Exposure latitude
In this test we look to see how tolerant of pushing exposure the a7R II's Raw files are. We've done this by exposing our scene with increasingly lower exposures, then pushed them back to the correct brightness using Adobe Camera Raw. Examining what happens in the shadows allows you to assess the exposure latitude (essentially the dynamic range) of the Raw files.
Because the changes in noise are primarily caused by shot noise and this is mainly determined by the amount of light the camera has had access to, the results are only directly comparable between cameras of the same sensor size. However, this will also be the case in real-world shooting if you're limited by what shutter speed you can keep steady, so this test gives you an idea of the amount of processing latitude different formats give.
The Nikon D810 is the a7R II's most obvious peer (same sensor size, similar pixel count), and you can see that the Sony's performance is very similar. Even after a 5EV push, it would be hard to call between the two. The a7R II is possibly a little noisier if you push all the way to 6EV, but the difference is small enough that it could be experimental error.
By way of comparison, you can only push the Canon EOS 5DS R by 2EV before it starts to exhibit more noise than the Sony, which suggests it is adding a fair amount of its own downstream read noise - limiting the degree of processing latitude that you get to work with.
ISO Invariance
A camera with a very low noise floor is able to capture a large amount of dynamic range, since it will add very little noise to the detail captured in the shadow regions of the image. This has an interesting implication: it minimizes the need to amplify the sensor's signal in order to keep it above that noise floor (which is what ISO amplification conventionally does). This provides an alternate way of working in situations that would traditionally demand higher ISO settings.
Here we've done something that may seem counter-intuitive: we've used the same aperture and shutter speed at different ISO settings to see how much difference there is between shooting at a particular ISO setting (and using hardware amplification) and digitally correcting the brightness, later.
There's an increasing amount of noise visible in the pushed low ISO shots, meaning the a7R II is not, strictly speaking, ISO invariant (or, at least, not across its whole ISO range*). Shooting at ISO 640 and above gives a better performance than shooting at low ISOs and pushing (which becomes visible at ISO 800 in this test). This would indicate that the camera is adding a little noise which is then overcome by applying a little amplification.
However, as we saw further up the page, the camera is adding very little noise - its low ISO files are extremely flexible. What's happening is that the a7R II, like the a7S before it, has more than one conversion gain mode, meaning that it is adding a tiny bit of additional amplification somewhere early on in the process, to push all the collected data above its very slight read noise.
This means that there is a (small) noise cost to shooting at below ISO 640 and pushing during post processing, but not because there's lots of read noise at low ISOs, but that the high ISOs have been optimized to overcome what little read noise there is. This explains how the a7R II has managed to record the best high-ISO DR that DxO has yet measured.
What does this mean, creatively? Rishi Sanyal has looked at the a7R II's sensor performance, and what it means for your photography, in more detail in a separate article. Click here to read what he found.
* You could describe the camera as being ISO invariant over two stretches of its ISO range: The ISO 100-500 is invariant in the same way we've seen on previous cameras, but now there's a second region: ISO 640 and upwards, where a further improvement is offered.
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