EF 300 f/2.8 L IS II with 1.4x OR EF 400 f/4 L DO ???

[Vern]

EDIT: Checking back for info on the v3 extenders I find 50 and 75% ratings for the impact on AF speed. I suppose this is a testament to the speed of the AF drive in the 300 mm lens that I never noticed any practical difference.

I haven't noticed the AF speed reduction either and have used the 2X III + 300 2.8 II for photographing actively feeding hummers w success. I guess it might show up in my fast dog application in the other recent thread on 5DMK III AF performance. However, I'm not likely to run the experiment - too many variables, too little time.

[Harfang]

Thanks Guys, I'm pretty sure I will go with the 300 since a get a fast lens with good optical qualities with extenders albeit in a heavier package. I find this setup more versatile.
I will try to rent the 400 before I make my purchase just to be sure....

Thanks again for the feedback

André