Raw Dynamic Range: 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 to keep it above that noise floor at higher ISOs.
To illustrate this 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. Essentially, we've done a more complete version of the exposure latitude test outlined on the previous page, showing the results of pushes all the way from 6 EV to 1 EV for ISOs 100 to 3200.
We've become accustomed to Sony's Exmor sensors being 'ISO-invariant,' meaning it makes no difference whether you choose to amplify the signal electronically (by increasing the ISO in the camera) or by increasing the brightness of the image when you process the Raw file. This isn't the case here - the camera's noise floor is a tad bit too high.
With the a7S, shooting a low ISO and pushing only becomes equal to shooting natively at ISO 6400 once you reach ISOs somewhere between 800 and 1600. If you shoot at a lower ISO than this, you'll end up introducing the sensor's noise into your image if you underexpose and push-process. This is a similar (though less dramatic) situation to the one we've seen from many Canon sensors, such as the one in the Canon EOS 6D, shown here at a common output size.
Contrast this with 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 compared to its 36MP sibling the a7R, the a7S falls behind, with the a7R showing better performance for pushed ISO 100 and 200 files compared to the a7S.
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 a7S, that threshold appears to be 1600. This information can help improve your images in higher contrast scenarios.
Above ISO 1600, 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. With the a7S, you can get a better image by leaving the camera at ISO 1600 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 1600).
For a very high dynamic range scene, though, you'll still benefit from shooting at base ISO, because that's where the camera has the greatest dynamic range.
Again, this 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.
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