The truth about teleconverters

[Tom3]

what do you mean by 'pressure on the AF system'? if something breaks because it functions a lot, it was crap to start with

 

[davebmck]

OK, just for the heck of it, I put my 18-200 on my camera and set the aperture to 6.3 at full zoom and took a picture. Of course the autofocus works fine as expected. When I review the picture it show the aperture as 6.3, so the camera knows it is at 6.3. The display shows 6.3 as well. Then I took the camera into a room with low light where an exposure of 1 second was necessary to get a good exposure and the camera auotfocused there as well.

What I can assume from this is that if you put the 1.4 TC on the camera it will probably autofocus at 6.3, especially if the light is fairly good. I have been planning to get a 1.4 TC to use with my 100-400 mm, so when I get it, I will try it with the 18-200 and post the results here. Don't get in a hurry for this info though, as it will probably be after Christmas when I get it.

 

[tsaraleksi]

As I said, the ability to AF at 6.3 is a specific property of that and a couple other Sigma lenses that tell the camera they are an f/5.6 lens until it comes time to take the picture, when they behave as though they have been stopped down to f/6.3 from f/5.6. A 100-400 with a TC will not AF in the same circumstance. (without fiddling and mucking around with it-- like I said, it's possible but requires taping pins and whathaveyou).

 

[davebmck]

A 100-400 with a TC will not AF in the same circumstance. (without fiddling and mucking around with it-- like I said, it's possible but requires taping pins and whathaveyou).

Correct, but I can live without autofocus in this case.

 

[gsgary]

As I said, the ability to AF at 6.3 is a specific property of that and a couple other Sigma lenses that tell the camera they are an f/5.6 lens until it comes time to take the picture, when they behave as though they have been stopped down to f/6.3 from f/5.6. A 100-400 with a TC will not AF in the same circumstance. (without fiddling and mucking around with it-- like I said, it's possible but requires taping pins and whathaveyou).

and the focus will hunt and be very slow