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I assume the AF motor is the same on the Canon 600 and 800 lenses. Would the 800 5.6 focus better (quicker, locking) than the 600 4 with a 1.4 tc which makes max aperture 5.6? My guess is that the extra linkage by adding a tc might degrade af a tiny bit but am curious what others have to say.
in short no, that's not the case.
If you are talking about servo drive speed at full range , it depends on the camera body used. I have not measured this as accurately as Jacob has, here are his results.
http://www.naturescapes.net/forums/v...?f=57&t=234695
you can see that the full-range servo drive speed with 600II + 1.4III is similar to 800 and with the 2XIII it is twice as fast as the 800+1.4 when used with a 1DX. With the 1D4 however it is slower... these numbers would change dramatically if you use the limiter switch on the lens. the lock on is almost instantaneous in that case with the 600
With the EOS 1DX the full-range servo drive speed does not decrease when using a series II super-telephoto lens AND a series III TC. If you are using older bodies it will decrease with each TC added. My understanding is that with the old bodies, Camera firmware reduces servo drive to achieve better accuracy when adding a TC (if you tape the TC pins AF remains fast but jittery and unstable). with the new module in the 1DX Canon have achieved remarkable speed and precision at the same time.
Keep in mind, however servo drive speed is only one factor in overall AF performance. there are many other factors that determine overall AF performance for a challenging subject such as birds in flight.
hope this helps
I did not mention I use Nikon. I did not mention it because I assumed whatever the answer would be - both same, 800 faster than 600 with 1.4, or 800 slower than 600 with 1.4 - it would be the same for all cameras. Surprised.
It could be very different for Nikon as their AF electronics is vastly different...
As Arash says, it's very dependent on the body used with Canon. I've got a loaner 1D X from Canon along with a 500mm f/4L IS II and my own EF 2.0X TC-III. On my 5D MkIII, the AF speed is usually around a second or more when using the lens and the TC combined. OTOH, with the 1D X, the AF is almost instantaneous no matter what lens/TC combination I use. It's certainly well within the .2 sec measured by Jacob. If it does so well with the 500mm operating at 1,000mm, the 800mm is going to be instant with the 1D X body.
Doesn't Nikon have a brand new 800mm? I'd be curious to see how fast it will AF, but I would assume it's very good on their pro body and not quite so good with the 7100.
Nikon has a new 800 lens. It was announced in February but I have seen one nor read reviews yet.
