Sony A7S Field Test Part II

Dynamic Range Performance & Video Recording

By William Brawley| Posted: 10/27/2014

Düsseldorf, Germany. Photo has been adjusted in Adobe Lightroom. Click image to view original.
Sony A7S + FE 24-70mm F4 Zeiss: 24mm, f/5.6, 1/800s, ISO 100, +0.3EVs.

Apart from the absolutely stunning high ISO performance of the new Sony A7S camera, another hallmark feature Sony is touting its dynamic range performance. At a stated 15.3-stops of dynamic range, the A7S should give photographers plenty of flexibility to reign in those highlights without clipping and lift up and reveal shadow details without a lot of noise. The primary reason why I personally shoot in RAW is the extra latitude it gives me to adjust images as I see fit, especially shadow and highlight details, as well as white balance, sharpness and noise reduction. In this installment of my A7S Field Test, I'll put the dynamic range capabilities of the camera to the test. I took the A7S along with me to Photokina in Germany, and I made sure to capture some challenging photos with strong highlights and deep shadows.

And last but not least, I'll go into detail with the other major feature of the A7S: video recording. With the specially-designed "4K-optimized" sensor with full-pixel readout, as well as a new higher quality XAVC-S video format, like the Panasonic GH4 I reviewed earlier this year, the Sony A7S is clearly designed to do double duty for both stills and video shooters. Let's see how it stacks up.

Cologne, Germany. Photo has been adjusted in Adobe Lightroom. Click image to view original.
Sony A7S + FE 24-70mm F4 Zeiss: 69mm, f/5.6, 1/800s, ISO 100.

Let there be light. And shadow.

With only 12 megapixels, the Sony A7S is certainly not the camera for those needing to create humongous prints or advertising photographers that typically work with 40-50MP cameras, however, as I mentioned in my first Field Test section, the A7S does make for an excellent photojournalist, street, or travel photographer's camera -- it's lightweight, stealthy, and has amazing high ISO capabilities. Coupled with its astonishing high ISO performance, the dynamic range provided by Sony's large-pixeled, full-frame sensor is equally impressive.

Walking around in Germany with the A7S, I was able to snap a variety of scenes, many of which had strong contrast, with bright skies and deep, dark shadow areas. I was interested to see just how much massaging the A7S files could take in terms of boosting up deep shadows without introducing too much noise. I typically don't do much or any post-processing to the gallery images I shoot for my reviews, as I want to present simply what the camera is able to capture rather than tweaking the photos to my heart's content. However, in this case, adjusting and fine-tuning images were definitely called for, though I pushed some of them beyond what I would normally do to probe the limits.

Original Photo, above. (Cologne, Germany)
Sony A7S + FE 24-70mm F4 Zeiss: 24mm, f/5.6, 1/1250s, ISO 100.

Adjusted Photo, below.
Photo with shadows and highlights strongly adjusted in Adobe Lightroom 5 using the RAW file. In the 100% crop above, you can see that even when strongly boosting shadow levels, relatively low noise is revealed when using Lightroom's default level of noise reduction.

As I've experienced with other cameras in RAW format, it's amazing just how flexible RAW images are to tweaks, adjustments and modifications compared to the straight-out-of-camera JPEGs. Importing a selection of RAW files from the Sony A7S into Adobe Lightroom 5, I had absolutely no problems pulling down bright highlights and revealing details in the sky, for example. However, as with any camera, if you overexpose too much, highlights are forever blown, and exposure or highlights sliders can only do so much.

For instance, I shot a test shot with a bright daylight sky at 0EV and then again +1EV. While the 0EV shots did show some minor highlight clipping in Lightroom, I was easily able to tone them down for a proper exposure without any clipped areas. However, with the +1EV shot, there was quite a large area with full 100% RGB levels, and even though I pulled the highlight level down to where it wasn't indicating red clipping warnings in Lightroom, the RGB values were still around 96% and looked overblown to my eye.

With shadow detail, the Sony A7S is very impressive, which is what I was expecting given the touted performance of this sensor. Shooting in daylight, most of these shots were at ISO 100, and therefore there was a lot of leeway when bringing up the shadow level and revealing detail without introducing noise. Many times on various sunny shots, I could view images at 100% without feeling the need to apply any additional noise reduction over Lightroom's default settings.

