jcs

Hey Luke- where is this documented? I did a brief search and found no math for Slog2 and its S-gamut. If the A7S's Slog2/S-gamut is custom for the camera, has Sony created A7S LUTs/matrices for conversion to linear/709? sunyata- thanks, that's the gamma curve from the PDF; could create something close with curves by hand, however it appears the issue is with S-gamut (since Cinema Color Mode appears to do better than S-gamut). Ultimately, we'd like to understand how the transforms work and what we need to get the proper conversions for the A7S in Premiere Pro and FCPX etc. (for most folks; Resolve would also be good too if the Slog2/S-gamut is different for the A7S)).

I posted the 3D cubes to show what 3D LUTs look like when rendered. Understanding what happens to color spaces when the 3D RGB space is warped is helpful for understanding color transform issues. While the cubes are rendered with piece-wise linear line segments, the interpolation can be smooth/curved using tricubic interpolation (I wrote that software before general-purpose GPUs existed- tricubic interpolation is a bit slow in CPUs). The first image shows the 3D LUT / cube, and the second image shows color swatches which represent a distorted 3D LUT: the green areas on the edges show where clipping has occurred. I found it helpful to visualize the 3D LUT and color swatches that cut through the 3D cube to better understand what the transform was doing as well as prevent strange issues when generalizing a transform to work on all possible input images. JG- thanks for posting the LUTs. Terrence those look pretty good! sunyata- Samuel H. (creator of the flaat profiles etc.) mentioned that S-gamut was non-linear. I haven't researched it; the transformation you posted is affine linear- though if applied after a non-linear slog2 to linear 1-D LUT the final linear transform will have a global change to color (the same way adjusting WB in post would work). I'll give your and JG's 3D LUTs a try with Slog2+S-gamut which shows yellow splotches (before any transformations) and see if they go away. Thanks for posting the LUTs! [edit] Slog3 info: http://www.nzcine.com/wp-content/uploads/2014/07/TechnicalSummary_for_S-Gamut3Cine_S-Gamut3_S-Log3_V1_00.pdf (also shows full pseudocode for Slog3 to linear). Slog3+S-gamut3 is independent of WB transform while Slog2+S-gamut is not. Slog3 is more accurate. Why did Sony put Slog2 in the A7S instead of Slog3? ;)

It appears the issue with Slog2 and S-gamut with the A7S is related to the nonlinear S-gamut Color Mode. My current work-around is to use Cinema Color Mode. If there is a significant benefit to using S-gamut, it appears some kind of 3D LUT is needed to fix it. I few years ago I wrote custom software to do 3D color correction (for stereoscopic 3D images). The 3D color cube was directly displayed and could be edited, and it was also possible to render color swatch slices to check for issues with clipping (cube elements outside the unit cube space) or other mapping issues (such as color merging leading to banding/quantization). Color cube display (unedited / unit-normalized cube shown (actual distorted cube for current color correction not shown)): Color swatches (cube slices) showing range/clipping issues: It may be possible to fix the S-gamut color mode issue by creating a 3D LUT which fixes the green/yellow splotches in skintones and perhaps also the magenta splotches in shadows. Since green and magenta are color complements, it kind of has the feel from physics describing a form of instability. That is, the system is stable in a narrow range, and when outside that range it becomes unstable, going extremely green or magenta, depending on which side of the parabolic curve center the color(s) are on. [edit: hosted pics on my server]

We wanted to try compositing basic 3D VFX in the background with GH4 4K video (inspired by videogames I used to write). I used 3DS Max to create a simple scene with an alpha channel and tried to composite everything in Premiere Pro CC. Unfortunately, PPro (and AE) have issues with the 3DSMax alpha channel (not a premultiplied color issue) and the low levels of the alpha channel appear to get clipped, cutting off subtle glows, etc., making correct compositing impossible. So I moved the background plate into 3DS Max and rendered the 3D elements directly on top. I had originally planned for a much more complex 3D scene, including having objects come out of the background and go around her head, but the time spent trying to work around the compositing issue took too much time.

Definitely a 3D color problem with S-Gamut Color Mode (with Slog2). WB, including in post, can't fix it: most of the skin looks correct but there are patches of green/yellow that will need qualifier-level post work to fix. When the whole frame is off (pbloom's yellow cast), WB should be able to fix it. I found Cinema Color Mode with Slog2 to not have the yellow splotch issue as with S-Gamut (which is an approximation of Sony's Pro version- perhaps that's the issue). The FS700 did (does) better in daylight- it's continuous spectrum light. Try shooting in artificial light- that's a bigger challenge for Sony sensors/color-science (I haven't used the A7S in daylight yet- we've been using the GH4- all positive comments re:color and skintones). I will fix the green skintones in another cut- I think the A7S did pretty well for AWB given the tricky lighting conditions.

