Sony Alpha 7R II: Real-world ISO invariance study
The recently announced Sony Alpha 7R II packs a lot of groundbreaking features, and one of its highlights is the new full-frame 42.4MP BSI-CMOS sensor. Sony sensors tend to show very high Raw dynamic range due to low noise characteristics, but we wanted to take a closer look at exactly how little noise the camera itself contributes to images. So we performed our typical 'ISO-invariance' analysis on the camera, but with a real-world scene. Have a look at the results for yourself.
Recently, we've been testing the 'ISO-invariance' of cameras in a controlled manner, as a means of comparing Raw dynamic range across cameras. Before we move on to the Sony a7R II results, we thought we'd do a quick refresher here.
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 means you can push those shadow regions and make them visible, allowing you to expose your scene for the highlights, and 'rescue' shadows later. Cameras with lower dynamic range provide less of an ability to do so, meaning that pushed files look worse than files exposed 'properly' at the time of capture.
All of this has an interesting implication: a camera with a very low noise floor (high dynamic range) 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. In a nutshell, it means that once you've determined your optimal shutter speed and aperture settings, you can forego the higher ISO setting - which really only brightens your file in-camera - and adjust your exposure later.
Why would you do this? To protect highlights. In fact, I used this methodology to achieve the shot taken on the Nikon D810, a highly ISO-invariant camera, below:
In our ISO-invariance test, we do something that may seem counter-intuitive: we use the same aperture and shutter speed at different ISO settings, and then equalize brightness of all resulting Raw files in post-processing. Recall that your ISO setting in your camera is simply an in-camera 'brightening' slider of sorts, so what we're essentially looking at is: what is the difference between brightening in-camera by increasing ISO (and using hardware, mostly analog amplification) vs. brightening after-the-fact in your Raw converter (digital correction)?
Below, you'll see a bunch of shots at different ISOs. Note that all shots X EV below ISO 6400 were adjusted +X EV in ACR. For example, the ISO 100 shot was pushed by 6 EV, ISO 200 by 5EV, and so on. An 'ISO-invariant' camera shows little cost to boosting a low ISO file in post-processing, while an 'ISO-variant' camera (which has lower dynamic range) shows significant cost to performing this exposure adjustment in post-processing as opposed to in-camera via setting your ISO to a higher setting.
As you can see above: there's a very modest visual difference* in noise between shooting at ISO 6400, compared with using the camera's base ISO (100) and digitally pushing later. Why does this matter? Well, for a start, it shows that the sensor is contributing very low levels of downstream read noise, which is impressive in itself. But it also opens up the option to use the shutter speed and aperture value you'd usually use for ISO 6400, while staying at ISO 200 and using a much lower level of amplification and, then, selectively brightening your image later. Using a lower level of amplification means that highlight detail is less likely to get over-amplified and blown-out. And you can see this in the signage: lower ISO settings retain more detail in the bright sign, whereas all detail is blown in the ISO 6400 shot.
Now, we're not saying there's no cost to keeping your ISO low and brightening in post.** We're saying that the cost of a 6 EV push of an ISO 100 shot (vs ISO 6400) is only a mere half a stop or thereabouts , with almost no visible cost in .*** Meanwhile, you give yourself no less than 6 EV highlight headroom by decreasing your ISO to 100.
Speaking of that sign, take a look at the unpleasant artifacts around it. With the a7R II, Sony continues the tradition of lossy Raw files, with no option for lossless 14-bit Raw. This isn't going to affect all your photographs, but it can rear its unsightly head at high contrast boundaries of pushed files. The area around the sign in the ISO 100 + 6EV and ISO 200 + 5EV shots have artifacts not present in the higher ISO shots. These posterization artifacts are due to Sony's lossy compression, which you can read more about in Iliah Borg's excellent study here.
We think this Raw compression is particularly a shame, especially when you consider how ISO-invariant this camera is otherwise. As you saw above, the pushed ISO 200 shot has very little additional noise compared to the ISO 6400 shot. This is because of the high dynamic range, and ISO-invariance, of the camera, which means that realistically you can save much of your image brightening for post-processing as opposed to increasing your ISO setting in-camera. However, you'll be limited in your ability to do so because of Raw compression, essentially meaning the camera isn't as ISO-invariant as it could be.
As we've come to expect from Sony sensors, the a7R II's high dynamic range means the camera is fairly ISO-invariant. This means you can often save image brightening for after-the-fact, affording yourself (potentially) stops of highlight headroom by keeping analog amplification low in-camera. You can do this by shooting in Manual, holding your shutter speed and aperture at whatever is needed for the higher ISO your low-light situation demands, but then simply dialing down your ISO setting.
Sadly, Raw compression means you'll be limited in your ability to do so, due to the potential for posterization artifacts around high contrast edges.
One thing to note: on many cameras, working in the manner we're suggesting may be awkward, because it can mean trying to operate with a very dark preview (because the camera thinks you're under-exposing when you dial down that ISO setting). But on Sony's newer cameras, you can employ the ultra-flat S-Log2 gamma curve from the Picture Profile menu. This super-flat tone curve ends up representing relatively dark captured tones as mid-tones, which allows you to see the wider dynamic range the camera and our eyes are capable of when shooting in the manner we're suggesting.
* Remember to turn off 'Shading Compensation' so you don't get weird artifacts near the edges, even in Raw (scroll back up to the widget after clicking this link).
** In terms of noise levels, there actually is some cost to pushing in post-processing as opposed to performing analog amplification in-camera (leaving aside discussion of Raw compression). In this case, this can be both a good and bad thing. Even with the best cameras with the lowest noise floors (e.g. Nikon D810), we see a bit of a noise cost to performing amplification in post vs. in-camera, and this is due to some residual downstream read noise, likely including some quantization noise due to insufficient ADC bit-depth for the large full-well capacities of such cameras. Part of the noise cost with the a7R II, though, is likely due to an additional benefit of the higher ISO settings on this camera: a higher conversion gain. Bill Claff's data on the a7R II vs the a7R indicate that at ISO 640 and above, a higher analog gain is applied fairly upstream in the signal pathway, which can help overcome the finite downstream read noise that, while incredibly low in Sony sensors, still is non-zero. In this case, then, 'ISO-variance' is actually a good thing, with the camera only truly ISO invariant from ISO 640 and above. Confused? It's a bit much to elaborate upon here, so let's discuss in the comments below!
*** For reference, here's what 1 EV differences in noise performance look like between different ISO settings on a D750, or what a 1 and 1/3 EV difference in noise looks like between formats (D750 vs D7100 in the link provided). The noise performance cost we see in shadows in our a7R II ISO 6400 vs. ISO 100 + 6EV shots appears to be less, to our eyes, than any of the differences you see here, hence our 'merely half a stop or thereabouts' comment.
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