Canon 70-200 f/4 L IS with 1.4x or Canon 70-300 f/4-5.6 L IS or 300 f/4 L IS over Tamron 70-300 VC?

Earth Art wrote:

The Canon 70-200 with TC vIII is better than the Tamron

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=404&Camera=453&Sample=0&FLI=7&API=1&LensComp=757&CameraComp=453&SampleComp=0&FLIComp=4&APIComp=1

The 300mm F4 is a little sharper than the Canon with TC III, but not by much

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=404&Camera=453&Sample=0&FLI=7&API=1&LensComp=111&CameraComp=0&FLIComp=0&APIComp=0

Play around with the tools above to see for yourself. The Tamron is pretty weak at the long end.

I'd take TDP with a grain of salt, especially Tamron results. I swear it looks like he drops his Tamrons before testing.

I'd check out photozone.de test results first.

I've had them all and with my copies (and ultra careful testing that took a long time to carry out) the 70-300L definitely had the best look and microcontrast and least CA at 300mm f/5.6, next was the Tamron 70-300 VC and worst was the 70-200 f/4 IS + 1.4x TC III.

At 70mm wide open the 70-300L was clearly the best (although it actually had the most lateral CA), then the tamron then the 70-200 f/4 IS (although this did have the least lateral CA, but after correction it simply had less resolution than the others, especially compared to the 70-300L).

In the mid-range the 70-200 f/4 IS was best and then the 70-300L with the Tamron definitely trailing.

At 200mm the 70-300L did a touch better than the 70-200 f/4 IS which did definitely better than the Tamron. The 70-300L had the least lateral CA at 200mm.

Interestingly they all performed most similarly at FF edges! and showed the greatest differences center and mid-frame!

basically my 70-300l compared to the 70-200 f/4 IS (with 1.4x TC  III for the 280/300 comparison) did, wide open (or both set to whatever the widest the 70-300L could hit) and center frame (other than for CA which was FF edge measured):

better but with more CA at 70mm

basically the same in all regards at 100mm

worse at 135mm

almost as good at 160mm

a little bit better at 200mm with less CA

much better at 280/300mm with much less CA

over the 70-300 range the 70-300L won the largest portion of the range although over 70-200 the 70-200 f/4 IS won the largest portion of the range (although not at the perhaps more critical shortest and longest ends).