teleconverter 1.4x iii autofocus problem

[jsfarnsw]

my 100-400 ii won't autofocus with a 1.4x iii on an EOS 70D.  When I install the teleconverter, the camera automatically switches from AF Servo to M Focus.  

Solutions?

[jrhoffman75]

I am pretty sure the problem is related to the fact that when you add the 1.4 teleconverter to the f/4.5 lens the max aperture becomes smaller than f/5.6.

[jsfarnsw]

that doesn't make sense, because this converter and lens are supposed to work together with AI.  

[jrhoffman75]

They will on a 1D series camera, and I believe in LiveView. But for through the lens AF the non-pro cameras have an f/5.6 limit.

See this article: http://photo.stackexchange.com/questions/16399/what-apertures-are-required-to-enable-autofocus-inclu...

[RobertTheFat]

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@jsfarnsw wrote:

my 100-400 ii won't autofocus with a 1.4x iii on an EOS 70D.  When I install the teleconverter, the camera automatically switches from AF Servo to M Focus.  

Solutions?

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None, I think. A 70D won't autofocus at an aperture smaller than f/5.6. (A few cameras will autofocus at f/8, but the 70D is not one of them.) The teleconverter cuts the maximum aperture of your f/4.5 lens to (if my math is correct) f/6.3.

[jsfarnsw]

Thanks everyone.  I did not realize the AF systems required that much light to work properly. 

[ebiggs1]

You can alway manual focus it.

Some cameras can have one or some of their contact pins disabled and the camera will AF (put tape over them).  I don't know if this is true on a 70D.  And I don't recommend doing it but you may search and find out for sure.

If it does work it will be very slow and not really all that accurate.  Just MF it, like the good ole days!  

[amfoto1 · Accepted solution]

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@jsfarnsw wrote:

Thanks everyone.  I did not realize the AF systems required that much light to work properly. 

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AF needs adequate light to work reasonably fast and reliably.

Do an internet search about teleconverters and taping up some of the contacts. This "fools" the camera into not realizing the converter is there, so that it will still try to AF the lens. Just don't expect focus to perform very well. It will be slower and in lower light situations or with low contrast subjects it may hunt a lot and even fail to achieve focus. There's a reason Canon sets a limit and turns it off, when a lens/teleconverter combo exceeds the rated AF capabilities.

You'll have to find a website where someone illustrates which pins to tape up. I haven't done the tape trick myself... I simpy don't bother with TCs on lenses where it would exceed the light levels, or in a few cases I just manually focus (500/4 with a 2X on it, for example).

7D Mark II (not the original one), 5D Mark III (not the II or original), all 1-series (film and digital) and the EOS-3 film cameras are "f8 capable", meaning that they can still AF (mostly with one center AF point only... 5DIII can use a small group of points in the center I believe) when a lens + TC makes for an f8 or larger aperture. All other Canon cameras, including 70D are "f5.6 capable".

The 100-400mm is f4.5-5.6 already... so adding even a weaker 1.4X, makes for an f6.3-f8 combo that exceeds your camera's design capabilities. But some folks manage to make it work to a limited extent with a few small, carefully placed pieces of Scotch tape!

Note: There are a few third party lenses that are f6.3 even without any TC, yet still AF... probably by similarly "fooling" the camera.

***********

Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER 

[ebiggs1]

"Note: There are a few third party lenses that are f6.3 even without any TC, yet still AF... probably by similarly "fooling" the camera."

And you are correct.  The big 150-600mm Tamron and Sigam both report f5 to the camera.  Even though it is at 600mm and f6.3.  I hear rumors that Canon is testing a 600mm f6.3 prime.  It will interesting to see how they handle it.

[RobertTheFat]

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@ebiggs1 wrote:

"Note: There are a few third party lenses that are f6.3 even without any TC, yet still AF... probably by similarly "fooling" the camera."

And you are correct.  The big 150-600mm Tamron and Sigam both report f5 to the camera.  Even though it is at 600mm and f6.3.  I hear rumors that Canon is testing a 600mm f6.3 prime.  It will interesting to see how they handle it.

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Does it matter? Anyone who can afford one will already have at least one of the f/8-capable cameras. Of course any extender will throw it over the limit, but who needs an extender with a 600mm lens?