Even with a 1.4X TC III its image quality is on par with the best Sigma and Tamron have to offer, and without a TC its image quality is unmatched, except by the EF 70-200 f/2.8L IS II. Add to that a short minimum focus distance and the EF 100-400 L IS II is the best lens in the 100-560mm range.
EF 1.4x III and EF2x III extenders are recommended for use with EOS Digital Cameras. *1: EOS-1D Mark II, EOS-1Ds Mark II, EOS-1D Mark II N, EOS-1D, EOS-1Ds. *2: EOS650, EOS620, EOS750, EOS750 QD, EOS850, EOS850 QD, EOS700, EOS10S/10, EOS REBEL/1000, EOS REBEL S/1000 F, EOS REBEL S QD/1000 FN QD, EOS ELAN/100, EOS630/600, EOS RT, EOS-1, EOS As mentioned, the extender won't physically fit the EF 70-300mm L f/4-5.6 IS USM II, so IMO if you are going to spring for the EF 100-400mm L f/4.5-5.6 IS USM II, I would go for the RF 100-500mm L f/4.5-7.1 IS USM. I agree with Trevor that the EF 70-300L II is a great lens.
hi Blake, i agree with most of your suggestions! my canon 1.4xIII extender and 600 f4.0II work quite good, it is the 2xIII + 600 II that turn pretty mushy! i don't have any issue with my rev. III extenders and 300 II. but like you have noted, i should send my extenders and 600 II to canon and have them calibrate the gear and hoping the issue go
EF (full-frame) lenses have a red dot, EF-S (APS-C sensor) lenses have a white dot. The EF-S lenses extend further into the camera body so will NOT mount to an EF TC. I have no problems mounting the Sigma 1.4 and 2.0 TCs to my (EF mount) Sigma 70-200 2.8 for use on my 500D but the TCs are useless on my (EF-S) kit lenses and my 60mm macro.
I have a 60D and a Canon EF 1.4 Extender II, I was wondering if it would be compatible with the Canon EF 70-300mm f/4.0-5.6 IS USM Lens? As others have said, the short answer is no. You can buy a Kenko 1.4x which will fit, but you probably wouldn't be happy with the results. . 338 325 188 111 133 196 377 260 133

canon extender ef 1.4 x ii compatibility