Tamron announced a new lens in progress late last week when I was out of town, certainly looks interesting for the spotting and airshow application provided the AF speed is up to snuff. At 150-600 f5-6.3 it'll be interesting to see how it compares to the longer Sigmas and the Canon 100-400 or even some of the higher end primes with TCs attached.
http://www.dpreview.com/news/2013/11...telephoto-zoom
Bookmarks