Quote Originally Posted by JJphoto


.... just want to show how the 2X TC work with the 300mm 2.8, I think the IQ is still not too bad, but the AF is pretty slower, I couldn't track the turns in flight with it. Maybe it will fairly catch the bigger slower moving birds like egrets, but havn't tried it yet. thinking the version II super telephoto+TCs III will do the better job, but how better can it go?


7D ,600mm ,f5.6,1/800,-2/3 expo. comp., ISO 200


JJ,


Both, Version II and Version III extenders will reduce AF Drive Speed in order to improve AF Performance. The 1.4X extenders reduce AF Drive Speed by 50% and the 2X extenders reduce AF Drive Speed by 75%. That's a pretty big hit for the 2x. I had no idea that it was that much myself.


Source (towards the bottom of the page): http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=3508





Here is a comparison of Image Sharpness: 300mm f/2.8L IS with 2X converter @600mm f/5.6 vs. 600mm f/4L IS @ f/5.6


http://www.The-Digital-Picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=249&Camera=453&Sample=0&am p;FLI=2&API=2&LensComp=336&CameraComp= 453&SampleComp=0&FLIComp=0&APIComp=2





Rich