I've been reading a great deal about extenders. One of the side effects of using extenders is that it stops down your aperture by a stop and 2 stops, for the 1.4x and 2.0x respectively. So this means if I use my 70-200 f/4 with a 1.4 extender it'll essentially become a 98-280 f/5.6, and with a 2x it'll be 140-400 f/8.
Since I use a 60D, which is obviously a non-1D series body, AF will only work with the 1.4x extender when using the 70-200 f/4. Using a 2x will result in a max aperture of f/8, and AF does not work at this point.
This all made sense to me until it came to me that I have a Tamron 18-270 f/3.5-6.3. At the 270mm end, the max aperture becomes f/6.3, which is 1/3-stop below the minimum of f/5.6. Yet without being aware of this I've been using this lens with full AF functionality - even at 270mm, f/6.3.
So what's the science behind this? Or is it the lens that's reporting f/6.3 while actually being at 5.6?