Cologne, Germany. Photo has been adjusted in Adobe Lightroom. Click image to view original.
Sony A7S + FE 24-70mm F4 Zeiss: 24mm, f/5.6, 1/4000s, ISO 100.

Of course, editing and adjusting photos solely from the Sony A7S is all fine and good, but while I was very impressed with the dynamic range of the images from the Sony, I wanted to see how they compared to those from other cameras. Back in the States, I took some test shots comparing the Sony A7S to a couple of its big competitors, the Canon 5D Mark III, the Panasonic GH4 as well as the compact Sony RX1.

As previously mentioned, Sony claims 15.3 f-stops of dynamic range for the A7S, however when you take a look at DxOMark's sensor ratings and in particular their dynamic range test score, things get a bit interesting. We're not sure exactly how Sony calculates dynamic range, but DxOMark's scores are a little surprising, indicating that the A7S has a maximum dynamic range of "only" 13.2 stops while the regular 24MP Sony A7 camera gets a DxO score of 14.2 stops -- a whole stop better at dynamic range! Similarly, the small, full-frame RX1 gets a 14.3 f-stop rating (the same score as the Nikon D800E).

This is partly because DxOMark's scores default to "Print" performance, where each camera's output is normalized to 8 megapixels, roughly corresponding to an 8x12 print at 300 DPI. This means higher-resolution cameras have an advantage over lower-resolution models because the normalization process averages out some of the noise proportional to the amount of downsampling required. This is confirmed by looking at DxOMark's "Screen" results (no normalization), which show the above three cameras are more closely matched at base ISO, with the A7S scoring 12.8, the A7 scoring 13.2 and the RX1 13.5 stops. Also note that the A7S does much better at higher ISOs than the other two Sonys, offering well over a two stop advantage by the time you reach ISO 25,600.

Still, the test results aren't nearly as high as Sony's claim, but perhaps the company does not take noise into consideration in its dynamic range measurements while DxO Labs uses a signal-to-noise (SNR) threshold of one when defining the lower boundary of acceptable luminance noise in their scores. Regardless of the apparent discrepancy, though, the Sony A7S's dynamic range is impressive particularly when it comes to shadow noise, as we'll see below.

In the images shown below, we have a bright, overcast sky as well as a very dark covered driveway. All were shot with these exposure settings: f/5.6, 1/100s, base ISO*. Converting each of these shots in Adobe Camera Raw all with the exact same levels of adjustments, all four cameras did very well in terms of bringing out shadow detail, as I was able to pull out impressive detail in the seemingly all-black area of the covered carport. However, when looking closely some discrepancies arise, and, to my eye, the Sony cameras come out on top.

*The Panasonic GH4's base ISO is 200.

Sony A7S Dynamic Range Comparisons

Sony A7S Original Shot. Exposure: f/5.6, 1/100s, ISO 100

With all four images, the RAW files were adjusted exactly the same in Adobe Camera Raw:
+5EV, No RAW sharpening or noise reduction applied. (Click images below for original JPEGs.)
Sony A7S Boosted (RAW)
Canon 5D Mark III Boosted (RAW)
Panasonic GH4 Boosted (3:2 ratio crop) (RAW)
Sony RX1 Boosted (RAW)
Sony A7S 100% Crop
Canon 5D Mark III 100% Crop
Panasonic GH4 100% Crop
Sony RX1 100% Crop

Compared to each other, brightened shadow detail from the Sony A7S looks the most "natural" to my eyes, with relatively low noise and more natural colors. While all four images were shot at base ISO (100 for the A7S, RX1 and 5D3, ISO 200 for the GH4), the crops clearly display a fair amount of noise when boosting shadows to such an extreme degree. The noise from the A7S image appears much less severe than the other cameras, especially the 5D Mark III, which displays much more grain and stronger chroma noise. The GH4 image has a finer grain to its chroma noise, and still manages to hold its own quite well with a lot of detail, even compared to the much larger, full-frame sensors from the other cameras. The RX1, the oldest camera in the bunch, nevertheless has the best DxOMark rating for dynamic range which is about more than just shadow noise. However it comes in at a close second place in my little comparison test here. The luminance and chroma noise is a not nearly as strong as the 5D3 or GH4, but stronger than the A7S. Boosting up the exposure as much as I did in these shots also accentuated some lens flare in some of the shots, most notably in the GH4 shots (using the 12-35mm f/2.8 lens).

