I'd welcome such a sub forum as well! +1
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The focal length divided by the aperture (ie f/ number) isn't just total light availability: it also defines the angle of the incoming cone of light that focusses onto the film/chip.
This has caused a lot of confusion, with people wondering why we can't use an f/8 lens in bright sunlight even though focussing at f/5.6 works in near-dark conditions.
Phase detect AF (the good and fast one that we all love) samples from two points in the incoming light cone, and the phase difference between them shows the amount and direction of focus needed. (Obviously this is per focus point, we're just looking at the centre point). Out of focus images show an image shift depending on which part of the incoming light cone is being looked at.
The further apart you sample in this light cone, the better the parallax and the easier it is to focus.
- Most cameras sample the cone at the equivalent of f/5.6 (39.3 degree light cone)
- Some also do f/2.8 (wider cone, so accurate and fast when available).
- f/8 (28.1 degree light cone) is rare and expensive because the cone is narrow and the difference between images at each focus sensor is relatively small.
Different sensors are chosen depending on the lens attached and its current zoom (which usually affects the f/ number). This is why your camera cares about getting aperture info.
There is no reason it couldn't TRY to use its narrowest AF and report to you what it thinks is going on.
There is however a business reason: Why would Canon want to make it easier/nicer for you to use non-EF lenses?