How to ruin a first impression: connect a Sony a7RIII body to a Sony 100-400mm lens, turn on, look through viewfinder, half-press shutter button and NO AUTOFOCUS! Try again and it worked. Play for the next two days and find that 10% of the time there is no AF when the shutter or AF-ON buttons are depressed. No rhyme or reason to the loss of AF. However, AF works fine on Canon lenses connected to Sony camera via Metabones adapter. 

Go into research mode. Search Mr. Google on web- no hits. Call supplier, Vistek in Toronto (888-365-1777) and speak to Service Dept.- no solution and suggest I call Service Centre. Call Sony Manufacturing and Technology Centre (Service Centre for Canada, 905-666-7669)- no solution, suggest I call tech support. Call Sony Tech Support (877-899-7669)- no solution but they set up a case number and a repair order. Call supplier, Vistek again and suggest a test at the Ottawa store and a possible lens exchange (I will be there next week). They agree (thanks Bo from Service Dept.). Continue researching Mr. Google to no avail. Posted a question on a Dpreview forum dedicated to Sony- no answer

Then I chanced upon a review article of the 100-400 lens by Thom Hogan here. Read half-way through, then came back and read this statement by Thom:

"How's it Handle?
Look, I like function buttons on the lens. They can be quite useful. Or not. Set at defaults I kept trying to figure out why the camera would stop focusing every now and then. Turns out that my support hand under the lens kept hitting the bottom function button. I'm not even sure why there's a button on the bottom with a lens like this. The left hand button falls naturally under the left thumb when you're hand-holding this lens, but the bottom button is right about where your hand is likely to be, at least part of your hand. On a monopod, that bottom button can be very close mount/pod."

BINGO! My natural position is to hold a tele-lens with it cradled below by my left hand, but in doing so I was inadvertently pressing the bottom Focus Hold button on the lens and disabling AF. I quickly re-programmed the buttons to do nothing (nice that you can do this on the newer Sonys). Crap, I was relieved, but as all this unnecessary fussing sunk in I wondered why this "issue" wasn't all over the web, why Sony Tech Support or the Service Centre didn't have this answer at their finger-tips. Surely Thom Hogan and I are not the only ones to have experienced this?!

I hope this blog contributes to putting the issue out there.