All in all, I was very impressed by the dynamic range capabilities of the Sony A7S. I was able to shoot photos in high contrast lighting with bright highlights that were easily toned back to reveal detail, as well as boost up shadow areas quite a bit without introducing a lot of noise that would impact fine detail.

Cologne, Germany. Photo has been adjusted in Adobe Lightroom. Click image to view original.
Sony A7S + FE 24-70mm F4 Zeiss: 70mm, f/4, 1/640s, ISO 100.

Silent Shooting

Now, despite me spouting the virtues of the Silent Shooting mode on the A7S, I have come across a couple of downsides, and one that has a noticeable impact on dynamic range and image quality, surprisingly enough. First of all, in the instruction manual, Sony indicates that by using the all electronic shutter -- Silent Shooting mode -- there is a strong risk of rolling shutter artifacts due to the fact that the camera reads the data off the sensor while being exposed (similar to movie mode). While I never experienced this in my time shooting with the camera, I also didn't shoot many subjects that would indicate this issue. However, if you plan to shoot fast-moving objects like motor sports, runners, or perhaps shooting out of a moving vehicle, it'd be best to switch back to mechanical shutter mode.

Now, the more interesting phenomenon (at least to me) with these two shutter modes is the effect of shutter type on dynamic range and noise. It turns out images shot in Silent Shooting mode display less dynamic range and more noise in the shadows. (Electronic shutter modes often capture at a lower bit-depth to facilitate faster readout which itself may contribute to higher noise, and EXIF metadata confirms the Sony produces 12-bit RAW files in Silent Shooting mode versus 14-bit files with the mechanical shutter.) Reading around the web about this issue from other photographers, I took a couple sample shots and ran them through Photoshop and Adobe Camera Raw, to see if I could find a discernible difference. While the in-camera JPEG shots of the scene below look practically identical, the RAW images on the other hand, are a different story. Processed the same as the dynamic range comparison shots above, the image captured with the mechanical shutter (aka Silent Shooting mode off) shows less noise in the strongly boosted shadows, though it does have a more purple-ish color cast. With the Silent Shooting mode shot, however, there's noticeably more chroma noise and a bit more contrast, though less, if any, of the purple coloring in the boosted shadow areas.

Click for JPEG or RAW file.
Click for JPEG or RAW file.
Both images were shot in RAW+JPEG and the RAW files were processed with Adobe Camera Raw with a +5EV boost without any noise reduction or sharpening applied.

HD and beyond: video recording with the A7S

4K Video Recording

While the Sony A7S may look like its higher-resolution A7/A7R siblings, the "S" version is purpose-built not only as a still cameras, but as a high-end 4K- and HD-shooting video camera. The special sensor is designed specifically with 4K and low-light video shooting in mind. 4K video is captured with almost the full width of the 12.2MP full-frame sensor without line skipping, pixel binning or resampling. Line skipping or pixel binning is typically utilized on other cameras since their higher resolution sensors have significantly more pixels, which would make reading the entire sensor and/or processing the full resolution too slow for video frame rates. The downsides to line skipping and pixel binning are a reduction in very fine detail and an increased risk of introducing moiré and aliasing artifacts into the video footage.

While the A7S is designed to shoot 4K video, one of the big frustrations to shooting in this format with the A7S is its inability to record 4K internally to the memory card. Instead, you must use an external 4K recorder, which not only increases the cost of ownership for a "Sony A7S 4K Shooting System," but also the simple logistics of going out to shoot. I can't just grab the camera and go shoot 4K video, like I did with the Panasonic GH4. Instead, I'd need to carry along an extra device, plus extra cables, batteries and ultimately a larger bag or backpack. It's clear, however, that the Sony A7S is geared very much more toward professional or serious filmmakers with planned productions rather than weekend warriors or casual photographers looking to grab a camera and go capture ultra-high resolution video.

