Log Gamma, Sony FS7 Workflow and Fire Dancers

A while back we met up with the dancers of the Aries Fire Arts collective to record a performance. This will be married to interview footage for our ongoing portrait series. This was shot at the Isis Oasis Retreat in Geyserville, California. The camera of choice was the Sony PMW-FS7. It’s great low light performance, coupled with variable frame rates and amazing dynamic range, made it an obvious choice. First I’m going to talk about some background, which might get technical. I like understanding what’s going on in detail, and as a physicist I’d rather see more math and graphs than most people. But I’ll try to keep those short. After the technical discussion, I’ll do a section on how to use this practically with the FS7, followed by a write up on the event. So skip by headers if you’re so inclined.

Background: How Does S-Log Work?

Imagine you’re in a dark room, and you light a candle. The room is now 1 candle bright. It’s not good enough to read your manuscripts, so you light a second candle. The room is now 2 candles bright, and it might be enough. You notice that the light doubles immediately.

Now imagine you’re in a room with 100 candles, and you light one more. Did you notice the change to 101? What about 102? At want point does your mind recognize the brightness of the room changed? It takes a while. On top of that, if you lit another 100 candles the room wouldn’t look twice as bright to you, even though the actual light energy doubled. And this is the basic concept of logarithmic scales.

loglinearIf everything were linear, we would expect to see a steady increase. But since it’s not, we see that, as the value increases, each additional change becomes less and less important. This is something used a lot in physical sciences, especially optics and electronics, as a way of preventing exponential functions from getting unwieldy too quickly. It dampens responses, something your brain is constantly doing with input. It might not be strictly logarithmic, but it’s a reasonably close model.

So what about cameras? By default, cameras just use a linear model. Sensors, after all, only know how many photons hit a given spot. The problem is that the lower end of this curve (called a ‘gamma curve’ in camera parlance) doesn’t get very much data, certainly not compared to the higher end. Especially true when you consider that most of your data is going to live on the left half of the curve anyway. This is not at all how the human eye perceives things, meaning a lot of that data on the highlights is wasted.

And that’s where logarithmic gamma curves come in. In S-Log, Cineon and other colorspaces, the video is encoded logarithmically instead of linearly. This means you get a lot more data in the shadows than you otherwise would. The highlights have slightly reduced data, but it’s not all that noticeable. The result is that your camera captures the information in a huge dynamic range, getting detail in shadows that would have been lost on other, lesser cameras. The sensor has to be up to this task, of course, and the FS7 is.

When you get video out of this, it looks pretty bad. Grainy, gray, washed out, etc. But that’s because your display is playing back as though the encoding were linear, since that’s how videos work. If you apply an opposing curve, you can recover all the detail you want and make a pretty picture. This can be done with predetermined Look Up Tables, or LUTs, or put in manually. The data is still there either way, and can be recovered either way.


FS7 and S-Log Workflowdemo-reel_4

Theory’s done. Here’s the easy part. Do you have an FS7 or similar, and want to know what to do with it? S-Log: Use it. If you have it, there’s honestly no reason not to. Worst case scenario is that you end up with the same image you had to begin with, but now you have options. Once you have the footage, you can either grade as normal or, to make it really fast, drop a LUT on it. The best ones I’ve found are from Sound Dogs at http://framedogs.com/sony-fs7-luts-and-you-nle/ . They also have a good write up on use. Lumetri Color in Premiere has the ability to use a custom LUT directly. You can also manually set your curves until you get the look you want, of course. See the image for an example of the raw file (top) and the version with the LUT applied.

The LCD on the FS7, and I’m sure a number of other cameras, includes the ability to apply color correction via LUTs directly to the previewed image. Some external monitors even allow you to cycle different looks. This can be very useful for making sure you’re getting the look you want.

The only real difference between shooting with a log space and a linear space is that, since you have all that lovely shadow and midtone information, you can safely expose a little lower on the highlights to keep the details you need, confident you can bring up the shadows as needed. I should note that exposure on these cameras, especially in Cine-EI mode is an entirely different subject. So I’m not going to include details on that here, or how it affects dynamic range and noise. Just be satisfied that S-Log3 will improve the versatility of your footage, even if you don’t change anything else.


Our Firedancer Shoot

fireIf you just wanted some S-Log and Sony FS7 how to info, thanks for reading. But if you’re interested in how we used it, look ahead.

I’ve wanted to shoot this particular portrait for a long time. Fire dancing is cool. My favorite photo that I’ve ever taken was of a performer doing a fire breathing act. Once we had access to this camera, I knew we needed to move forward. That was about six months ago. Things got busy, and more importantly it was extremely difficult to find a good site. Liability issues, interest and cost all factored in. Eventually the people at Isis Oasis in Geyserville contacted me. They were very nice, and seemed excited for the show. Most importantly, they had an Egyptian temple on site that would serve as the perfect backdrop (pictured).

The fire provided a lot of light, but my goal with this was to make sure that we didn’t get the usual amateur video, in which the fire is all you can see. That meant bringing all the lights we can. We were lucky enough to setup near a work shed, which had industrial circuits to handle all of our lights.

Four lights were placed front right and left, two on each side. Each held five fluorescent lamps, with and without softboxes. The largest, camera left, was raised high to provide ambient lighting. That was the key side. In addition to this we turned on all the practicals on the temple, and placed one fixture (seen in the picture) to backlight the dancers. In the back on camera left were a mix of fluorescent and LED lights. One LED panel was placed on the ground, tilted down to provide dynamic lighting streaks on the ground. This was used to add a bit of depth. All of the back lights had some kind of interference, such as the branches on camera right, for that same purpose. We ran the performance three times, then broke into other routines. The last run involved using the FS7 with the shoulder mount, which was actually pretty comfortable… though the fire got a little hot. The GH4 served as backup, and got reasonable footage. It could have done the job with more even lighting, but the FS7 still wins. We went back and fourth between 4K and high frame rate, depending on the shots needed.

Here’s the result:


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