增距鏡 = Teleconverter ? If you wanna use the teleconverter for your kts lens 17-85mm f3.5-5.6, I think you would not get what you want !

2x teleconverter will double up your lens focal ( 2x 85 = 170 then multiply with crop sensor 1.6x = 272mm ) this look good for you, you will get 272mm but this 2x teleconverter also will double up your aperture ( 2x 5.6 = 11.2 ), the problem come now, your 400D body AF ( auto focus ) will not work !!! ( for Canon enry level DSLR only support up f/8 only ).

It not not a proper way to buy a teleconverter for short range lens.

please visit to http://www.photomalaysia.com - insides this forum got a canon user sub forum

引用:

原帖由 dumpdump 於 2008-5-15 21:14 發表
after searching info, final decide sEF 70-200mm f/2.8L IS USm + Canon Extender EF 2X II   is it good choice?

Go for it, but have to pay more !!! A very good combination.