Sony A7S 4K Daytime Sample Video
3840 x 2160, MOV, 30 fps, Apple ProRes 4:2:2
Download Original (4.77GB MOV)

Logistics aside, 4K footage from the Sony A7S looks absolutely stunning. If you're used to viewing a lot of 1080p footage, as I am, the jump to 4K resolution is amazing. I had the brief opportunity to borrow a Blackmagic Hyperdeck Studio Pro recorder to capture some 4K footage with the A7S, which allowed me to record video at a much higher bitrate than the typical internally-recorded videos from other cameras, such as the Panasonic GH4 and Canon 5D Mark III. While the 5D Mark III tops out around 91Mbps and the GH4 at a notably higher 200Mbps for 1080p (and 100Mbps for 4K), the A7S outputs an uncompressed 8-bit 4:2:2 4K-resolution (Ultra HD 3840 x 2160) signal. Capturing and recording this signal with the Blackmagic recorder in Apple ProRes format ended up with an approximate bitrate of a whopping 589Mbps. (Needless to say, you'll be burning through SSDs quickly. For instance, a 42 second outdoor clip shot at 4K on the A7S was 4.77GB!)

Full HD Video Recording

In addition to 4K video recording, the A7S also includes a newer, higher quality video format called XAVC S for recording 1080p and 720p high definition video. The "S" version of the Sony-developed XAVC recording format is technically designated as the "consumer" version, and can support only up to 3840 x 2160 resolution (though not on the A7S) and uses the more user-friendly MP4 file container as opposed to the pro-oriented MXF container. What you end up with on the Sony A7S is a higher-quality 50Mbps video format with up to a 60p frame rate in a much nicer, computer-friendly MP4 file than the AVCHD-format video with a maximum 28Mbps bitrate and MTS container files from older Sony cameras. (However, the AVCHD video format option still exists on the A7S.)

There's an interesting caveat, though, to recording in XAVC S on the A7S, and other newer Sony cameras: You'll need some of the latest and greatest memory cards, SDXC, in fact. Even using super-fast, 32GB SDHC cards in the A7S, which work fine for photographs or other video formats, the XAVC S option is off-limits (and displays a big warning telling you your memory card is not supported for XAVC S) unless you pop in a 64GB or bigger "eXtended Capacity" SD card. Using a non-supported card, if you ignore the warning, you won't be able to record any video at all until you change to one of the other two formats. This isn't such a big deal nowadays as memory cards become cheaper and cheaper, though. It is something to note, however, that you might need to upgrade your storage if you want to shoot XAVC S video.

Video Image Quality & Dynamic Range

Sony A7S XAVC S 1080p Sample Video
1920 x 1080, MP4, 60 fps, 50Mbps
Download Original (242MB MP4)

The image quality of Full HD video out of the Sony A7S is excellent. Using the Sony FE Zeiss 24-70mm f/4 lens, I was able to capture sharp videos with lots of fine detail and great colors. Without using the special Picture Profile feature, the dynamic range in the video above was very good with the ability to pull out a good amount of shadow detail, though I noticed a stronger propensity to clip highlights in strong sunlight, especially with the Multi Metering Mode (and not shooting on full Manual exposure mode in video). Dialing down the exposure compensation helped a bit, though, without underexposing too much, but I still had a few hot spots.

Of course, a compressed video file will not provide you the same amount of information and flexibility to adjust exposure and shadow/highlight details as RAW image files. And even the 4K uncompressed video output, while a much higher quality video, is not the same as RAW video.

However, while the A7S may not offer RAW video recording or output, there's another trick up its proverbial sleeve, one that professional and advanced videographers looking to retain the most processing latitude possible with their footage will appreciate: Sony's S-Log2 gamma curve. A feature typically found on high-end cinema cameras, the A7S includes Sony's Picture Profile feature, including gamma curve adjustments, with the S-Log2 gamma curve offering very low contrast and very high dynamic range similar to the gamma curve adjustments from Technicolor CineStyle on Canon DSLRs and CineLikeD profile on the Panasonic GH4. And while the footage straight out of the camera looks extremely flat with S-Log2, it's designed to be post-processed and color graded to give the final video the desired look. Similar to how a RAW file for a still photo allows the photographer to tweak and massage the image with a lot of flexibility, Sony's S-Log2 gamma curve gives cinematographers and video editors similar creative freedom.

I'll start right now and say that I am not a professional colorist, and while I understand the concept of color grading, I have very little experience professional or otherwise with extensive image editing of video footage. However, I was curious as to how much different the default "No Picture Profile" look of A7S video was to S-Log2 footage.

