Ask Dave, July 23rd - Autofocus & TCs

I think this gets me all caught up on my weekly Ask Dave blog posts. Work has been extremely busy (we are launching a new website), so that is my excuse, I'll try to avoid falling behind again.

Ask Dave, July 23rd
I get tons of questions each week, I respond as quickly as possible, and I might just answer them here too. If you have questions you can email me at Learn@DaveShumway.com.

Question:
From... Debbie
So I hear these new TCs are great, but can I use them on my 100-400/5.6L IS?

Answer:
The new TCs are great, I assume you read my blog post about them a while ago.

Unfortunately they still don't work (AF) with an f/5.6 lens and a non 1 series camera. The 1.4TC III still slows your lens by one stop, and the 2.0TC III still slows your lens by two stops. What that means is if you have an f/2.8 lens and you place a 2.0TC III on it, the lens becomes an f/5.6 and a 7D can still autofocus that. Take a f/4.0 lens and place the same 2.0TC III on it it is then a f/8.0 lens and a 7D can not autofocus that, but a 1D IV can. Even as amazing as the 1D IV is, even it cannot autofocus a lens like the 100-400 with a 2.0TC III.

You can use some tricks to get a non 1 series camera to autofocus with a lens and TC combination that is f/8.0. By placing tape on the pins that communicate with the camera you can trick it to thinking there is no TC and it will still attempt to autofocus the combination (it is really slow). Unfortunately I have never seen anyone use a 2.0TC with the 100-400 and have any Canon camera autofocus the combination.

Remember that you can always manually focus your lens regardless of the lenses maximum aperture; but the darker the lens that harder it will be to see and manually focus your lens accurately.

p.s. from my experience all Canon cameras will attempt to autofocus any lens and TC combination (even at f/11) in liveView mode. It is slow and not really accurate, but it works (and usable for video).