Which Tele-convertor best6 for Canon 350 D ? Will that using as Auto focusing?

Answer 1

As fhotoace said any Canon teleconverter will work just fine.

I'll expand on his comment about the teleconverter "eating light" though. What that means is a 2X teleconverter will not only double the focal length of your lens but it also doubles the effective f-stop. Attaching the teleconverter to your lens when its at f8 causes the lens to perform as though it were at f16 - 2 stops slower, requiring a slower shutter speed or higher ISO to compensate.

ISO 100 @ f8 @ 1/250 sec.

2X teleconverter, ISO 100 @ f16 (effective f-stop) @ 1/60 sec.

2X teleconverter, ISO 400 @ f16 (effective f-stop) @ 1/250 sec.