As you can see below in a sunny day shot, the standard image has high contrast with the carport roof looking extremely underexposed and bright highlights in the clouds clipped. However, with the S-Log2 profile enabled, the unedited video looks completely different with extremely low contrast, fewer clipped highlights and significantly more detail in the shadow areas!

Sony A7S XAVC S 1080p framegrab (resized) -- Picture Profile disabled
Sony A7S XAVC S 1080p framegrab (resized) -- Picture Profile enabled with SLog2 Gamma, ungraded
Sony A7S XAVC S 1080p framegrab (resized) -- Picture Profile enabled with S-Log2 Gamma, graded

It's interesting to note that when you enable the S-Log2 Picture Profile, the lowest ISO setting available is 3200. While this "native ISO" is said to offer optimal dynamic range when used with the S-Log2 gamma curve, such a high base ISO can make it a bit more difficult to get a proper exposure, especially in bright areas, without resorting to very small apertures or ND filters. Some dedicated video cameras, such as the Canon Cinema EOS C300, which itself has a relatively high ISO of 850 for its "Canon Log" gamma settings, have built-in ND filters to help balance the exposure (the Canon, for instance, has three).

Note: For details on how S-Log2 gamma works and how best to use it on the Sony A7S, see this excellent article by Alister Chapman, a professional videographer and a Sony Independent Certified Expert.

Also interesting is that Picture Profiles apply to still images as well, but only if you haven't enabled Silent Shooting mode (electronic shutter), which I found a bit odd since movies use electronic shutter. I discovered this while investigating the earlier mechanical vs. electronic shutter noise issue, when I noticed the live image preview in the LCD was much flatter with far less contrast when I turned off Silent Shooting mode. I soon realized that despite having the camera in one of the PASM shooting modes, the A7S was applying the Picture Profile setting that I had left enabled in Movie mode, in this case, S-Log2. In fact, I could access the Picture Profile sub-menu as if were in Movie mode, but ONLY if I turned off Silent Shooting mode -- otherwise, this sub-menu was grayed out. Checking the Sony A7S instruction manual, it makes no mention of Picture Profiles being applicable at all for still photography, only that you can "change settings such as color and tone when recording movies." However, the A7S Help guide does mention Picture Profiles can affect still images as well as limit what still image features are available.

Furthermore, taking some test shots around the office, I noticed this not only affects JPEGs, but RAW images as well. The "S-Log2" JPEG looked basically like a still frame of ungraded video footage. However, the RAW files were different. With the Picture Profile disabled, the RAW file looked, more or less, like its JPEG counterpart, whereas the RAW file from the S-Log2 profile was very underexposed, despite the JPEG image looking much brighter. So be aware that you may have to turn off Picture Profiles you intended only for movies when taking stills.

High ISO Video Quality

Sony A7S Still Life ISO Series Video
3840 x 2160, MOV, 30 fps, Apple ProRes 4:2:2 -- converted to 1080p due to bandwidth & file size.
Download Original (1080p) - (286.7MB MP4)

Overall, I was very impressed with the video quality from the A7S, externally captured or internally recorded, especially at base ISO. Of course, I'd expect video from a high-end camera like the A7S to look excellent when shooting at lower ISOs, but one of the primary features for the A7S is its super-high ISO capability. To see just how well the A7S can handle high ISOs in video, I hooked up the 4K recorder and shot an ISO series of our standard Still Life target, as well as giving it a little "torture test " to see how it does in an extremely low light environment.

With a maximum ISO sensitivity of 409,600, the Sony A7S is difficult to shoot in our lab because even if you crank the shutter speed up to 1/8000s and the aperture down to f/22, with our standard lighting setup, eventually the video becomes over exposed. At IS0 204,800, I had to start adding ND filters to keep the exposure in-check.

Sony A7S 4K Low Light "Candle" Video
3840 x 2160, MOV, 30 fps, Apple ProRes 4:2:2
Download Availble Soon

As I asked in Part I of the Field Test, while the A7S can climb the ISO scale like no other, are the shots, or video in this case, even usable? At maximum ISO levels, video from the camera is extremely noisy with both heavy luminance and chroma noise. The heavy noise is also constantly "shimmering" like the white noise pattern of analog TV. At ISOs 204,800 and 409,600, the visible noise pattern distorts the video quite a bit. However, shooting an exclusively candlelit scene at ISO 51,200 or even ISO 102,400, the noise is still very much under control and lots of fine details are still visible in the scene. Stunningly impressive!

