Canon 300mm f2.8L IS USM V1 with 1.4x TC III or 2x TC III?

CyberDyneSystems wrote in post #18213544
For what it's worth,

- I did not find the Version III T-Cs to show ANY improvement either in performance (af speed acquisition) or IQ when mated to a version I IS super tele (in my case the 500mm f/4L IS, and the 200mm f/2L IS)

- I did see the performance increase when the MkIII were paired with BOTH a VII tele lens (500mm F/4L IS VII) , AND a fully modern AF system such as 1Dx, 1Dx2 and 5D4.

- I saw no performance boost vs. VI and MkII combo on 1D4 or 7D2!

- Always get the 1.4x first. You will love how the lens works with that T-Con.

- 2X is better saved for a "when you need it" vs. an "always on"

Were it me, I'd save $ and get the MkII T-Cons. A pair would cost about what one MkIII cost.

Another advantage to the MkII, the 2X "stacks" behind a 1.4x. I have on occasion needed that ability!

Ditto on everything he said

Just my 2p!

I use the Canon 300 F2.8 L IS (Mk1) on both 7D2 and 1DX (previously 1D4) bodies. I upgraded my 1.4 Mk2 to a 1.4 Mk3 and, frankly, saw little difference - if had had to say which was better then I honestly couldn't say though the Mk3 does seem to perform better on my 100-400 Mk2.

As regards the 2 x extenders? I was not happy with my 2 x Mk2 (and a couple of other Mk2s) that I tried on any of my lenses. They were OK for AF but unless the light was just right I was better off cropping. When I sold it and bought the Canon 2 x Mk3 I suddenly had a useful accessory - even on F4 lenses. I cannot remember if there was any worthwhile improvement in the AF performance (on the 1D4) but the IQ was noticeably better on the camera screen - let alone the monitor. With the 1DX the Mk3 does AF better but this is probably down to the newer camera. I don't have any Mk2 Superteles but if you are wanting a 2 x extender I would not consider the Mk2 - I just didn't bother with mine due to the IQ loss. The Mk3 works well with the 300 F4 L IS and 600 F4 L IS Mk1 and excellently with the 300 F2.8 L IS Mk1 - it is quite handy on my 100 F2.8 Macro as well.

A 1.4 extender on your 300 F2.8 is a very worthwhile accessory as there is little cost in AF and IQ - Mk2 or 3. A 2 x is a bit more demanding for light and has slower AF - I feel the Mk3 is the only choice here.

I sold my 1.4X & 2X MKIIs after getting the MKIIIs. My newer lenses & bodies definitely focus faster with them.
When I had a 300 f/2.8L IS MKI I got good performance from my MKII extenders on my 1DIV.
I find the IQ and focus speed to be better with the MKIII extenders, especially the 2X.
I would start with the 1.4X, and get the MKIII so you won't have to upgrade down the road.
The only advantage I saw was the ability to stack the MKIIs. (Which I rarely used.)