Conclusion

The Tamron 100-400mm F/4.5-6.3 Di VC USD is an excellent alternative to the equivalent lenses from Canon (EF 100-400mm f/4.5-5.6L IS II USM) and Nikon (Nikon 80-400mm f/4.5-5.6G ED VR), offering 90% of the performance at 50% of the price. The maximum aperture of f/6.3 at 300-400mm is a stop slower than the competition, though, and the lens isn't overly sharp at 400mm, while the optional tripod mount is both costly and a little rudimentary - otherwise, there's a lot to like about this lens. 

The Tamron 100-400mm F/4.5-6.3 Di VC USD offers excellent image quality overall, with smooth bokeh and very good sharpness, an effective and versatile image stabilisation system, and a fast, quiet and reliable auto-focus system. Tamron's optic is also smaller and lighter than the Canon and Nikon equivalents, something that handheld shooters will appreciate.

Image quality is very good throughout most of the focal and aperture range, with excellent sharpness consistently achieved by stopping-down one stop from the maximum aperture. Chromatic aberrations are well controlled and flare is only an issue when shooting directly into the sun. The only real optical issues are some obvious vignetting at the maximum apertures and some mild pincushioning at the 100mm focal length.

In summary, this is a versatile lens that, even with having to add the cost of the tripod mount, is still substantially cheaper than the Canon and Nikon alternatives, yet it gives very little away in terms of features, build quality and performance. Highly recommended!

Ratings (out of 5)
Design	4.5
Features	4
Ease-of-use	4.5
Image quality	4.5
Value for money	5