The Sony A7s outputs ‘clean’ 4K via HDMI (Micro Type-D connector). Here are the specifications:
|PAL||25p||25p, 50i, 50p|
|NTSC||24p, 30p||24p, 60i, 60p|
|Data Rate MB/s||500||250|
|Wrapper||HDMI 2.0 Baseband Protocol|
|Color Space||x.v.Color (xvYCC)|
|Gamma||Rec. 709, S-Log2, others|
What in the world is x.v.Color or xvYCC?
xvYCC or x.v.Color (same thing) is a proprietary color space developed by Sony that has the following characteristics:
- Similar to Rec. 709 so that it can be displayed on current broadcast systems, however –
- It has an extended range that goes beyond studio swing. Rec.709 displays 55% of the Munsell colors whereas xvYCC can display 100% of the colors.
- Sony 4K Blu-ray disks will support xvYCC.
- Most modern GPUs already support xvYCC.
- Already an official part of the HDMI protocol.
Just to be clear:
- In stills mode, the Sony A7s shoots in sRGB mode (with sYCC gamut a.k.a the sensor gamut) or AdobeRGB mode.
- In video mode, it shoots HD as well as UHD in xvYCC mode. Both are full swing.
Whether you’re working with 1080p or 4K, you must take care in your color transformations while working in xvYCC mode. If you work under a studio swing environment, then you must take this into consideration or risk clipping.
What is S-Log2?
S-Log2 is a gamma curve that keeps the image flat for grading. Sony claims it has 1300% more dynamic range (15.3 stops of DR). What the heck does that mean?
You’ve probably heard of people using 18% middle grey to expose cameras. In S-Log2, the middle grey is at 32%. The ‘usual’ 90% white level is at 59%. So, guess where 100% white should fall? 1300%. In other words, Sony claims that the A7s can record about 13 times more data than Rec. 709. The key word is ‘can’. Theoretically. However, practically, the dynamic range is limited by the gamut and noise characteristics of the sensor.
Here’s a graph showing S-Log2 compared to S-Log3 (newer gamma found in the Sony F5/F55/F65):
If you look at where Sony claims the white and black points clip, you’ll see the curve lies in the region from -10 to +6, or 16-stops of DR. They claim S-Log3 has 1.5 more stops of DR than S-Log2. Let’s not fail to see that the above graph is plotted on a 10-bit signal in probably RGB space, without chroma subsampling or compression. An 8-bit signal subsampled at 4:2:2 will probably reduce the dynamic range to about 11 stops.
The Sony F55, shooting RAW, has about 14 stops of real latitude. So does the Arri Alexa. I would place the Sony A7s slightly better than the Canon 5D Mark III (10.5 stops), but not as good as the Blackmagic Cinema camera (13 stops). This is borne out in the footage I’ve seen so far. This could be why Sony has decided S-Log2 is good enough for the A7s. S-Log3 would have negligible benefits, thought it still might come as a firmware update.
What is interesting to see is that S-Log2 doesn’t have a smooth shoulder (highlight roll-off). This should tell you that you must try to avoid clipping highlights under all circumstances. In the shadows the Sony A7s will do great, judging by the performance of similar Sony sensors in the D800, A7r, etc.
S-Log2 is not the only gamma available. Here are the others:
- Cine1-4 – these are possibly Cinegammas, from 1 to 4. This is roughly 460%.
- ITU709 – this is Rec. 709
- ITU709 [800%] – this is probably Hypergamma. It offers more range than Rec. 709 and Cinegamma but less than S-Log2
I will have more information once I test the camera. Just because one gamma has a higher percentage value doesn’t mean it’s better. Various gammas respond differently to grading, which you MUST do if shooting in any gamma other than Rec. 709. E.g., I prefer the way S-Log2 grades over S-Log3. Others might prefer a different gamma. That’s why it’s all there, and you get to test and choose the one you like the most.
