Raw Dynamic Range

By Rishi Sanyal

ISO Invariance

It's not just high dynamic range scenes that benefit from a camera with high (base ISO) dynamic range and a low noise floor: it can also reduce the need to amplify the sensor's signal (by increasing the ISO setting) to keep it above that noise floor. That can afford considerable benefits: by saving the brightening for post-processing as opposed to hiking up the ISO in your camera, you can give yourself significant highlight headroom. Why? Because brightening via ISO amplification in-camera often clips bright tones to white. ISO 200, for example, clips every pixel that is more than half 'full' at the end of an exposure. ISO 400 clips every pixel that is more than a quarter 'full', and so on.

Cameras with low noise floors allow you to essentially bypass the ISO amplifier, so that you can selectively brighten dark tones - while saving bright tones - in post-processing. Cameras with higher noise floors require amplification in-camera to bring the image signal above this noise floor, since brightening in post amplifies both the image data as well as the noise floor. Brightening in-camera by amplifying the image data before it hits the noise floor, of course, doesn't amplify that noise floor.

To test cameras for their noise floor and, therefore, their ability to save brightening for post-processing as opposed to in-camera, we've done something that may seem counter-intuitive: we've shot multiple images at the same exposure settings but using different ISO settings. The Raw files have then been brightness matched. The ISO 6400 shots, then, have 6 EV of hardware amplification performed in-camera, while the ISO 100 shots have 6 EV of digital 'amplification' performed in Raw processing software (ACR). A high dynamic range, low noise camera will show little to no difference between these shots, whereas a noisy camera will show a significant difference.

The a7 II performs relatively well here, but not quite as well as we've come to expect from Sony Exmor sensors. Shooting a lower ISO setting and pushing only matches native ISO 6400 when you use ISOs between 400 and 800. Shoot with ISOs lower than this, while midtones (the grays you see in the above crops) will look comparable to natively shooting at ISO 6400, you'll end up introducing some noise into your shadows if you underexpose and push-process. However, to put this in perspective, the a7 II far outperforms what we see from the Canon EOS 6D and 5D Mark III in this test.

It's informative to contrast the a7 II's performance against the Sony sensor in the Nikon D750, where there's essentially no difference between electronic and software amplification once you get above ISO 200 (even ISO 100 pushed is quite acceptable). Even compared to its 36MP sibling, the a7R, the a7 II falls behind, with the a7R showing better performance for pushed ISO 100 to 400 files compared to the a7 II. In fact, the a7 II shows only slightly better performance than the 12MP a7S here, which we've previously shown to be less ISO-invariant than the a7R or Nikon D750.

Do note that it's the difference between a single camera's pushed ISO vs. native ISO 6400 that we're analyzing here; the a7 II's additional noise over the a7S is because of its poorer overall ISO performance, which is a separate issue. That is to say, the a7 II is still outperforming the a7S slightly in terms of ISO-invariance because the difference between ISO 100 pushed and ISO 6400 is greater for the a7S than it is for the a7 II.

These results mirror quite well previous findings from DXO that indicate that the dynamic range of the a7 II sits squarely between that of the a7S and a7R.

What does this mean?

This information can help you decide when you should increase ISO on the camera and when you can under-expose by selecting a lower ISO and push later, in software. In the case of the a7 II, that threshold appears to be ISO 800. This information can help improve your images in higher contrast scenarios.

Above ISO 800, in terms of noise, there's no difference between increasing the ISO setting in the camera, which applies electronic amplification, and pushing the files later in post processing. However, there is a difference in terms of highlight capture. Amplifying the sensor signal by increasing your ISO setting risks pushing the brightest tones so far that some of them 'clip' and can't be recorded. Under-exposing by selecting a lower ISO prevents this from happening. You can still use the shutter speed and aperture you would have at the higher ISO setting, but under-expose by lowering the ISO, thereby retaining more highlight information. With the a7 II, you can essentially get a better image by leaving the camera at ISO 800 and reducing exposure by however much you feel is necessary to retain highlights, then increasing the brightness when you process your Raw files (instead of increasing the brightness by increasing ISO above 800).

This will effectively allow you to obtain greater dynamic range in low light, greater than what you might have gotten had you natively shot at a higher ISO. You can even lower the ISO all the way to 100 and pay a small noise cost in your shadows, with the benefit of much more highlight range. This noise cost will be larger than what you might get from Sony's own a7R, or a Nikon D750.

The method we've outlined here to obtain greater dynamic range in scenarios typically requiring higher ISOs isn't the way most camera makers expect you to work, so the camera's JPEGs and all its metering and exposure assessment tools that are based around those JPEGs become pretty-much useless. However, this way of shooting confers an image quality advantage if you make the effort to work around these inconveniences.