Autofocus explained
How does autofocus work, and why is lens aperture so important?
Autofocus works by looking for the closest subject that can be rendered in focus. It does this by evaluating the subject for contrast, maximum contrast is when the subject is in focus. With DSLR cameras the individual AF point is sensitive to a line of contrast, either vertical or horizontal; cross-type AF points are sensitive to both horizontal and vertical lines. If you have no contrast the camera will not be able to focus, you’ll know this if you’ve tried focussing on a blank painted wall.
Early implementations of focus in Live View worked by maximizing the contrast of the subject, changing focus iteratively to achieve max contrast. When more AF points are active, the camera has more information to process, and it is possible that it will find a closer subject than the one you intend. This is often seen on EOS M and
EOS R cameras where the AF points cover almost the entire image area.
In reduced light levels, subject contrast is reduced, making it harder to detect accurate focus. If your lens has a slower max aperture, say f/5.6 or more, it becomes harder for AF to function. DSLR AF sensors need the light to be perpendicular to the AF point to detect contrast. It’s for this reason that using a tele-converter on some lenses, or using long lenses with f/6.3 or slower apertures often causes the camera to focus more slowly, if at all. Bright light on high contrast subjects helps, but in low light AF performs better with fast aperture lenses.
Taking a look at the EOS R, AF can be used successfully in very low light levels, but it needs the help of fast aperture lenses. Remember an f/2.8 aperture lets 2x the light in an f/4 aperture, and f/1.4 lets in 8x the amount. For the best performance you need well lit, high contrast subjects and fast aperture lenses. Automation gets ever more intelligent and works well, but giving it the best chance is the path to sharp shots.