CONVERTER Question

[GyRob]

i just read this
Regarding the Sigma 1.4 converter

( The high optical performance is maintained and the AF speed is not affected Regarding the Sigma 1.4 converter )

ive read that the Canon 1.4 convert is said to slow focus down so should i buy the Sigma ?
Rob.

[Cadwell]

The Sigma convertor does slow AF speed, just like the Canon. I know this from personal experience. Well, to be technically correct, the camera slows the AF drive speed when it detects a teleconverter. 50-67% for a 1.4 x and up to 75% for a 2x.

[GyRob]

THANKS for reply i thought it seemd strange.
I dont supose you can fool the camera into not knowing a converter is fitted :) that would be good.
Rob

[csnudelman]

The quickest and most accerate focusing mode with the 20D requires a f/2.8 or larger maximun opening. My 135mm L f/2 lens becomes a f/2.8 with this converter and thus stays within this high performance mode. A f/2.8 lens would become an effective f/4 and so when using the converter it would not be able to take advantage of the camera's best AF mode. :p

[Cadwell]

THANKS for reply i thought it seemd strange.
I dont supose you can fool the camera into not knowing a converter is fitted :) that would be good.
Rob

Yes, you can. It's a thing called the "tape trick" where you apply sticky tape over 3 pins on the TC. This prevents the camera from knowing the TC is there.

It is not, however, a great idea.

I've tried it on three fast focusing lenses (500mm f/4.5, 70-200L and 100-400L) and you can hear the lenses judder into focus. The camera over-shoots the focus point, adjusts back too far, shoots forward, backward, forward and eventually gets to good focus. It's at that point you see why Canon slows the focus down with a TC - oh, and it takes longer than the slowed focus from the camera knowing a TC is present. You can also imagine that you're not doing your lens focus motor a power of good at that point.

[GyRob]

shame that is not a good idea :( and the 135 is not long enough for jets , even my 300f4 is a little short but its hopeless with the 1.4 fitted. need to win the loto .
Rob.