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Message: <blockquote data-quote="BeerBelly" data-source="post: 421035" data-attributes="member: 35439">I'm aware of the fact you need to include the TC loss in the calculations. I'm also aware of the fact I tried it and it works...Often these long zooms (like the 150-600 or the older 150-500 from Sigma) represent their wide open aparature to the body as f5.6 instead of f6.3 to allow AF on older bodies that do not focus down to f8. This is also the reason AF works with the TC, since 1.4 x 5.6 is f8. The reason why your 2x TC didn't work is because it is a 2x TC...the arithmetic here is f5.6 x 2, getting a number of f11.2 (too low to AF).Stopping down to achieving sharpness doesn't come into play, since the lens always focuses wide open (to let in the most light possible) and stops down only to take the shot.</blockquote>