[JJ_SO]

"Speed" was an unlucky choice of term from my side. "Fast lenses" are a common expression and my "speed" is referring to the max. aperture. Which also has an impact on focus precision or the ability of the focus unit to see something to focus at in lowlight conditions. But of course, f/4 or f/2.8 don't make that superhuge difference. Nikon i.e. says something about the aperture and which AF-sensors are compatible with f/8. What they don't specifiy so clearly is, when even f/4 is too dark.

 

So, with the disadvatage of a CDAF system needing more light to focus quick and precise, fast lenses are welcome to improve AF.