Recently I was DP'ing a shoot for the return of a BBC primetime comedy show. As we were getting set up I had the usual pre-shoot conversation with the client... what's your workflow, what resolution would like, what gamma settings would you like, etc... and as often is the case, the client didn't know. I always like to establish if the client has a preferential setup before I make a suggestion, as they understand their workflow better than anyone.
In this instance, they wanted something that would be fairly straightforward to work with, they could get a clean key from and wouldn't require a great deal of extra post-production to finish, as it wasn't going to a grader. So I was asked - "What do you suggest?".
Now having shot quite a bit of green screen content in the past, and obviously having shot very similar content for the show last year, I know what I'd usually suggest but I thought the best thing to do would be to shoot a couple of quick screen tests with my AC - Marius Ulevicius before committing to anything. As we were shooting content with talent, we've only got the one opportunity to get it right so I wanted to ensure the client could see exactly what they were going to get beforehand. So these test below really were just for that.
Now I have to preface this whole article with the fact that I'm not an experienced camera engineer. I don't have the technical knowledge of someone like Alister Chapman. I certainly don't have the post-production skills of the studio the shoot was for. This was a very quick test to show the client, so pretty much the only variable to change between each scene test was the gamma setting so I know there are slight exposure issues and WB issues on some of the gamma settings. However, I've been shooting with the FS7 since it came out. I've shot numerous green screen shoots in that time. I've shot with just about every gamma setting that the camera has and I thoroughly believe that each setting has a time and a place. So...
What Gamma Setting Should you Use on the FS7 for Green Screen?
Now baseline settings for all these tests:
- 4K 4096 x 2160
- 180˚ shutter angle
- Native ISO (0db)
- White Balance set from a grey card (apart from the Log settings, where I think it was set to daylight - by accident)
- Lit by 2 x 5K tungstens through 2 x 12'x12' half silks with 6 x space lights above
- All the Hyper Gamma settings were tested with the F55 709 matrix
Straight out the camera, this is what I got:
So looking at these on a broadcast monitor with a waveform, it's quite clear, STD 5 Rec 709 is punchy and clear. Everything is within range. It looks saturated. HG 1 & 2 are less vibrant but slightly more natural. There's the obvious exposure adjustment I should have made between them. HG 3 has more vibrant mid-tones but I don't think the skintone looks too off. HG 4, although under exposed on my test, looks a bit flatter than HG 3. HG 7, although over exposed on my test, has good dynamic range. And HG 8 seems very similar to STD 5 Rec 709 but with slightly more range.
I also tested various Cine EI settings too:
Which by the looks of things, are quite yellowy, so I assume I had the WB selector on 5800K instead of the 3600K it should be. This impeded results slightly, but gave us an idea of how it would key. I've very quickly stuck a LUT (SLOG3.cine - the offical Sony LC-709A, SLOG3 - none and SLOG2 - the official Sony LC-709) on and done some very basic colour correction (I appreciate it's very rough...):
So none of these Log scenes look particularly easy to work with... even if they had the correct white balance, the tones are all very similar even though I have boosted contrast on them.
All of these can be compared in this video below:
How do they key?
Now for the key... the aim for this was to very quickly do this, with as little additional processing as possible. I'm sure a clean, natural key could be taken from each of these settings fairly quickly. But I wanted to test how each compare... So in After Effects I've used Keylight and eye-dropped a colour sample from close to the hair line. I've then slightly adjusted Screen Gain and Screen Balance to get the optimum key. And here are the results (I'll post a link at the bottom to the original high-res images):
So with STD5 Rec 709 I've got a clean key around Marius. The shadows need some tweaking to save them being lost in his jeans, around the edge of the frame and on the floor of the infinity curve.
My brief observations were: HG 1 is slightly under exposed so there's less transparency in his jeans but more shadows in the rest of the image, where it should be keyed. HG 2 has got better exposure and I think has keyed better than STD5, although quite dull skin tone. HG 3 has the best key so far and maintains a fairly pleasant skin tone. HG 4 is slightly under exposed and showing more shadows in the screens. HG 7 is over exposed but showing more of the floor. HG 8 has keyed nicely with reasonably nice skin tones.
And for the Log versions:
So you can see straight out of the camera, these are very tough to work with, with no initial processing...
Now with the LUTs and basic colour correction, SLog3 with the S-Gamut3.cine keys OK but the image doesn't look great and there's a lot of noise... SLog3 with S-Gamut3 doesn't key quite as well. SLog2 doesn't key well and there's tons of noise in there. I think with a lot of work, these are salvageable, but I don't think I have many client who'd put the time and patience into it.
I've pulled all of those into a short reel here with close ups at 400%:
So on our shoot, we quickly flicked through these and threw down a quick key (in a similar way). We opted for HG 3 for what we needed, as we felt we could get the cleanest key. I think STD 5 would have been fine for what we needed really. Alternatively HG 8 would have worked well.
So as for the theory behind this... STD 5 Rec 709 will replicate around 7 stops of light. As we're in a controlled environment, our brightest source on screen isn't much more than skin tone (well spectral reflections) and the darkest point being dark clothing. 7 stops should be enough to authentically replicate that. Hyper Gamma is around 10-stops with HG 3 accentuating the mid-tones. This means that there is more detail in skin tone and the green screen, which helps with the key. Whilst S-Log captures 14-stops, but other compromises are made to the image quality to do this. A lot more work is required to match the quality of STD 5 or HG 3, straight out of the box.
The full size JPGs can be viewed here.