By Macgregor and Enrique Pacheco - Originally published in spanish on dslrmagazine.com
Those with years of photographic experience, are aware that we a currently living in the crest of a technology wave. Never before has the latest news and advances in photography been so quick and continuous, affecting both the way we shoot or record our images. Even the transition from analog and digital seems slow and progressive when compared with the dramatic emergence of new sensors and software in recent years.
It seems that every 2 or 3 years, the Japanese manufacturers get on the market (sometimes with great effort) and others almost by pure luck, a product that create a vortex in the audiovisual sector.
It happened with the firsts DSLRs that were able to record video -almost five years ago. And it seems to be happening again with the new mirrorless UHD from Sony: the Sony A7s and its ability to record 4K, encode Slog2 and reach ISO levels never before seen.
Sony A7s: Dynamic Range
The reader seeking a clear, concise and clear answer regarding the number of “stops” dynamic range of Sony A7s, may be disappointed after reading this article. the dynamic range of a camera is sometimes a lot more subjective than it seems.
This value is strongly related with the noise, which is key to judging whether or not the usability of the last stops towards the shadows.
Sony A7s vs Panasonic GH4 in CINE4 mode pro ISO 200 the difference of detail in highlights in the sky speaks for itself.
What about the S-log? Certainly the ability to record in S-Log with the Sony A7s is one of the big reasons why the A7s is causing such a stir. Recording in log means having a more equal distribution of the information allocated to each stops, which in theory preserves the highlights better, as well as creates a more adequate image for grading.
But as you can see both in the video test in the chart, the S-log2 gamma curve only seems to move the dynamic range towards the highlights at the expense of significantly increasing the noise in the shadows. Therefore, although we believe will be very useful in situations where we need those extra steps in highlights its use should not be generalized to all work since, as in most situations, it will be more convenient to record at a lower ISO and achieve a cleaner image. Perhaps at this point we should remember that Slog only works with ISO settings higher than 3200.
As you can see in the table and the video above, CINE4 offers a a dynamic range as wide as the S-log profile but with much less noise. Our recommendation would therefore would be to use Slog only when you plan on color grading the footage in a professional environment; otherwise, you might be OK choosing the CINE4 profile (you may even consider underexposing one stop to better preserve the highlights).
Sony A7s: noise
A lot has been said about the low noise levels of Sony A7s. Again, we must emphasize that noise is rather subjective. That said, we can say that the noise levels of Sony A7s are certainly the lowest we’ve ever seen so far.
Of course, the larger full frame 12Mpx Sony sensor helps for this low level of noise.
We want to emphasize again that the A7s tends to create a fair amount of noise especially in the shadows in S-log2 mode. While this is greatly lessened once grading or applying the corresponding LUT, for users looking to achieve the cleanest image possible we recommend trying other profiles such as CINE4, which also allows us to work at lower ISO levels, down to ISO 100 (ISO 200 in the case of CINE4), Therefore we must choose between a more cinematic dynamic range with more noise, or a less rich range in highlights with a much lower level of noise.
Sony A7s: resolution
One of the great strengths of the Sony A7s is its ability to output UHD video. That is 3840 x 2160px, which is 4 times larger than FullHD, 1920×1080.
Unfortunately, the A7s has no internal 4K recording, needing an external recorder for this. The problem is that the Sony only has a 4K output through HDMI, and currently there are no HDMI 4K portable recorder’s on the market.
Soon several options will be available on the market, one of the most interesting prodcuts will be the Shogun from Atomos (shown above), which will also feature a touch screen and a variety of different recording formats.
The only option available to us at the moment to capture the native resolution of the Sony A7s, is the Blackmagic UltraStudio 4K capture card. This is a desktop rack (shown below) that connects to your Mac computer through a Thunderbolt port.
At 4K resolution, the Sony delivers a crisp, rich detail, with few digital artifacts such as aliasing or moiré.
Compared to the Lumix GH4 we must note that although the A7s images might appear sharper, actually both cameras seem to capture the same level of detail.
Sony A7s: codec XAVC-S
Here’s one of the greatest improvements regarding Sony A7s: the codec..
Previous cameras recorded internally on AVCHD codec, which had a low transfer rate of 25Mbps, which produced highly compressed images, with poor quality and color artifacts.
Finally Sony has listened to the requests of users and has implemented a much more slid codec in the A7s: XAVC-S. Now the A7s can internally record at a maximum resolution of 1920×1080 at 50Mbps with this codec, which offers higher quality than AVCHD although it still samples color at 4: 2: 0 and records at 8 bits of color depth.
XAVC-S (the amateur version of XAVC, which reaches up to 12 bits and 4:4:4), although not perfect, offers a very acceptable quality / size ratio. It seems to be solid enough to let us do color corrections without the fear of “breaking” the image. For these tests we have used Davinci Resolve, getting fantastic results with XAVC-S from the Sony A7s.
Regardless of the internal recording, the A7s has “clean” HDMI outputs, both 1080p HD and UHD 4K. However both options are limited to 8 bits although at a significantly improved 4: 2: 2 color sampling.
An interesting factor for those who only need a 1080p finish is being able to capture in 4K and then downsample to 1080p in post. Thus not only achieve an image of higerh quality with virtually no compression artifacts, but also oversampling from 4: 2: 2 to 4: 4: 4.
Sony A7s: moiré and aliasing
It seems that moiré and aliasing will no longer ruin our productions. Finally, manufacturers have listened to the demands of users and have fixed the annoying and persistent problems that almost all DSLR cameras suffer.
The A7s Sony is the first CSC camera that makes a complete reading of the sensor when recording video in both Full HD and UHD. This is due in large part to the fact that the 12 Megapixel sensor. Such low-density photodiode sensor lets the A7s use every pixel to form the final image, so there is no line skipping. Line skipping is what produces those unwanted digital artifacts, the most common are the moire and aliasing, which are almost always shown in areas of fine detail or closely spaced lines.
In our tests, both the Sony A7s as GH4 Lumix produce great results. However, the Lumix shows a tendency to produce color aliasing in some cases and we've seen moire artifcats on the A7s in occasions as well.
Sony A7s: Conclusions
This is a camera with unique characteristics in terms of video recording, a fantastic response at high ISO levels, and a huge dynamic range. If we want to be able to record in 4K, an external recorder is definitely necessary, making the whole kit more expensive than cameras like the Panasonic GH4 or even the 4K Blackmagic. It is always difficult to make a comparison, since both prices and features differ from model to model.
We want to thank Camaralia, Adrian Cabello and ePlató for their assistance to perform this review.