A TC20E becomes an electrified extension tube

Started Sep 26, 2012 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,779
Re: Electrical mod

mmalkin15 wrote:

You are correct that the aperture is reported correctly when pin 10 is cut but autofocus hunts rapidly back and forth around the proper focal point and cannot lock in at all with my 70-200 - likely too much gain in the motor feedback loop.

Yes, and this is what one expects to happen if pin 10 goes o/c and the teleconverter still has its x2 optics. Apparently, simply adding that much extension to the lens also compromises the AF servo loop stability. That's surprising; I should look into this further next week after I'm back from my photo trip.

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