If lighting/noise is an issue, you can shoot at 1/FPS for the shutter: e.g. 240 fps with a 1/240th shutter (vs. 1/480).

Yes, 1080p60 APS-C. The lighting was very challenging: red and blue spotlights with a white spot on the models. Red+Blue = Magenta. I do see what appears to be excess green on some models (the color complement of magenta), however I didn't change the camera settings. It's correctable in post; I wanted to show how the camera performed with a wide variety of skin tones (and no setting changes). AWB was used, however with constantly changing lighting conditions, I did not have had time to make changes during the live event. All in all, I think the A7S did a fine job (for a Sony ;) ). The FS700 was (is) tricky with skin tones; the A7S is an improvement. Not up to Panasonic, Canon, or ARRI, but moving in the right direction. Even with the challenging color science, the talent really likes the A7S look. For low light, shallow DOF, the A7S is a nice complement to the GH4 for 'tinycam' high quality shooting. Once the community figures out how to deal with the color issues (ideally in-camera), it's usefulness will improve.

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Our first shoot with the Sony A7S. Shot 1080p60 1/60th shutter with Picture Profile 7 (Slog2), with Color Mode changed to Cinema. Graded in Premiere Pro CC using color curves and saturation, the same setting applied to all clips. The goal was to see how well the A7S handled skintones under very challenging, changing lighting conditions: low light, multicolor spots, etc. NR was only used on the first clip (very very low light). Shot handheld using the Sony SEL18-200mm APS-C lens (FS700 kit lens), AUTO ISO, AWB, AF (center spot focus area), IS, manual zoom, built in mics. With this lens and settings, the A7S makes an excellent low-light live/event camera.

We felt the best way to really learn how to use the GH4 was to shoot a commercial. All the GH4 shots were in 1080p VFR 60fps mode, Natural picture style, Panasonic 12-35 F2.8 lens, gen 1 Fader ND 77mm (+ adapter), and Flycam Nano DSLR steadicam. The pool shots were iPhone 5S 720p 120fps, and underwater shots with GoPro Hero 3 Black. Edited in Premiere Pro CC with heavy use of the new masking tools. Neat Video was used for denoising. Really pleased that WMG (Warner Music Group) and youtube worked out a deal to allow their music to be used (along with an ad). https://www.youtube.com/watch?v=Hc2LXH_MsQQ https://www.youtube.com/watch?v=Hc2LXH_MsQQ Editing a fashion show shot on the A7S next (will show a wide variety of skintones in low light- shot event style with 'auto everything'- worked pretty well).

Slog2 gamma with Cinema Color Mode worked well in low light for a recent fashion shoot. There were a wide variety of skintones and very challenging lighting- red and blue lights mixed with white, etc. I'll post some footage in the next few days.

With an IS lens handheld RS isn't a major issue, especially if using APS-C mode (less RS). I'll post example video soon.

Using picture profile 7 on the A7S uses Slog2 Gamma and S-Gamut Color Mode. This can result in green/yellow splotches on skintones after converting Slog2 into something that looks close to Rec709. WB settings in camera or after correction in post don't appear to matter: only certain shades of skintones have the color issues. While this can be fixed with qualifier color range correction tools, that's a lot of extra work. I found that changing the Color Mode to Cinema appears to fix the color issue (Pro Color Mode also looks pretty good). It's not clear how this might affect dynamic range (figure primarily related to gamma/luma), as that's a major point of using Slog2, however so far the results look pretty good.

http://www.dvxuser.com/V6/showthread.php?325824-Panasonic-discovered-a-fix-for-the-audio-buzz!

Yeah you'd think they'd prevent it, but they don't.

Right- you can go as low as 1/50th and still get smooth motion. Shutter >= fps for smooth motion: 50fps: >= 1/50 shutter, 60fps >= 1/60, 120fps >= 1/120, 240fps >= 1/240.

Yes, the A7S will shoot "4K" in S35 mode (cropped and scaled up to 4K; less detail than FF).

A reminder to check the shutter speed before shooting 60p to use a 24p slow motion. I had just shot 60fps VFR mode GH4 (sets the shutter automatically) and when shooting with the A7S in 60p I forgot to change the shutter to 1/60 (or faster) and left the shutter at 1/50th. The re-interpreted 24p was not smooth, and even pixel interpolation couldn't fix it. Remember to set the shutter to 1/60th or higher when shooting 60p for 24p slow motion.

