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I got my Canon EF100-400 IS lens for bird photography, and it is a fantastic lens for all-round bird and wildlife photography. Image quality is excellent as you'd expect from a Canon L series lens, and the image stabilisation works very well - it is possible to handhold the lens down to 1/100 and still get very good bird pictures provided the subject isn't moving much. Weighing in at over 1.5kg including the tripod collar (supplied) there's no getting away from it, this is a heavy lens. That is the price of performance unfortunately, and it does add considerably to the weight of the kit-bag. Bird and wildlife photographers always want more reach, and adding canon's 1.4x teleconverter is the obvious way to do it. Unfortunately this combination loses another stop making the 400 end f8. Even masking the contacts to fool my Canon 350D into trying to autofocus with it gave poor results. The autofocus took so long to settle that the combination was almost useless for bird photography, it was quicker to switch to manual focus and use the focus confirmation beep. The lens is f5.6 at the long end, which is a little bit slow, meaning that running at ISO400 or higher is often necessary on overcast days to get the shutter speed down. This lens is an excellent combination for mid-level DSLR bird and wildlife photography, fast enough to get good shots of birds in flight and long enough to get good pictures of birds at typical binocular range - if you need a scope to see a bird properly then it is a little bit too far away and a longer lens would be needed, though the EF100-400 will still get a good record shot. Note that battery life is not particularly relevant to this product - there are no batteries in it. Canon's IS does draw a little power to stabilise one of the optical elements but I have not noticed it shorten the DSLR battery life relative to other lenses.
This is a great lens - make no mistake about it. Whilst it's not cheap, it is an excellent "maid of all work" zoom lens for wildlife, sport and aviation photography - and pretty much anything else that requires a decent piece of glass attached to your Canon DSLR camera. It does have its limitations, mind you, and you're not totally guaranteed a magazine-quality shot every time you hit the shutter release, but it's easier to take great photos than bad ones with it. I have had my 100-400mm IS lens for just under 4 years now and it has been used in conjunction with my Canon EOS-300D and -350D DSLRs plus my new Canon EOS-40D. It will fit any pro-sumer Canon DSLR - the above-mentioned ones plus the newer -400D and -450Ds, plus the higher-end -10D, -20D, -30D, -50D and the 1-series. New prices are around the £1100-1300 mark - I saw a second hand one in my local camera shop nearly four years ago for around £800 and jumped at the chance (they were still around the £1200 mark then). It had hardly been used at the time, but I've made up for that now.... Second hand ones appear on eBay and other sites from time to time - if you see one, they're a worthwhile investment. As an aviation photographer who specialises in low-level jet shots (i.e. no sky in the picture whatsoever), I needed a big lens that was relatively easy to carry up mountains and steep hills, had a solid build quality (as it would be in a camera backpack, being jostled around whilst climbing said slopes) and delivered excellent results with ease of use (no second chances to get that all important shot!). With the f/4.5-5.6 L 100-400mm USM IS lens, I wasn't disappointed at all. Build quality is superb, the lens is solid yet easily operated. The push-pull zoom mechanism may fool some people but I find it exceptionally easy to use. A locking collar on the lens can slow the zoom movement or lock it completely. The tripod attachment is fastened to another collar, which can be removed to save weight or, in my case, turned 180 degrees so to provide a convenient carrying handle when picking it up. There are two Image Stablising (IS) modes - effectively for whether you are moving (e.g. on a boat) or your target is. The IS can also be turned off and manual focus used, but I've never had to do this - the auto-focus works very quickly and quietly, never quite needing to "hunt" (e.g. focus in and out constantly, not quite finding the correct setting) for the subject in my experience. It does struggle a bit in darker conditions (it is an f/4-f/5.6, after all), so taking pictures of dark-coloured jets in already dark mountain valleys doesn't always work too well. Experience brighter conditions and subjects and you're laughing. There is little to no noise at all from the auto-focus, with only the occasional almost inaudible rattle from the gyroscopes inside the lens when its working particularly hard. Some people say you can use this lens with a Canon x1.4 or x2 extender - you can, but you will have to tape over the contact pins in the camera body as there are issues with the auto-focus when the camera detects a f/5.6 lens attached. In addition, you lose a full f stop with the x1.4 converter and two with the x2. Due to the crop factor, bolting this lens to a Canon DSLR such as the -350D or -40D increases it effectively to a 160-640mm lens, making it extremely useful. Some claim the lens is a little "soft" around the edges at the 400mm limit in terms of picture quality but I've yet to experience this for myself. The lens hood is easy to attach, is sturdy and does not fall off. It also serves as something of a bumper in the event that you drop the lens(!) - far better to break/crack the hood than the lens itself! The only wear and tear the lens has suffered in the three and a half years I've owned it is a couple of little pieces of white paint flaking off the tripod attachment - and that's it. Some may say this lens is heavy but I've lugged it all over the mountains and hills of Scotland, Northumberland, the Lakes and North Wales in my Lowepro backpack and I don't even realise it's there. People who hold the 100-400mm lens for the first time say it's heavy, but that is just a case of getting used to it. I have hand-held it all day at airshows and air displays, etc., without problems. Do not be tempted to carry the camera/lens combination by the camera body only, however - despite the feeling of weightlessness after a while, the 100-400mm is still a fair weight and you will damage either the lens mounting on the lens or the camera body itself. Two hands are therefore needed, one on the body and one on the lens - personally I hold the lens itself with my left hand placed underneath and just behind the hood attached to the front end. It is an extremely comfortable lens to hold and use. Yes, it may be pricey at over the £1000 mark, but you're getting a very good piece of kit for your money - it may be the only "good" piece of glass you'll ever need for your Canon DSLR set-up. It may struggle a little in bad light as it only goes down to f/4.5 - splash out on a Canon f/2.8 300mm and an extender (and re-mortgage the house while you're at it - they're £3000!) if you need the extra stop. This lens certainly turns heads, being a Canon L-series 'white lens', so you'll look the part even if your technique doesn't quite come up to scratch! I've lost count of the number of people who have asked about the lens, or wanted to swop with their little point-n-shoots! Seriously, though, you could do worse than buy this lens. I have never regretted spending the money on it, and the results I get justify my decision every time. If anyone wants further information, simply drop me a message. (Much-modified version of a review posted on Ciao a couple of years ago.)