Which 70-200 f2.8?
From what I've heard the Mark 1 Sigma is the sharpest of the Sigmas, and the current Tamron is sharper than the current Sigma. I don't know much about the Tamron with TCs, but the Mark 1 Sigma works very well with the 1.4x TC, not quite so well with the 2x.
Tim
AF - Pentax K5, Sigma 10-20/4-5.6, Tamron 17-50/2.8, Sigma 30/1.4, Sigma 70-200/2.8, Tamron 70-300/4-5.6
MF - Vivitar CF 28/2.8, Tamron AD2 90/2.5, MTO 1000/11
Stuff - Metz 58 AF1, Cactus v4, Nikon SB24, Raynox 150, Sigma 1.4x TC, Sigma 2x TC, Kenko 2x macro TC, Redsnapper 283 tripod, iMac 27”, Macbook Pro 17”, iPad, iPhone 3G
Flickr • Fluidr • PPG • Street • Portfolio site
Feel free to edit any of my posted photos! If I post a photo for critique, I want brutal honesty. If you don't like it, please say so and tell me why!

Bret
my pics: link
my kit: K3, K5, K-01, DA 18-55, D-FA50 macro, Siggy 30/1.4, 100-300/f4, 70-200/2.8, Samsung 12-24/f4, Tamron 17-50, and lots of other bits.
some Tamron stuff with K20D
http://www.flickr.com/photos/greypoint/sets/72157607272018577/
some Tamron stuff with K-5
http://www.flickr.com/photos/greypoint/sets/72157626720338568/
some Sigma stuff
http://www.flickr.com/photos/greypoint/sets/72157625452542277/

Gary
Armed with a K3, some M, A, FA, DA, and star lens. With an eye open for "just one more lens".
My PPG link

Regards
Sam-Joseph
Pentax K7, Sigma 17-70 f2.8-4.5, Sigma 70-200 f2.8 APO EX, Sigma 70-300 APO, Sigma 1.4x TC, Vivitar 2x TC. Takumar 135mm f2.5, SMC Pentax A 50mm 1:1.7, SMC Pentax -M 1:4 200mm, Pentax X70
sam-joseph
Member
Newcastle, NSW, Australia
I've read the earlier Sigma is the pick of the bunch, but the Tamron gets a good write up as well, optically speaking. Some have criticized the slow focus of the Tamron, but I believe that would be typical of other mounts who rely on the lens-based motor. Can anyone comment on the AF speed and accuracy of the Tamron on a K7, and which would be the best teleconverter. I will be looking to sell the Tokina 80-400, which is mostly used for shooting the kid's soccer matches, hence the interest in a (2x ?) TC.
Regards
Sam-Joseph
Pentax K7, Sigma 17-70 f2.8-4.5, Sigma 70-200 f2.8 APO EX, Sigma 70-300 APO, Sigma 1.4x TC, Vivitar 2x TC. Takumar 135mm f2.5, SMC Pentax A 50mm 1:1.7, SMC Pentax -M 1:4 200mm, Pentax X70