Tamron 150-600mm f/6.3

[neuroanatomist]

I can see the AF being an issue. Once you are passed the focal length where it goes from f/5.6 to f/6.3, all but your center point AF won't work.

Nope, the AF system will work as with an f/5.6 lens, because the lens lies to the camera and says it's f/5.6 even though it's really f/6.3.

[Kayaker72]

Nope, the AF system will work as with an f/5.6 lens, because the lens lies to the camera and says it's f/5.6 even though it's really f/6.3.

So with the Tamron all the AF points will work after the lens is f/6.3, or just the center point? I was thinking it may just be the center point (with the assistance of the points surrounding the center).

Edit...just reread your post and I think that is exactly what you are confirming. Interesting. So, are the issues we are hearing about with AF from ~500(410?)-600 mm isn't due to being reduced to the center point on 5DIII/1DX. It is due to less light hitting the AF points but the camera still using those AF points?

Edit #2: I think I was having an idiotic moment with this post. Fortunately, I didn't operate any heavy machinery yesterday.

[neuroanatomist]

Edit...just reread your post and I think that is exactly what you are confirming. Interesting. So, are the issues we are hearing about with AF from ~500(410?)-600 mm isn't due to being reduced to the center point on 5DIII/1DX. It is due to less light hitting the AF points but the camera still using those AF points?

Yes, they'll all work. With your 5DIII, an f/5.6 lens and 1.4x TC, only the center point (and 4 expansion points) are active because that's how the camera is programmed to behave with an f/8 lens. If you tape the three pins specific to the TC contacts, you'll find that the camera ignores the TC, thinks there's an f/5.6 lens attached, and you'll get all 61 points with the center 21 being cross type. Same idea with the f/6.3, but no tape required because the lens is doing it's own masking.

As you say, the AF points don't work as well with the narrower aperture - it's not so much the amount of light, because they don't work as well even in full sun, it's more that the narrower aperture doesn't provide as large a physical spread of light (to drive the phase difference) from a given point. When you look at an AF sensor, the f/2.8 lines are more widely spaced than the f/5.6 lines - that's the wider baseline that provides greter accuracy, but also requires a wider aperture to spread the light sufficiently.