The A7 and A7R are both 36MP but the latest A7s is only 12MP its sensor is specifically designed for low light.
It can take pictures of black cats in a cellar at midnight apparently.
Now I have been looking into this system:
A7 24Mpix to be the allrounder.
A7r 36 Mpix, same DXO score as the D800! The high resolution monster.
A7s 12 Mpix, The darkness monster and full info 4K video on FF (with external storage though), the big pixel solution.
=> btw does anyone have a site (I found it on snapsort but did not believe their numbers) where you can see the size of pixels on the sensors? Calculation that worked for the Hasselblad 60Mpix = 6 Micometer; brings the A7s to 8.5 Micrometer. Why would you have bigger pixels then the Hasselblad? (Snapsort did bring Airy Disks and diffusion better in my knowledge)
The main advantages of this A7 series are multiple:
- lightest and smallest FF around
- performance that can follow (beat) the best FF
- relatively low price for performance
- you can work with about any lens (if you accept to do some things manually)
This does not make them the thing to have:
- AF is notoriously random (and lenses with it are limited, Canon lenses can AF, but not very well)
- battery life is limited
- it outperforms on some things, but not by much and at the price of some other oddities (menu is one of them).
- less sturdy (A7 only) and less weather proof
So conclusion:
You need to shoot in low light with a lot of action: you need to put the money down for a D4s.
Low light less action, some nostalgia: buy a Df so you keep some modern features
High resolution: put the money down for a D810
You want to use your old (e.g. Leica mount Nikkor) lenses manually on a high performing system that is very portable and not too expensive, the old fashioned way: Yes the Sony A7 series will do the job better then anything else (model depending on your requirement).
Just a comparison chart that seems to explain the low light performance best (higher general DXO scores do better at low ISO, but generally small pixels do worse afterwards):
The story I got: Do you want to take a picture of a black ninja holding a black cat in a dark closet, well if you can do it at ISO 16000 the D4S and the Df do well, but the A7s starts to beat them and keeps that level up to 51200K. Put a Leica Noctilux on the A7s and you will still be able to take pictures of the financial darkness you put yourself into. D4s does a bit better in low ISO and the DF is the low light champion till ISO 3200.