Sony a7 III dynamic range and high ISO improve over its predecessor
Sony recently announced the a7 III, a comparatively affordable full frame mirrorless camera that incorporates a host of advanced features derived from the a9 and a7R III. The combination of price point and feature set makes it attractive to both enthusiasts and pros, particularly those looking to get into full frame or perhaps even make the switch to mirrorless. While we've already shot quite a bit with it and offered our thoughts on the camera as a whole, we hadn't had a chance to take a deep dive into its image quality performance.
And we know many of you are wondering: what's the dynamic range like? The high ISO performance?
Let's take a look.
Low light (high ISO) performance
Low light performance has improved markedly over the a7 II, putting it more or less in-line with the a7R III (and therefore a9) when images are viewed at the same size (we've downsized the a7R III shot to 24MP). These are 100% crops here (if you're viewing on a smartphone or Retina / 4K display, see this footnote* below). Roll over the captions, or click on any of the images to view our full studio scene images for each camera.
This is a great result, but also comes as no surprise: noise performance is broadly determined by a combination of sensor size and technology, and we've recently seen some significant improvements to sensor technology made by Sony. In particular, the backside-illuminated (BSI) and dual gain architecture of most recent Sony sensors helps squeeze every last bit of performance out of these already low noise imaging chips. Furthermore, the original a7 and a7 II lagged in high ISO performance, often failing to surpass the best APS-C sensors.
Dynamic range vs. the a7R III
The a7 III more or less matches the base ISO dynamic range of the a7R III, when both are viewed at common size (we've normalized all our graphs to 8MP). That means both cameras will give you similar ability to make use of (brighten) shadows in Raw files if you want to show a wider dynamic range than shown with the default tone curve. And as long as you're shooting uncompressed Raw, performance is no different whether you're shooting Single or Continuous drive.
In numbers, that's 14.6 EV and 14.8 EV for the a7 III and a7R III, respectively, which falls within our margin of error. You might see a difference in extreme pushes or exposure adjustments, but it's not likely to be photographically relevant.
Note the jump in dynamic range at ISO 640 for both cameras. That's essentially the camera's second 'base' ISO, where the second stage of the dual-gain architecture kicks in. At ISOs 640 and above, most recent Sony sensors use a higher gain mode that essentially amplifies the signal at the pixel-level to get it above the (already pretty low) noise floor.** In laymen's terms, that just means 'more picture, less noise', particularly in shadows – hence the increase in dynamic range.
Our analysis shows the a7 III to just edge out the a7R III at these higher ISOs, albeit only by about 0.3 EV (which happens to be right around our margin of error). You might see this in the deepest shadows – in fact, if you look very closely at the darkest patch in our ISO 25,600 rollover above, you can kind of see a tad bit less noise in the a7 III, but is that photographically relevant? Up to you.
... but it shows a marked improvement over its predecessor
While base ISO dynamic range remains the same as its predecessor, the dual-gain design brings a marked improvement at high ISO. Shadows at high ISO will be notably cleaner on the a7 III, and that's before you consider the better overall high ISO performance – even in brighter tones – likely due to either a more efficient sensor or lower upstream read noise.
Compressed continuous drive performance
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. Dynamic range catches up at higher ISOs, though never quite matches the performance of 14-bit readout. Even at ISO 6400, 12-bit files are roughly 0.4 EV behind - though this is unlikely to significantly impact your photography. The differences at lower ISOs and at ISO 640, on the other hand, you might notice in more extreme pushes.
|a7 III Uncompressed (orange) vs. Compressed 12-bit (light orange) performance. We're not sure about the jumps at ISO 160 and 800, but for the most part there's a drop in dynamic range at lower ISOs that more or less evens out at the higher ISOs.|
In Single drive mode, compressed Raw continues to use 14-bit sensor readout, so measured roughly the same dynamic range as Uncompressed (it dropped 0.1 EV, but that's within our margin of error).
