AF Point Sensitivity with Narrow Aperture Lenses

[Waddizzle]

I guess that I've been thinking too much again.

I've run up against one of those, "Now how is that supposed to work," issues.  Except for the high end models, most of Canon's DSLRs cannot focus at f/8.  The cameras seem to top out at f/5.6. 

But, some lenses, like a Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary, can stop down to f/6.3 at the long end of the zoom range.  So, how is the AF systemsupposed to work properly when the lens stops down that far? 

I've used the "big Siggy" on camera bodies that focus to f/5.6 and others that can focus at f/8.  The f/8 camera bodies seem to focus more accurately at maximum zoom than the f/5.6 bodies.  Granted, the focus systems are not the same, which probably explains most of the differences. 

But, how are the f/5.6 bodies supposed to auto focus when the lens itself stops down to f/6.3?  It seems to work, but why?

[Waddizzle]

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@ebiggs1 wrote:

It has to do with accuracy and cross-type focusing.  In order to make sure you get accurate and reasonably quick AF, Canon limits it to f5.6.  Except on the higher end cameras where the extra grunt can be added.

Sigma and Tamron lenses report f5 to the camera so they will work.  The actual aperture is recorded when you shoot.

If you want to test this you can tape three of the pins on a tele converter and try it.  This will fool the camera and it will think you have what ever lens you have attached.  So if it is a f5.6 lens the camera will still think it is a f5.6 lens.  The lens will AF albeit slowly and maybe not to accurately even though the converter changed the f5.6 to f8.

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Thanks, I've been curious as to why I seemingly get softer images with an f/5.6 body, compared to an f/8 body, at the long end of the 150-600.  I've seen it enough to know it's not my imagination.