it is sharp and focuses correctly. When I shoot say a hawk in a tree with the sun at 11 o'clock and the sky seen through the branches, although the bird is clear and I shot the bird with centre point focus (and I am quite still - shot over 3000 shots of black kites with my rebel and the same lens) and I can swear I did aim right the camera somehow does not focus on the bird...

When I shoot with my 24-105, the camera focuses correctly. So the camera does not seem to be at fault. It just occurred to me that I should probably clean the contacts. However, if that does not fix the problem, I am thinking of taking the lens to the Canon repair centre and have them look at it. The lens is almost 5 years old and has never been serviced.

Does anybody have any other suggestions? Has anybody experienced anything similar?

Look forward to your advice.

Thanks in advance.