Attached Link: http://www.sigma-imaging-uk.com/lenses/telezoom/170-500mm.htm

Talking of blur and shake in another thread made me think...

I've been using a Sigma 70-300mm and it's nice, AF is fast and smooth etc etc. 
Needing a litte extra range, got a 1.4x teleconvertor from my local box shifter. 

Wether it was that or a fault, the 300mm started making loud scary wrenching grinding noises. The box shifter was no help whatsoever, intially even refusing a refund. Fortunately Sigma where good enough to repair the 300 for free, but I'm a bit wary of using another T/C in case the same problem happens again.

Now I see the box shifter is offering a new Sigma 170-500 (sony fit - link above) for £550, and the local Jessops has agreed to price match.

Question is what are these lenses like? I see the weight is around 5lbs. and about 30mm (a foot) fully extended, using a tripod is out and I'm not sure about a monopod.

I've read because of he digital effect the 300 becomes a 450, so logically a 500 would be like a 800. but what the effects of shake at that level ?

Also are there any other brands [I've heard of Tamrom] that would do the job at a similar price or level ?
 

Pinky - you left the lens cap of your mind on again.