Other settings in the Sony A7s
Gamma is an important way to manipulate the image. Here are other ways:
|Picture Profiles (7 options)||Black level, Gamma, Black Gamma, Knee, Color Mode, Color Level, Color Phase, Color Depth, Detail|
|Creative Styles||Standard, Vivid, Neutral, Clear, Deep, Light, Portrait, Landscape, Sunset, Night Scene, Autumn Leaves, Black & White, Sepia (Contrast -3 to +3 steps, Saturation -3 to +3 steps, Sharpness -3 to +3 steps)|
|Picture Effects||Posterization (Color, B/W), Pop Color, Retro Photo, Partial Color (R, G, B, Y), High Contrast Monochrome, Toy Camera, Soft Focus, Soft High-key, HDR Painting, Rich-tone Monochrome, Miniature, Watercolor, Illustration|
|White Balance||Auto WB / Daylight / Shade / Cloudy / Incandescent / Fluorescent (Warm White / Cool White / Day White / Daylight) / Flash / Color Temperature (2500 to 9900K) & Color Filter* (G7 to M7: 15 steps, A7 to B7: 15 steps) / Custom / Underwater|
|Noise Reduction||High ISO NR: Normal/Low/Off selectable|
|Lens Compensation**||Peripheral shading, chromatic aberration, distortion|
- *This allows you to add ‘tints’ by changing the colors in minute increments
- **Only for Sony-approved lenses.
Mere mortals have spent years with their Sony cameras without knowing about all the styles provided. However, it would be wise to test all of them thoroughly. The combination of the above is what makes a ‘secret sauce’ for each cameraman.
How to get maximum video quality from the Sony A7s
It’s simple really. Based on what I know so far (which could change!), here’s where I would start for maximum image quality for the Sony A7s:
- Shoot at ISO 3200
- If you go beyond f/11 diffraction kicks in, reducing resolution
- Gamma – S-Log2, and don’t touch any other profile setting
- Neutral style, though I would also test Contrast -3, Saturation 0 and Sharpness -3 to 0.
- Picture effect – Strictly off!!
- Expose skin at 40-50% IRE (advice from Alister Chapman), and be extra careful of clipping highlights (the internal Zebra function will assist greatly here).
- High ISO Noise Reduction: Off
- White Balance: Custom (2500 to 9900K)
- Manual mode and manual exposure
- Lens compensation on for Sony lenses; off for everything else.
External Recorders and Monitors for the Sony A7s
At the time of this writing, there’s only one officially confirmed recorder that accepts 4K from the Sony A7s, the Atomos Shogun ($1,995, 2.5″ SSD media, 7″ 1920×1200). However, the following recorders should have the same ability soon:
- Convergent Design Odyssey 7Q ($2,295, 2.5″ SSD media, 7.7″ 1280 x 800 monitor)
- Aja Ki Pro Quad ($3,995, 2.5″ SSD media, really small monitor)
The Shogun is really the go-to recorder not only because it is the smallest but it has a really good high-resolution screen that’s great for pulling focus. One ‘scary’ prospect about HDMI is that the cable connector really does not lock in. Sony provides a cable lock with the A7s, and William Thomson from Atomos told me Atomos will come out with their own cable lock (and a single-source power supply that will power the recorder as well as the camera and microphones). I would not use the recorder without a cable lock. It is way too easy for the HDMI cable to slip out.
The go-to codec to record the uncompressed signal is Prores HQ (4:2:2). It will have a maximum data rate of 880 Mbps (110 MB/s), though real-world data rates will be lower based on the frame rate and content. This is how it translates to drive space:
- 6.6 GB/min
- 396 GB/hour
- A 120 GB SSD drive will hold about 18 minutes of 4K footage
- A 1TB SSD drive will hold about 3 hours
This data rate will be hard on your computers, so you’ll need fast drives for on-set copying and duplication. A 7,200 rpm drive should be able to sustain about 110 MB/s so you should get real-time copying. Playback would definitely demand a slightly better drive or SSD or RAID solution. I would only grade in a 32-bit environment, to avoid problems with banding and posterization due to the 8-bit codec.
The Shogun provides a LUT but I was not able to test it on the demo. We’ll have to wait till IBC 2014 for the official launch for a detailed review of the color options available. For now, I suggest you switch to Rec. 709 for a quick overview, and then go back to shooing S-Log2. Be careful of the zebras, they will be different for both.
As far as plain-old monitoring is concerned, we’ll have to wait for official word on which monitors will accept the HDMI 2.0 signal. If 4K recording is your thing, it makes sense to opt for the Atomos Shogun, and you get the best of all worlds. You also get focus peaking, zebras, levels and everything else (not passed on, the HDMI is clean signal only) from within the recorder.
To connect the recorder or monitor, the weapon of choice is the Hot-shoe Monitor Mount (from $5 to way more):
In Part Six, we’ll look at batteries, power and audio.