Occasionally, an image taken with the 100-400L may need a bit extra sharpening, but then, so does almost every long lens, which is what people often tend to forget. It also can focus quite close, and with the 500D achromatic close focus doublet it even reaches 1:1 zoomed in at 400 mm at a very decent distance from the lens, great for chasing dragonflies, and other easily disturbed creatures.
As to the IS, the balance of the lens really helps a lot too, provided one holds it the way it was intended. The focusing and zoom ring are coupled, in the sense that when you zoom, the focusing room moves with it, and vice versa (independent of focusing, however). That means that one can hold it by the focusing ring for one-handed operation for both zooming and focusing, if the latter is required, f.e., to turn the focusing ring just that little bit because the AF system decided to focus on a little twig just next to the main subject, or because the wind was blowing the object just a little bit away, or just to put the sharpest or focusing point just a bit further ahead or backwards. It also makes zooming and focusing very fast, zoom out to find a subject, zoom in to magnify and frame. And th efocusing ring also happens to eb the perfect spot to hold th elens for very good balnce. Despite the 2-stop IS, I managed to shoot at 1/45s at 400 mm with my APS-C cameras when I still had those for a > 50% return rate in sharp photographs. With IS off I needed ~ 1/750s or faster. I also tested this against a Sigma 170-500 and a few Tamron 200-500s, and with those at 500 mm I needed a minimum of 1/1000s to 1/1200s to get a reasonably sharp handheld picture. I got the Tamron first, but returned it two weeks later, got the 100-400L despite it really being out of my budget by a fair amount, but I have never looked back.
It is the only zoom lens left in my arsenal, and for a reason. I consider it a great lens for longer distance macros and closeups (even if that sounds contradictory <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />), for landscapes, sport, wildlife, flowers and candids (despite the white colour).
The biggest problem really is focusing accurately, i.e., from a user and user error perspective, especially in a dynamic environment, as DoF even at F/5.6 at longer FLs is rather thin <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. Get it just a bit wrong, and th epicture will not be optimally sharp.
A lot of people seem to have problems with the zoom mechanism, but I do think that if one gets used to it, one realizes that for such a lens there is no better way, especially for handheld photography, because of the balance and extremely fast method of working this makes possible.
The picture below I had on the main wall of my living room for a while now, printed at 60 cm X 90 cm. It certainly beats the winter blues <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' /> :
And here one taken with 1D Mk III, juvenile songthrush, about 1/3 crop of total image:
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....