Photography often tends to be about compromise and wildlife/bird photography is no exception. Even if one is fortunate enough to have a relatively large budget, there is no ideal solution. Often photographers get obsessed with the image quality (sharpness, contrast, bokeh, colors) of a lens without giving enough thought to its usability or portability.
The Canon 400mm f/4 IS DO is one lens which seems to frequently stir this debate. Its huge price tag puts its very close to the 500 f/4 IS, yet its image quality is measurably inferior to the 500 as is its 'reach'. So one would wonder why the heck would you get the 400 DO? The answer to which is 4.2 pounds. The DO is infinitely more hand holdable due to its considerably smaller size and weight. The fact is the 400 DO will produce an infinitely better image if the 500 stays put at home.
Though the 400 DO may not have the superlative image quality of its other super telephoto brothers, its no slouch either. This picture was taken with a relatively slow shutter speed, hand held and with a teleconverter. Even with some quick and rough post processing to boost contrast and sharpening, the resulting image quality seems quite acceptable to me.
Technical Specs: Canon 1Ds3, 400 DO + 1.4x TC (560mm equiv.), f/7.1, 1/160s, ISO 400.