[jon pafford]

I have a tamron 1.4 teleconverter and a sigma apo dg 170-500 lenes. When I use this combination, the camera, ask me for the input focal length. What input focal length should I use on the camera?

[Canada_Rockies]

That is a very tough question. The problem with zoom lenses and setting SR focal lengths is a really difficult one to get around. One should really change the SR focal length every time one changes the zoom setting, but that is just not a practical solution. The trouble comes in when the selected focal length is much longer than the actual effective focal length. The SR system will overcompensate and actually come up with worse effects than if it were off.

In your case, (170*1.4 = 238) would be the minimum setting and 700mm the maximum. When I use my 70-210, I set the SR to 100mm. It seems to work OK the few times I have used the lens at the wide end, but I really have not used it enough to be certain.

One philosophy I have considered is to set the SR to the upper end of the zoom range and turn it off when I go wide on the grounds that I am shakier at 200mm than at 100mm. The 70-210 is the only lens I have that requires this sort of guessing, so it does not come up very often in my shooting.

[jmc7104]

The only reason the camera wants the focal length is for shake reduction and if you are using a 1.4TC shooting at 500mm you should be on a tripod with sr turned off. I have the 50-500 plus 1.7TC and even shooting from the truck resting the lens on the door I turn sr off and engine also. jim