>>4469144This goes over everything in detail
https://www.photonstophotos.net/MarianneOelund/AutoFocus_System_Design/AutoFocus_System_Design.htmConceptually, it might be easiest to consider the AF module as having a "virtual" aperture of f5.6, that all light gets funneled through. It was hard for me to grasp that faster aperture, which normally = more light, doesn't matter in this case, it's like you're only using the light from the f5.6 portion of the lens. I'm pretty sure later Nikon bodies, and for sure many Canon bodies, had f2.8 AF points.
Mirrorless (or live view on DSLR) is totally different though. In this case you do get less light hitting the AF sensor when not focusing wide open. The practical benefit is better DoF and exposure preview, and Nikon opted for that while putting their focus on making the AF better despite less light.
Depending on the lens' maximum aperture, the aperture used when shooting, and the bodies you're comparing, even despite this "flaw", in many circumstances, the Nikon would still be better off. Nice that we're getting more options with the Z8, but it's only going to take something like a Z5II, which is already basically class-leading, even better. Like it has a ~3.5EV advantage over an R6II for example, so if you're shooting an f1.4 lens at f5.6 or higher, the R6II has a 0.5EV stop advantage, but if you're shooting that f1.4 at anything under f4.5 or lower, the Z5II has the advantage. If you were using an f2, or f2.8 or f4 lens, a Z5II would always have the advantage.
