Sony a7 III Review
Raw Dynamic Range
- The Sony a7 III's 13.8 EV (14.6 EV 'print' normalized) dynamic range at base ISO is nearly class-leading
- High ISO dynamic range is class-leading, meaning cleaner pushed shadows under low light conditions
- There's no drop in dynamic range in continuous drive modes, unless you shoot compressed Raw
- You can use HLG to perfectly 'expose-to-the-right' (ETTR) when shooting Raw
- The a7 III has two ISO-invariant ranges: 100-500, and 640-51,200. For why that matters, read more below.
In this test we look to see how tolerant of pushing exposure the a7 III'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 this test 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.
The a7 III's extensive base ISO dynamic range - which we measured to be 13.8 EV thanks to its very low sensor noise floor - means its Raw files are very tolerant of extreme adjustments. It's a similar performance to other best-in-class cameras like the Nikon D750, but markedly better than Sony's own a9 or the Canon 6D Mark II. It also performs similarly . This means you can expose to protect highlights and selectively brighten your image later without worrying too much about noise creeping into your shadows and midtones.
Using HLG to ETTR
The wide exposure latitude the a7 III affords you doesn't mean you should underexpose your images any more than you need to; rather, for the best quality image, you want to maximize exposure as much as possible before clipping highlights worth preserving (a technique known as 'Expose to the Right' or ETTR).
Like the a7R III, the a7 III gives you an easy way to determine this clipping point in Raw: by enabling HLG (Hybrid Log Gamma) in a Picture Profile and turning on Zebras (set to 100+). Once you've done this, increase your exposure until you first see zebras in highlight regions you wish to preserve. This setting will get those highlights just short of clipping in the Raw, essentially preventing you from unnecessarily underexposing - and thereby increasing noise in - the Raw file based on the JPEG preview. HLG does not affect the Raw file. Read more in-depth coverage of this feature in our a7R III review.
A camera with a very low noise floor is able to capture a large amount of dynamic range, since it adds very little noise to the detail captured in the shadow regions of the image. This 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 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) vs. digitally correcting the brightness, later.
Due to its dual-gain architecture, the a7 III has two 'ISO-invariant' ranges: ISO 100-500, and ISO 640-51,200. That's why in the comparison above you don't see much of noise difference between ISO 400 and ISO 100 pushed, or between ISO 6400 and ISO 800 pushed.
There is a difference though between a properly exposed ISO 6400 shot and an ISO 100 shot exposed for a proper ISO 6400 midtone exposure and then brightened in-post. That's because of the a7 III's excellent high ISO performance: at ISOs 640 and above, the sensor switches each pixel to a higher gain circuit, effectively kicking up amplification at the sensor level and helping the camera overcome the already low levels of read noise.
Our friend Bill Claff has actually measured any noise benefit to - for the same exposure - using a higher ISO in-camera vs. brightening a lower ISO Raw in post. He's found that there's essentially no benefit to using ISO 500 vs. boosting an ISO 100 Raw 2.3 EV, and no benefit to using ISO 51,200 vs. boosting an ISO 640 Raw 6.3EV in post. There is, however, nearly a stop improvement in shadow noise going from ISO 500 to ISO 640, because of the gain switch at ISO 640.
What does all this mean for your exposure decisions? If your midtone exposure demands ISO 400 but you're worried about clipping highlights, you're better off keeping your exposure settings the same but dialing the camera back to ISO 100 and then selectively brightening the Raw later. This affords you 2 EV extra highlight headroom, with no extra noise in shadows or midtones. If on the other hand your midtone exposure demands ISO 6400, you're better off keeping the same shutter speed and aperture and dialing the ISO down to ISO 640, affording you 3.3 EV extra highlight headroom at no noise cost.
High ISO dynamic range
The dual gain circuitry in the a7 III - and most recent Sony sensors - not only improves general noise performance at high ISO, but also high ISO dynamic range. Below you can see our measured dynamic range increase the a7 III (dual gain) displays over the a7 II (non-dual gain) at ISOs above 500. You'll see this as cleaner (pushed) shadows at higher ISOs.
Compressed continuous drive performance
Like the a7R III, the a7 III can maintain its full dynamic range in continuous drive modes, thanks to 14-bit readout in bursts when shooting uncompressed Raw. If you shoot compressed Raw, the camera drops to 12-bit sensor readout in continuous drive modes. This negatively impacts dynamic range, dropping 1.4 EV at base ISO and roughly 1 EV at ISO 640.
|a7 III Uncompressed (orange) vs. Compressed 12-bit (light orange) performance. There's a drop in dynamic range at lower ISOs that more or less evens out at the higher ISOs.|
Dynamic range catches up at higher ISOs, but never quite matches the performance of 14-bit readout. To put this in perspective, though, the compressed Raw performance you see here roughly matches the dynamic range of the a9 in any mode.
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from No 6
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