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Canon DSLR bodies prevent poor auto-focus performance by disabling it completely when the lens reports maximum aperture at chosen focal length smaller than f/5.6. This implies that some lenses will not auto-focus on a teleconverter, e.g. the popular Canon EF 100-400 f/4.5-5.6L, or any f/4 lens on a TC stronger than 1.4×.

In an answer, Matt Grum mentions that

There's a well known trick whereby you tape the pins on your teleconverter to prevent it from reporting the true aperture of the lens+converter combo so that AF remains enabled.

Which pins have to be taped? Are there any other consequences than just enabling AF, e.g. how will that taping affect focal length stored in EXIF info?

If there are any other workarounds, e.g. using alternative firmware, those options would be welcome too. Let's skip the obvious option of moving closer and removing TC :)

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1  
This is an interesting question. The presumtion is that the camera can still focus but just does not want too? –  Itai Sep 27 '12 at 17:55
1  
Yes. Of course, locking focus is harder with a small aperture, especially in low light or a peripheral AF point, but Canon bodies won't even try beyond f/5.6. Older 1D models went up to f/8. –  Imre Sep 27 '12 at 18:10
    
What I thought is that it had to do with the distance between pairs of Phase-Detect sensors and that it the light simply won't fall far enough apart at those apertures. I imagine this limitation would not apply to Live-View. Does the camera refuse to focus in Live-View as well as those apertures? –  Itai Sep 27 '12 at 18:42

2 Answers 2

up vote 6 down vote accepted

Basically tape the last(left) three pins of the converter. EXIF distance data will likely be lost for most lenses.

You will find your answer at this link: Fred Miranda Tips Page

As quoted from the site:

Just place a small transparent piece of tape on the last 3 pins of the converter. The tape should be placed on the left hand side of the converter when looking at it from the lens connection side.

They also have a picture example of doing so if you need that guidance.

As far as other consequences to taping the pins, I found a bit of information and some anecdotal data for you as well from here:

You should be aware that the EXIF data will not be correct with regards to focal length, however.

Another thing to be aware of is that these three pins obviously do a little more than that with some lenses. Whilst my 100-400 is fine with a taped 1.4x my 70-200/f4 didn't like it at all. It hunted like crazy and hardly ever achieved focus lock. Without the tape it was as quick to focus (as far as I could tell) as without the teleconverter. The solution was to put the tape on the corresponding contacts on the back of the 100-400, now both lenses work just fine with the teleconverter. -chris maddock

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The Kenko teleconverters let you autofocus to F8. Arthur Morris talks about using a 1.4 on an 800mm and it working. I've heard the 2.0x tele works on F4 lenses but haven't tested it yet myself. interesting possibilities there, and you don't need to tape the pins.

http://www.birdsasart-blog.com/2012/08/30/mega-amazing-discovery-800-f5-6-1-4x-tc-canon-eos-1dx/

(I've tried the pin taping. it works, but it's a hack, I found it had a bad tendency to stop working in the field, and I wasn't every really happy with the setup or reliability. It wasn't something I wanted to have to rely on in a crunch).

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