And if you're confused about when the camera drops to 12-bit – which is the only time you'd see these drops in DR – the only combination that diverges from 14-bit is when you shoot compressed Raw in (any) continuous drive mode. All other combinations of Mechanical or Electronic shutter, drive mode or Raw type are 14-bit.
vs. a7R II
We threw this one in here because the a7 III and a7R II are currently being sold for roughly similar price (the latter is $400 more expensive), so we're aware of some discussion about choosing between the two. You're unlikely to notice our measured 0.2 EV higher base ISO dynamic range of the a7 III, but you might notice the 0.5 EV advantage at ISO 640. At higher ISOs the cameras even out.
Realistically though, there's not much difference between these cameras.
|a7 III (orange) vs a7R II (red) dynamic range. You might notice the 0.5 EV advantage of the a7 III at ISO 640, but for the most part performance is similar.|
Due to the dual-gain architecture, there are two 'ISO-invariant' ranges: ISO 100-500, and ISO 640-51,200. This means that 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.
Wait, does this mean I should shoot ISO 640 instead of 320?
No. Not necessarily.
If you have enough light to expose ISOs 200-500 correctly, you should use those ISOs. For example, say you can set a shutter speed and aperture to expose ISO 320 properly. You should not rather choose ISO 640 and shorten your exposure (to preserve highlights that the higher amplification of ISO 640 might clip). That would mean lower overall signal:noise ratio due to increased photon shot noise contribution, and would essentially have the same overall effect of shooting with a smaller (in this example: APS-C) sized sensor.
Recall that dynamic range is not everything, and generally the more light you collect, the better your image. Bill Claff's 'Photographic Dynamic Range' data for the a7R III, which uses a higher threshold for 'acceptable noise in shadows' and therefore considers total light captured more than our measurements, shows that ISO 100-400 outperform ISO 640 and higher. Dual-gain boosts low light performance, and shouldn't affect your exposure decisions any differently, other than perhaps biasing toward ISO 640 rather than 500 in low light.
We've summarized our results in numbers in the table below.
|ISO 100 (24MP)||ISO 100 (8MP)||ISO 640 (24MP)||ISO 640 (8MP)|
|a7 III||13.8 EV||14.6 EV||13.4 EV||14.2 EV|
|a7 III (compressed 12-bit)||12.4 EV||13.2 EV||12.3 EV||13.2 EV|
|a7 II||13.9 EV||14.7 EV||11.8 EV||12.6 EV|
|a7R III||14 EV||14.8 EV||13.1 EV||13.9 EV|
|a7R II||13.6 EV||14.4 EV||12.9 EV||13.7 EV|
|a9||12.6 EV||13.4 EV||12.4 EV||13.2 EV|
So what's the take-away? The a7 III's image quality more or less matches what we've come to expect from modern, well-performing full-frame sensors. There's really not much difference between the a7 III, the a7R III, the a7R II, or the Nikon D850 for that matter.
But if you're coming from one of the original a7 cameras, you'll notice the dramatic increase in low light performance. The a7 III bests its predecessors both in dynamic range and general noise performance at higher ISOs, thanks to a number of sensor improvements (efficiency, BSI, dual-gain). Interestingly, the a7 III, which we'd imagine shares a similar sensor to the a9 minus the stacked design, offers roughly 1 EV more dynamic range than that camera at ISOs 100 and 640 (the cameras even out at the highest ISOs). General noise performance of the a9 - if you're not pushing your files - is similar though.
The a7 III's image quality more or less matches what we've come to expect from modern, well-performing full-frame sensors
The a7 III offers great image quality performance at an affordable price point. That said, it's not image quality that sets this camera apart from its contemporaries but, rather, its significant other capabilities like autofocus, silent shooting, video and a number of other things we'll be delving into in our full review.
* Retina & smartphone optimized 100% crops:
** Technically speaking, it's not exactly more amplification. Rather, the sensor switches to a different circuit within the pixel that has different capacitance at the floating diffusion node. This essentially generates a larger voltage swing (signal) per photoelectron captured, which means the signal - your picture - is less affected by the noise floor of the sensor and electronics.
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