Canon EF 100-400mm f4.5-5.6L IS USM Telephoto Zoom Lens

My thoughts on the Canon EF 100-400mm lens which in my opinion is the single most useful travel lens in their product line - by David R. Beebe
This has been my go-to lens for many years now. It is a great all-in-one wildlife photography travel lens but it does present a number of compromises.

The biggest issue with the lens is its push-pull design. Because of having to equalize air pressure when extending or compressing the lens, this is not a weather sealed lens. You can draw fine dust into the lens which then can get onto the digital sensor of your camera body. Neither is it a lens for a wet environment. A heavy rain can mean disaster, even if in a rain cover is used, as humidity can be sucked into the lens which can fog the internal lens elements.

That said, there are many positive attributes too. Weight, great zoom range for wildlife, reasonable auto-focus performance, image stabilization for hand-held mobility and good glass are all on the plus side of the equation. The single best analysis I found on this lens was that it becomes much, much sharper at f8 or smaller. This is a significant difference and not to be ignored. If you are shooting with this lens, set your DSLR to AV at f8 and let the camera take care of the rest. The edge detail of the eyes of your wildlife subjects will improve significantly with all else being equal.