Is there an understandable explanation for this? If I were to play dumb, I could argue that AF speed should depend on working aperture, or *available* light, but it is determined by max aperture, whatever the working aperture is. Why is that? Your own words or a reference welcome.
The AF sensors are based on rangefinder technology. The maximum aperture also gives the maximum baselength the sensors can operate at. You can either have sensors spaced wide apart that focus very accurately but need a large aperture; or you can have them spaced closely together, for less accuracy, but can handle small apertures.
EOS-1 and -3 series have both types built in.
Consumer models only have the small f/ types.
Karen Nakamura www.photoethnography.com/ClassicCameras/
p.s. anyone who has experienced focusing prism "black-out" with an old fashioned SLR with a microprism focusing screen (such as a Pentax Spotmatic SPII) has experienced this. It has nothing to do with the amount of light (brightness) but the angle the light takes through the lens.
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