When a DSLR auto-focuses it does so with the aperture wide open, regardless of the aperture to be used for the photo capture. Once focus is achieved, it stops down the aperture to the desired size and takes the picture.
This is probably the answer you were looking for.
For example, if you had an F2.8 lens and set the aperture for the photo to F16, the camera would auto-focus with the aperture at its maximum of F2.8, then close down to F16 for the shutter release. If the max aperture of the lens (or effective max aperture, if used in combination with teleconverters) is F8 or smaller, it will not auto-focus (except on some pro bodies and, hopefully, the 7DII).
This is my understanding, anyway. Take anonymous internet info for what it's worth!
You can also see it: Select F16 of F22. If you look into the lens you cannot see the aperture. If you press the shutter release and you can see de aperture close (and open). If you change the aperture while looking into the lens you won't see any difference. Only when pressing the shutter release.