I agree with you about our ability to tell the differences when it comes to noise. The pixel peepers are an obnoxious bunch that don’t understand what photography is or how to take a good photograph to begin with. Sharpness and noise really have to be extreme to effect a properly set up compostition or image.
The point I’m making is that AF isn’t based on a single still frame. The ability of a camera to recognize certain patterns the instant data from the sensor is processed depends on a lot of factors. We’ve seen how fast moving subjects can fool AF, that low lighting can fool AF, and out of focus objects in the background and foreground can fool AF. In that microsecond where the camera hunts to lock AF and the shutter button is pressed, we get a completely out of focus image. In sports, particularly motorsports, this is pushed to an extreme.
We shoot on very long lenses, often higher aperture zooms, subjects that are moving almost 200mph, with objects crossing the frame, cars coming in and out of shadows, dirt and debris kicking up, etc. The more noise and resolution a sensor has, the more data points the AF system has to compute. I don’t know the specifics of how Canon is handling this on a hardware or software level, but I do know the raw image
matters.
The screenshot I took best illustrates this. I compared Sony and Canon’s flagship lower resolution cameras to their higher resolution cameras. This is one of about ten major differences in photojournalist/sports/action bodies that matter. I didn’t have control over the lenses, but at 100% zoom, the lower megapixel cameras meant for action show significantly less noise, and less data points a machine learning or AI AF system has to compute.
I doubt the AF is looking at full readout to begin with. I do know that lower megapixel sensors perform better in lowlight when it comes to noise because of the pixel size. Canon has a 2mp speciality sensor for extreme lowlight situations. The larger pixels collect more photons leading to less noise. I hope this helps clarify my position.