[JeffH]

Gerry, I've had my 100-400 for some 15 months now and initially had similar problems to you.

After posting some shots on a photographic forum I was advised by a semi-pro photographer to take some test shots of a brick wall at various focal lengths. I did so and he checked them over for me and concluded that one of the lens elements was probably out of line.

I took the lens back to the dealer (Jessops - I bought it on a price-match deal) and also showed them some of the photos I'd taken with it. Without any quibble whatsoever they exchanged the lens for a new one and there was an immediate and noticeable improvement.

Also, soon after buying my 30D and 100-400 I went on a couple of the Canon Experience seminars and I met another guy who'd had a similar problems.

My advice would be to take your lens back to the dealer in the first instance and seek a replacement.

Jeff