I need a new telephoto zoom lens for my Canon 40D. Research has now boiled down to one of those two: Canon EF 100-400mm f/4.5-5.6L is ii usm OR Tamron for Canon SP 150-600mm f/5-6.3 di vs usd I will use the lens mainly for surf photography, hence fast moving objects, fast shooting, difficult light (dusk, dawn, hard shadows in sunlight). I need: - long focal length - sharp pics - fast autofocus - wide aperture would be beneficial (wish I could afford a f/2.8) Not so important is: - weight I was keen to get a Canon, but to my surprise found out that the Tamron lens produces sharper photos! However it seems as if Canon has the better autofocus. At 400mm the two lenses work with the same maximum aperture, so I figured the Canon lens has no advantage over the Tamron. That leaves the following questions: - Will I have issues with any of the two lenses in poor light? - Will the Tamron autofocus be reliable enough (especially at dawn or dusk)? - How important is Canon's dust and water seal for my beach location?