My wife and I like to visit reserves and look at the local released red kites and I can often photograph wing tag numbers in flight which I know I cannot do regularly with my binoculars. I more often take my kit with me on walks on hills and cliffs of up to 7 miles, dropping the telescope for walks longer than that. I do climb gates and occasionally fences. I am interested in photographing birds and animals on my walks, not necessarily to professional standard, although good photographs are pleasing, but also to assist identification. I probably only recognise 50% of species of birds as they rise on my approach, so far away that I would need a 1200mm lens to get a reasonable photograph. I don't have the real birder's touch, so recognition is an issue for me. When they were handing out mind's eyes in 1945 I seem to have got left out, and the only photographic thing about me is my camera. I won't be doing tetrad census any time soon, although I would enjoy it. Using my binoculars or telescope I regularly note a set of features which are not present on any bird in the world, never mind NE Scotland, if I have not already recognised the species. Photographic identification is therefore important to me.
I have progressed this year from a Panasonic FZ18 with a TCON17 convertor, purchased last autumn, and opening my eyes to photographing birds if not photography, to a 40D with 2 IS kit lenses up to 250mm. Obviously reach is inufficient so I am training with my TCON17 attached to the front of the 250mm lens. This gives me about 400mm. The extended lens combination is about 10 inches long, but only the first 3 inches does not rotate when auto focussing so it is not easy to handle. The 1.7 teleconvertor does produce better results than the 250mm on it's own, and I am starting to have success using it. Off topic but I think interesting, I think I am more likely to damage the micromotor by letting my finger stray to the rotating part than due to the weight on the front, and I was not expecting it to give such reasonable results.
From my studies on the subject of next lens, it seems that there is a consensus that someone wanting reach should get one of the 400mm f5.6 lenses (No, this isn't another of those discussions), get better, and move on to the 500mm f4, or possibly the 800mm f5.6 if my wife has not divorced me already. The weight of the larger lenses worries me, and I'm concerned that the 500mm will not get taken out of the backpack very often, and 400mm is not enough. I think that I can devise a way to carry a reasonably protected mounted 400mm f4 DO IS with or without extender and get it ready to use in a few seconds, slower than my binoculars, but not stowed away. It can also be used with a 1.4 extender and autofocus, and manually focussed on a tripod with 2x extender. If I go the 400mm DO way, I save a lot of weight as my long lens kit will be a new 400mm DO and 2 extenders instead of a 400mm f5.6, a 500mm f4, one extender, 2425g instead of 5340g if canon extenders are used. There is a good weight saving of 2915g, therefore, and also a cost saving, if anything about Canon long lenses could be said to be a cost saving.
There is now the issue of the canon 400mm f4 DO IS USM lens. How good is it really? Although my needs are not that high, any photograph is improved by good contrast and sharpness. Many reviews of the DO lens place it litte higher than one of the 400mm f5.6 lenses. I have never seen one of these lenses and don't think I can hire one either. canon have not made it an L lens, which the 5.6s are. Does anyone have experience of the lens, does it work well with extenders, and would it meet my needs?
The other advantage for me of course is not having to choose which 400mm f5.6 lens to get.
Finally, sorry for the long post, but I already know that the DO lens is not as good as the 500mm f4 or the 400mm f2.8 lenses, and I wanted to get some more useful personal pointers than a simple request about the 400mm DO lens.