Area wise, a F.F. image sensor's area can fit 27.3 times type-1/2" sensors.
See "area normalized" column (1 vs. 0.0361)
Re: sensor used in A7s
Other than its large pixel pitch, the sensor technologies of its generation of SONY Exmor
is a significantly improved CMOS Image sensor (low power, per-pixel sense amplifier a.k.a. active pixels, built-in column parallel A/D, and performs both analog CDS and digital domain CDS.)
In the end, the net effect is the overall image-level S/N is so high such that it can sustain extreme signal boost (ISO boost) without being complaint as "too noisy".
This is the main reason that even a type-1/2" CCD (ICX418, ICX828, etc.) has the similar per-pixel area as A7s, and even ICX418AK, ICX828AK have complementary-color Bayer filter (CMY +G) than A7s' RGBG (primary-color) which let in more lights, A7s' can still afford offering extreme ISO boost.
For Mark (re: why A7s)
I think in the forum we are all looking for a product that can "catch" extreme low light for astro viewing purposes.
It seems to me A7s has potentials due to its extremely high ISO capability.
On the other hand, I am also curious that if there is a good summary write up
describing how to use A7s in a way that has the appearance of (almost) continuously taking images (few second exp time) and displaying them without too much delay.
Even if its not directly, immediately off the camera body, can PC-tether do it?