Framegrab of Sony A7S 4K footage at ISO 409,600. The sole light source is this single candle.
100% crop at ISO 409,600.
Framegrab of Sony A7S 4K footage at ISO 51,200.
100% crop at ISO 51,200.

Other image quality characteristics for video recording, such as rolling shutter and moiré artifacts, were rather standard fare or to be expected, I found. At 60p, videos had very little rolling shutter distortion (and even less on the 720/120p option), while 30p and 24p video did display some noticeable Jell-O-like effects. Moiré was very well controlled, and in typical day-to-day videos, I didn't experience much moiré or other aliasing artifacts. When trying to purposefully induce moiré, I shot some video of a fine metal mesh table and saw faint moiré patterns interfere when shooting at just the right angle and distance. It was visible in both 1080p video and 720p (though it was more noticeable at 720p).

Framegrab of 1080p60 XAVC S showing moiré.
Framegrab of 720p120 XAVC S showing moiré.

As we only had access to a 4K recorder for a short time, I have not tried to see moiré or rolling shutter artifacts at 4K resolution. However, at least in terms of moiré, I expect the A7S to do very well at this resolution, considering the sensor is purposely designed not to line-skip, pixel bin or resample when capturing 4K video. [We have requested another 4K recorder and will update this review with our findings as soon as possible.]

Final Thoughts on Shooting with the A7S

The Sony A7S is one heck of a camera, and I had a blast shooting with it. It's solidly built, lightweight and relatively compact, though having a full-frame sensor necessitates larger lenses, thus negating some of the advantages of a mirrorless system. (See the Sony FE 70-200mm F4 lens below.) The design is a modern yet stylish nod to the older film cameras from years past -- just a cool-looking camera.

Sony A7S + FE 70-200mm F4 OSS lens.

Functionally, I had very few issues or frustrations while shooting. My one main complaint was the design of the front sub-command dial, but after shooting with it more and more, I've grown used to it, which isn't all that surprising. Otherwise, it's a very comfortable camera with a nice, contoured grip that helps balance the camera effectively when using longer, heavier lenses.

And in terms of image quality, the uniquely low-resolution full-frame sensor captures excellent quality photos with sharp enough detail (unless you want to print billboard-sized photos), excellent colors and impressive dynamic range. Video quality is equally impressive, especially 4K video. However, I have to say that needing an external recorder to capture Ultra HD footage is a definite bummer, as it makes capturing such footage much more of a "production." While the Panasonic GH4, for example, can capture 4K internally, the larger sensor of the A7S and the processing needed to record and encode such footage likely generates too much heat for the A7S. I'll keep my fingers crossed that the second generation A7S has internal 4K recording!

Would I purchase one of these cameras myself? It's hard to say. On one hand, it's a very expensive camera that rings up at around $2,500, without a lens. Also, thanks to the full-frame sensor, despite being a mirrorless camera, FE lenses are inherently much larger than comparable Micro Four Thirds glass, for example. Lastly, the AF performance, while decent, was exactly that. Decent. Nowadays, many other mirrorless cameras include on-chip phase detect pixels, which should provide for quicker focusing. I found the low-light AF on the A7S to be a little on the slow side. However, to its credit, the A7S can autofocus in extremely dim conditions -- below 1/16 foot-candles, which is beyond what many DSLRs can do! As someone who also enjoys photographing wildlife, birds and other fast-moving and often-small subjects, faster AF speed is a quality I look for in my personal cameras, but if you want one of the best low-light focusing cameras, the A7S is a prime choice.

The dynamic range, for photos and video, is excellent, though based on DxOMark test results, the A7S is "out-ranged" by a number of other cameras, including the less expensive and higher-resolution Sony A7. For a more well-rounded experience, for landscapes as well as for some faster-moving action shots, the A7 might be a better choice.

On the other hand, I love the shoot-anywhere-in-any-lighting capabilities with the A7S' insane high ISO sensitivity, and the smaller 12MP images are a breeze to work with on most any computer and save a ton of space on hard drives and memory cards. Plus, the solid build and cool design make it easy and comfortable to use, and the completely silent shooting mode makes the Sony A7S great for candid street, travel and event photography.


Editor's Picks