Low Light Stills Performance

By Rishi Sanyal

a7S: King of low light stills?

Where the 'R' in a7R stood for resolution, the 'S' in the 12MP a7S stands for sensitivity. This purported increase in sensitivity comes at a cost: in the face of cameras with ever-increasing megapixels, the a7S comes in at a rather paltry 12MP. Lower resolution and higher sensitivity are certainly not unheard of - the Canon 1DX and Nikon D4S both top out at relatively modest resolutions of 18 MP and 16MP, respectively (though this is partly done in the name of speed). But is the resolution of a bygone era (Canon EOS 5D anyone?) a worthy tradeoff for the still image and video quality enhancements? It certainly appears to be for video, but we thought we'd put the a7S up against some of its immediate competition (a7R and 5D Mark III) in a real-world low light scene to see just how effective it is at low-light for stills.

But first, a bit of theory: lower resolution sensors can increase pixel-level performance (because bigger pixels capture more light), but it's typically total light gathering area across the entire sensor that is a major determinant of ISO performance, all else being equal. So, to see whether the a7S offers anything beyond the pixel-level benefit its lower resolution would lead you to expect, the higher resolution image is normalized to the resolution of the lower resolution camera. Ultimately, for the a7S to make sense to stills as well as video shooters, Sony's engineers need to have exploited some of the other advantages that well-designed larger pixels can potentially bring: for example, lower cumulative sensor read noise, icnreased effective sensor efficiency (due to less inter-pixel spacing), and so on.

High ISO shootout

We put the a7S up against the higher resolution a7R to see if the a7S offered any significant high ISO advantages over the a7R when the output of the a7R was downsized to that of the a7S. Furthermore, we pitted the a7S against a professional DSLR not too far outside the price range of the a7S: the Canon 5D Mark III.

We shot a night scene that included a range of tones from deep shadows to bright highlights to get a comprehensive idea of noise performance of these cameras at various ISOs. In order to level the playing field for all three the cameras, we shot all of them with the same lens (Canon 24-70 F4L IS), and kept aperture and shutter speed - that is, total exposure - consistent. Since the a7R and 5D Mark III don't offer ISO sensitivity settings above 25,600 and 102,400, respectively, these higher ISO shots were simulated by maxing out the ISO on each respective camera, adjusting shutter speed, and then digitally boosting exposure in ACR.

You can read our comprehensive analysis on the a7S' ISO performance based off of this test in the detailed article we've already published, but we'll summarize our thoughts below:

The Sony a7S enables the use of incredibly high ISOs, including some unavailable to most cameras. When shooting at incredibly low light levels, the a7S will most likely give you significantly better results than any camera that does not natively shoot at these ISOs. There are also advantages to be had over the higher native ISOs of other cameras in its class, particularly higher resolution ones. Although a Nikon D750 holds its own against the a7S at all its native ISOs, we see the a7S overtake the 36MP a7R (and D810, for that matter) in normalized shadow noise performance at ISOs above 6400. At more moderately high ISOs (6400 and below) as well as in brighter regions of images - where image quality is determined primarily by how much total light is captured - the normalized ISO performance of the a7S will be similar to that of high-performance cameras of its generation that share its sensor size (i.e. full-frame).

Autofocus in low light

Nikon D750:
Illumination -2.7EV
Sony a7s:
Illumination -5EV
(same exposure as D750 image)
Sony a7s:
Illumination -5 EV
(exposed to match D750 brightness)

The Sony a7S only offers contrast-detect AF (CDAF), which can't match the tracking abilities of cameras with phase-detect AF (PDAF). Hence, we don't consider it suitable for applications requiring pro-grade AF, and both the a7 and a7 II outperform it in terms of both speed and continuous AF.

However, of interest is Sony's claim that the a7S can focus down to -4 EV. We believe them. We performed a rudimentary low-light AF test similar to the one we performed for our D750 review, where we slowly lowered light level until the camera was no longer able to focus, then slowly increased it again to find the lowest light the AF system can focus in. We then calculated the exposure value needed to correctly expose the area being focused on.

On this basis we found the a7S to focus down to around -5 EV. The rollover above demonstrates how dark this is, in comparison to the Nikon D750 (which is rated down to -3 EV). The shot in the middle was given the same exposure as the Nikon, to illustrate the difference in illumination level. For the right-hand image we increased exposure to match the brightness of the Nikon image, which required settings of: 1/8s, F1.8, ISO 64,000. That's really, really dark.

But there's an important caveat here: focus was very slow, with a lot of hunting and intermittent failure. And this isn't surprising: CDAF needs to hunt to find the point of focus, essentially by trial-and-error, and this hunting slows down in low light. What this means in real-world terms is that if your subject is moving, or if you shake the camera too much, AF often fails because the subject (and point of focus) has moved while the AF system is still hunting. So even though the a7S can, technically, focus in dimmer light than any DSLR, in real-world use a D750 and 5D Mark III tend to outperform the a7S in low light. This is because their PDAF systems only need to make a few quick measurements to determine where to send the focus element of the lens.

Summary

While the Sony a7S is an incredibly capable low-light stills camera, it's not unique in its ability to offer acceptable images at incredibly high ISOs, with the 36MP a7R following the a7S' ISO performance fairly closely up to ISO 6400, and the Nikon D750 offering competitive ISO performance all the way up to ISO 51,200. The a7S does, however, allow one to shoot at ISOs that don't even exist on many other cameras, although even there, it's not unique in its ability, as a Nikon D4S manages to stay neck-to-neck against the a7S up to ISO 409,600.

A camera that renders images in very low light must be able to focus in such low light levels as well, and here the a7S does display a remarkable ability to focus in light levels lower than any camera we've tested. However, it's very slow, and intermittenly fails at focus at such low light levels, which isn't at all surprising. What this means, though, is that in slightly less dim situations where DSLRs can focus (e.g. EV -3 for the Nikon D750), their refined, dedicated phase-detect systems will typically outperform the a7S in speed and ability to acquire AF, especially for moving subjects.