Focus acquisition speed and accuracy
The Tamron 24-70mm f/2.8 features the Ultrasonic Silent Drive (USD) autofocus motor that is used in high-end Tamron lenses. It is very fast, accurate and produces very low noise during AF operation. In comparison, the Nikon 24-70mm f/2.8G is both faster and quieter, but not by a big margin. In fact, to date I have not seen any lens for the Nikon mount that focuses faster than the Nikon “trinity” (14-24mm, 24-70mm, 70-200mm). But this speed advantage is not that big of a deal, since the speed of the Tamron 24-70mm is more than adequate for any kind of photography. The key in autofocus performance is precision and low-light performance and the Tamron 24-70mm surely does not disappoint in both. Lola and I shot a few weddings with this lens and it performed quite well even in very dim environments.
Read more: http://photographylife.com/reviews/tamron-24-70mm-f2-8/2#ixzz2yCTtla3v