I'm not seeing any aliasing or moire at all in 1080p FF mode (only minor aliasing in crop mode and a bit more in 60p). The internal scaling from 4K FF (24fps) to 1080p is pretty good: looks like a decent scaler (at least bilinear, perhaps better (cubic, Lanczos etc.)). An interesting note regarding noise on the A7S: when looking at the display or viewfinder in low light, much noise is visible. As soon as video recording starts, the noise level drops significantly (appears to be temporal NR).

Color science (no greenish-yellow tint and strange color response in different lighting conditions (looks like a 3D color map kind of flaw)). Rolling shutter. Still shots with autofocus and Canon lenses. We have 6 cameras for 6 different uses: 1. FS700 for slow motion or when pro audio is needed in-camera. 2. 5D3 for stills (and/or RAW video) 3. GH4 for highest quality video (4K and 1080p) in good lighting conditions 4. A7S for low light and/or portable shallow DOF with compressed video (otherwise 5D3 RAW is far better) 5. GoPro for POV and underwater 6. iPhone 5S for pickup shots and getting a shot 'cause that's the camera we have on hand at the time Some day all these features will be in a cell phone, watch, or glasses.

The gen 1 SpeedBooster works on the A7S! (lol). It goes into APS-C mode (when in AUTO APS-C). The quality is pretty good in crop mode- perhaps a tiny bit of aliasing (and apparent increase in sharpness from said aliasing). Tested with the 24-104 F4L at 'F2.8'- worked pretty well. After shooting everything so far with the 50mm F1.4, the IS works great handheld. With APS-C mode turned OFF, one can use the 'Crazy Wide Super Vignette' mode ;). Gotta try this with the 16-35 F2.8. With A7S+SB that extra stop of light is like double the ISO, right, so ISO 410K is now 820K? ;)

That was shot in a very dark scene, just a few low-output incandescent lights above- way darker in real life. I let Photoshop try to auto-white balance (camera was set to AWB). Looking at the vectorscope in PPro, the skintones are pretty close to the skin tone line (but rotated slightly- not directly tracking the skintone line)... Something about the Sony sensor/color science is biased towards green (and when mixed with perhaps another red-bias we get yellow). I didn't spend much time trying to fix it to make a point: Sony color science is a challenge (have the same issue with the FS700). The A7S is a useful tool despite the issues- I'll spend time figuring out how to work around them (something I wouldn't need to do with Canon). Once I found that the Natural profile worked well with the GH4, and to not use the curve controls, I've gotten generally good results without much effort with the GH4. Hopefully we'll find similar settings with the A7S. Here's straight from the camera with just saturation reduced slightly: Always looking to learn new things- if anyone has a process to make this look better, would like to hear the steps (will definitely experiment with camera settings more- prefer in-camera accurate color vs. trying to fix in post (another reason why I prefer not to use anything like Slog unless no other choice due to conditions). Note also the Philip Bloom video above: it has a greenish-yellow tint.

GH4 with Voigtlander 25mm @ F.95, ISO 1600, Natural picture profile (+ post CC, Neat Video noise reduction) A7S with Metabones v4 adapter and Canon 50mm F1.4 @ F1.4, ISO 6400, picture profile 1 (+ post CC) The A7S really shines in low light (kind of like having Neat Video built into the camera). The GH4 with a very fast lens isn't too shabby, and cleans up well with Neat Video. I still prefer the GH4 color science, though I am still experimenting with the A7S settings (seeing too much green/yellow in skin tones). It's possible to get 20% slow motion with the GH4 4K: shoot 4K @ 30p and reinterpret as 24p in post (a subtle but useful effect).

Doing lower light tests with the GH4+Voigtlander 25mm @F.95 and the A7S with Canon 50mm F1.4 @ F2 (fairly equivalent in terms of FOV and DOF), the A7S does much better with noise control vs. the GH4 (as expected), however the GH4 does surprisingly well. In lower light shooting, the GH4 noise does go from nice monochromatic to colored, similar to the A7S (which is much less and better controlled). The GH4 is looking more detailed (4K => 1080p) and clear in bright light. The A7S is clearly better in low light; shooting handheld is tricky as rolling shutter is 30.5ms (vs 20.5ms for GH4 in 4K (13.7ms in 1080p)): http://www.dvxuser.com/V6/showthread.php?303559-Measuring-rolling-shutter-put-a-number-on-this-issue!