Introduction Tamron 17-70
As far as I know the Tamron 17-70 lens is not exist in the moment. There are other two lenses that are exist the Tamron 17-50 and the Sigma 17-70.
The Tamron 17-50
is an excellent lens with f/2.8 constant brightness which helps in low light situations like inside the room, or before sunset at a campfire. The Tamron 17-50 has a stabilized version as well, it is a little bigger lens, with image stabilizer. If we compare the two Tamron lenses to each other, the older non Vc version is sharper especially in the corners, but in real word the newer version is about near as good. The Vc helps in low light situations. The Vc version is more expensive though, and also a little bigger and heavier as well. The original version is surprisingly compact for the f/2.8 brightness. The older lens has 67mm filter size, the newer Vc version 72mm.
See the reviews of the Tamron lenses below:
Tamron 17-50 f/2.8 review
Tamron 17-50 f/2.8 Vc review
The Sigma 17-70 lens
has at least three versions:
The original version with normal micromotor without stabilizer with f/2.8-4.5 brightness, the newer Sigma 17-70 f/2.8-4 HSM OS with ultrasonic motor and stabilizer and the newest Sigma 17-70 f/2.8-4 HSM OS “C” contemporary lens with a little better performance and little better build quality. The first two lens is quite similar optically, but the newer more expensive lens has a little better brightness at the long end: f/4 vs f/4.5. The new Hsm focusing and the stabilizer makes it more useful, but much more expensive as well. The Sigma 17-70 has nice colors, but at larger apertures quite soft in the corners. Compare to kit lenses like Canon 18-55 is the Sigma has much better contrast and colors and center sharpness, but the kit lens is little sharper in the corners.
Tamron 17-50 vs Sigma 17-70
According to my opinion the Tamron 17-50 f/2.8 lens is superior optically, but the Sigma 17-70 has wider range, better autofocus (the newer versions) and better build quality. The Tamron is razor sharp at f/5.6. The only drawback of the Tamron 17-50m is the old noisy micromotor.