A7s/A7sii question - HD pixel binning?

[pilsburypie]

I'm aware the A7s/sii has the 12mp sensor for a reason - 4k just looses the top and bottom part of the sensor to get the 16:9 ratio and you have the correct amount of pixels (approx 8mp). This avoids the pixel binning and thus makes for a better image with less noise.

 

So what if you use these cameras just for 1080 HD? Does it use the full sensor as in 4k mode and bin pixels? Does it use just a smaller part of the sensor, kind of like super35 mode thus imparting a crop factor?

 

Interested to know as although it may seems daft, I will be likely recording a fair bit in HD due to 4k workflow and distribution.

[pilsburypie]

Sorry to bump, but does anyone know?