Video comparison: Internal
To give some idea of how the a7S performs, we've shot a series of clips of it compared with the Canon EOS 5D Mark III. Because they're shots of a static scene, they can't tell you anything about how the two cameras capture and compress motion, but they do highlight their abilities when it comes to detail capture and noise performance. The video below shows the cameras at increasing ISO settings, first in daylight-balanced lighting, then in low, tungsten light (around 3EV). This second run of ISO settings starts at ISO 800, since that is the lowest sensitivity setting at which the 5D III can achieve the correct brightness (even with its F1.4 wide-open).
The a7S performs well compared to the Canon 5D Mark III, another full frame camera renowned for its low light capabilities. This video compares the two cameras' ISO performance using internally recorded 1080p video. Click here to download original file. |
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As you can see from the low ISO sections, the a7S is capturing much finer detail than the 5D Mark III can. This difference is most visible if you closely compare the etching on the left-hand-side of the scene or the blue and white pattern on the back of the playing cards on the right: the a7S gives a crisp result in areas of the very finest detail.
A key thing to spot is the effect on the Siemens stars, near the center of the scene: the 5D Mark III produces a complex, symmetrical snowflake-like pattern from the finest detail, whereas the Sony just produces a smooth, featureless circle. This tells us a lot about how the cameras are operating.
The symmetrical patterns seen from the Canon show that it's sampling the scene with the same frequency in both axes, either by using all its pixels and downscaling or by 'binning' them together at an early stage. The same is true of the a7S, but it's then doing something rather clever: before downscaling to 1080 resolution, it's filtering out any detail that can't be correctly represented at the smaller output size.
This is the first time we've seen a camera do this - take advantage of the fact that it's over-sampling the scene by a factor of 2 (the minimum necessary to perfectly re-create the original image) to then filter-out the errors that come from trying to represent higher frequencies than the output resolution can correctly include. The result is the cleanest 1080 footage we've ever seen on a camera of this type, in terms of moiré avoidance.
As the sensitivity increases we begin to see the benefits of how good the Sony's sensor is. Even when compared to another full-frame camera, the a7S produces great results. The Canon's results become noisier as the ISO increases and the exposure levels drop, with them becoming extremely noisy by its maximum setting of ISO 25,600. However, the Sony doesn't produce the same amount of noise until at least ISO 102,400 - meaning it offers at least a 2 stop noise advantage over the EOS 5D Mark III in practical terms.
Video Comparison: External
With the use of an external recorder, the a7S can shoot 4K (UHD) footage. The footage is certainly detailed (though you lose the moiré-reducing benefits of oversampling), even compared to the internal 4K capture of rivals such as the Panasonic GH4 and NX1. In the video below we recorded our studio scene across the entire ISO range in 4K using the Atomos Shogun external recorder. As with the 1080p examples above, this video won't tell you anything about how the camera handles motion, but it can be informative when evaluating noise at different ISO sensitivities.
The a7S's 4k output can be captured if you use an external recorder, such as the Atomos Shogun. Click here to download original file. |
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The benefits of that big, well-designed sensor provide clean footage even at exceptionally high ISO.
We only recently started to test cameras with the Shogun, so we don't yet have a full 4K comparison between the a7S and other cameras when recording to this device. We plan to to a more comprehensive comparison between 4K cameras using the Shogun in the near future.
Super 35 / APS-C mode
Just as in stills mode, the a7S can shoot video in a cropped APS-C mode, using a central chunk of the sensor equivalent to the 'Super 35' format commonly used in videography. However, there are costs to using these modes: something worth bearing in mind before concluding that the a7S will work perfectly with cine lenses designed for the Super 35 format.
If you glance back to the section on why its full-sensor 1080 footage is so good (its ability to 2x oversample and intelligently downscale), you may spot a problem trying to offer 1080 and 4k output from a smaller crop of the sensor. There are two reasons why these crops can't match the quality of the full sensor footage: firstly using a smaller region of the sensor means you'll capture less light with the same exposure settings, and so end up with noisier results. This needn't be a problem, of course, you can just limit yourself to lower ISO settings.
More significantly, though, the way the video is made up will differ considerably: changing the way it looks. The APS-C area of the a7S's sensor is around 2560 x 1440 pixels. This means that the 1080 footage won't be oversampled to the same degree - so can't produce the same near-moiré-free outcome and, more troublingly, it means the camera is having to upsample to create 4k output.
The difference is probably tolerable when working at 1080 - the increase in grain and moiré is unlikely to be a problem for anyone but the most demanding videographers. However, switching back and forth from native 4k and awkwardly up-sized 2.5k isn't going to be terribly pretty, We'd suggest avoiding the APS-C/Super 35 mode if you're shooting 4k.
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