The 17-40 "L" is a stop slower than the 16-35mm f/2.8 L II, and that's basically the reason why it costs only half as much as its bigger brother. But otherwise there's not a great deal of difference between them. Some reviewers have found the 17-40 to actually be sharper at the wide end, and it may be less prone to lens flare. Either way, it's a great, sharp lens which delivers superb image quality and is a favourite of landscape photographers.