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Should I get 1.4x III or 2.0xIII extender - please help.

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lukemike:
Hi,
I am about to purchase either 1.4x III or 2.0x III canon extender. Initially it will be used with 135/f2.0 on 7D and in a couple of months with canon 300mm f2.8 IS II lens.
I used to have 1.4x II and used it with 70-200 f/2.8 IS II so I do know about the max aperture and AF speed differences when you use them on a given lens.
However I have no experience with 2.0x III and would like to know a bit more from people who actually used them with 135 f/2 and 300 f/2.8 IS II lenses.

Question the the users: IQ: if you take a shot with 1.4xIII  on and crop it to the size of the one taken with 2.0x converter will there be a significant gain in the detail (with these two lenses 135 and 300 mm). I vaguely remember reading a review that stated there is not much if any difference, but again I may be wrong. Looking at these samples seems to prove they do differ:
http://www.the-digital-picture.com/Reviews/Canon-Extender-EF-1.4x-III-Review.aspx
Further down the page there is a nice comparison of a flower carpel and there is some difference between 1.4 and 2.0 on The Canon 180 L Macro.

Second question AF performance. Main use of 300 mm and 135 with TCs will be birds and BIF. I remeber that 1.4II together with 70-200 f/2.8IS made AF significantly slower.
Is there a massive difference in AF speed reduction between 1.4xIII and 2.0xIII on 135mm and 300mm lenses.

Thank you for your help.

neuroanatomist:
The 1.4xIII will cut AF speed by 50%, the 2xIII will cut it by 75%.  I've used 600mm f/4L IS II with the 2xIII for BIF and the AF is fast enough, but only just - and YMMV, as I'm using a 1-series body (the higher voltage battery drives the lens AF motor faster that a 7D can).

As for IQ, the 2xIII will do fine on the 300 II - in fact, the combo is similar to the 600/4 MkI (although the MkII is better).  I suspect the 1.4xIII would be a better choice for the 135L, though - that will take a bigger IQ hit from the 2x than the supertele.

I did the test you describe with my 600 II and 1D X, and the 2xIII delivered better IQ than the 1.4xIII cropped to the same FoV.

I haven't tried that with my 135L, in fact, I've never even used a TC with the 135L (probably because I've got the 70-200/2.8L IS II).

Harv:
I can't speak to the performance with the 300 Ver. 2 but I have and regularly use both the 1.4xTC III and the 2xTC III with my 300 f/2.8L IS (Ver. 1) and my observations are as follows...

I know that Canon says the 1.4 will slow AF by 50% but to be perfectly honest, I hardly notice any slowing of the AF with that combination.  However with the 2x the AF is significantly slower unless I am pre-focused pretty close to where I need to be.

As for IQ, it only takes a very small hit with the 1.4 and I have no problem shooting wide open.  However, to get what I feel is acceptable IQ with the 2x, I have to stop the combination down to at least f/8.  Wide open, I definitely see a noticeable hit in IQ.


charlesa:
I have experience with the 2x III on the 70-200 mm II and the 400 mm f2.8 IS I, not too happy with IQ wide open at f5.6, and in most situations you cannot stop down to f8 to improve IQ, so would not be a combo to use often.

lukemike:
Thank you very much for your help. Based on what you are saying it seems to me that the 1.4xIII is going to be a better choice for my setup. Many thanks.

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