02-03-2016, 08:42 PM
Quote:"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.
Well, compared to the Sony 70-200mm f/4 (on an A7x) even the AF argument isn't valid neither in terms of C-AF nor P-AF which are both located on/obtained from the image sensor. The 1 f-stop advance of the full format sensor and identical equivalent DoF (on the sensor planr) compensate the f/4 vs f/2.8 effects here as well.
The AF sensitivity argument is only valid on DSLRs because the AF photodiodes in the viewfinder are identical on APS-C and FF DSLRs (comparatively speaking).