Canon 2x Extender III AF Performance

[Ayelike]

Hello,

I took my new 5D3 out for a field test at a motor sport event and was very impressed with the subject tracking capabilities. I was using my 70-200mm and the AF Servo maintained focus from the cars coming into view all the way to flying past me. Perfect.

After just going full-frame I'm thinking about getting a little more reach and am thinking about the 2x extender. I'm happy to take the hit on aperture and IQ (extender III with 70-200 IS II seem to work pretty well together) but my question is about the drop in AF speed.

I read somewhere that AF speed is reduced by 75% but what does that mean in the real world? The lens I want to use it on focuses pretty quickly so is quarter of the speed still ok or an age?

Also, once focus is acquired, will the 5D3 still be able to track the subject or will it drop out due to the AF slower speed?

I'd love to hear from anybody that has this combo and has used it for motor sports. Is the extender mainly for static subjects or those moving across the way - rather than subjects whizzing towards you with speed.

[jasium]

Peripherally-related anecdote: 

I've not shot with a 5D3, but I can vouch for the 70-200 II + 2xIII on a 1DX.  I tried it at a youth-league football game in evening, rainy, overcast light.  Uniforms were grey/black.  AI servo, AF mode 4 (erratic movement) untweaked,  'center 9' AF points selected.  The kids weren't nearly as fast your cars, but they generally follow less-predictable paths.  OOF rate <1% until they pass MFD.

With the 2xIII mounted, the initial focus lock feels substantially slower, but the following tracking felt (and worked) just fine.  I'm not in the motorsports scene, but 2xIIIs are cheap to rent - definitely encourage you to try one in your working environment.

[charlesa]

Use the 2x TC III on a 400 mm f/2.8 IS L I and the 70-200 mm f/2.8 L IS II on a 1DX, shooting birds, soccer, rugby and waterpolo. Yet to notice any problems with AF tracking.

[Razor2012]

The AF speed with the 1DX will be a little faster compared to the 5DIII because of the higher voltage, but not sure how much of a hit will be taken when using the 2XIII.

[Wilmark]

I have the same combination and yes the speed does take a significant hit. But it isnt so bad if you start with a good focus near the target using an object easy to focus on - ie with more contrast than the objects you are going to focus on. In others words it appears taht if the autofocus has to move a great distance to achieve focus - if feels like forever. After you lock on focus you need to be careful not to loose this focus with the background esp if the background if very far from your subjects. this is my experience trying to use this combo for BIF - its qute a hit and miss. My feeling is that the 1.5x conv is a far more useful combo.