It is important to consider that even though these cameras / sensors are currently the best available for low light shooting, you will still get noise. As you will see in the high ISO 100,000's the images from both cameras are close to disintegration. There will be a better performance for video obviously with the smaller output size, but for stills photography it strikes me that you need a really good reason to shoot at ISO 102800 and above.
The test was carried out using a Sigma 50mm f/1.4 ART lens on both cameras. To give some idea of a real world test I used the lens wide-open until the Nikon's top speed of 1/4000th. sec. meant I had to stop down. Now this is important for something like indoor sports when a high shutter speed is required to stop the action with, for example, a very long lens. so in that regard the Sony does have an advantage. Below are OOC jpgs. and there is a colour balance difference. But this test is primarily about noise performance and how well each camera suppresses it and leaves detail intact.
Even allowing for the fact that I had to upsize the Sony files, the A7s does very well compared to the Nikon. At the very highest settings it's a toss up as to which mess of colour and noise you prefer, but at the lower settings I certainly prefer the the results from the Sony.
Shooting raw however is a slightly different story. The Nikon files seem to improve somewhat compared to the jpgs.. This is what I've been noticing when I've been editing files from both cameras.
There is no doubt that while these are both remarkable sensors for the ultimate high ISO low light performance, the Sony comes out the best, though not by much. There is of course the Nikon D4s with the same sensor as the Df and the added functionality of that camera may produce marginal gains. However for video between the Df and A7s there is only one choice as the DF doesn't shoot video and overall, because of it's jpg. superiority the A7s has to be seen as the ultimate low light camera. Because there is also the silent shutter. And many low light situations are also low noise situations and a silent camera is a bonus.
The A7s is a remarkable photographic tool, allowing decent pictures and video to be shot in extraordinarily difficult situations, that would defeat pretty much every other camera. I can't see how anyone can argue that it isn't the best camera currently available for working at high ISO's. Every other review seems to take the same view.
Maybe one day Nikon will wake up to the fact that bit by bit other products from other companies are slowly eroding their customer base and the more cameras like the A7s appear, the faster that process will continue. Professionals like Nikons, of that there is no doubt, but it must be the case that many will be very impressed by what Sony can now offer and many may make some different purchasing choices to those they have made in the past in the light of that.
And Sony want this market. Their recently established professional network is evidence of that. As is the newly introduced A7 Mk II plus a some upgraded a-mount lenses which can of course be used on the FE series with an adapter. Plus it's not just the 'big lens' end of the market that the A7s works for. Imagine an A7s fitted with the Zeiss 55mm f/1.8 e-mount lens and think about what could be achieved with that. There's not a lot of places that are out of bounds, light wise, with that combination.
I'm still mightily impressed with the Df at lower ISO's and it does have an incredible dynamic range, but I'll leave that to another post. As far as the upper end of the ISO scale is concerned, then I have no hesitation in declaring the A7s the winner. It's not by a huge difference but it is there and it makes the camera incredibly useful for a variety of photographic situations. A superb camera capable of creating superb image files. And more than that a genuine advance on what's currently available. Something that can't be said for every camera release these days.
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There is an interesting article on the BBC website HERE, which deals with many of the issues above.