I am very interested in this lens for wildlife photography. i've looked at a couple of reviews that say it is not very good (compared to primes) at 400mm for distant subjects. It is not clear as to what is meant by distant subjects. Are we talking about 100 yards or more? I would like to hear from people who have used the new version and under what circumstances. Canon's 200-400 f/4 should be out later this year at a price tag probably close to $11,000 (vs. $7000). I wonder if I should wait?