FS7: Headroom

egproductions

egproductions

Veteran
When shooting with a 709 preview LUT and ISO is set to 2000ISO you are left with a lot of headroom and also a fair amount of noise (at least when exposing by eye..you can of course overexpose). My question is what ISO preview should you be shooting at in order to emulate no headroom without clipping? I would guess that it's 500 ISO since the camera can't go below that but I just want to confirm this because the last thing I want is to be shooting with a preview LUT at 500 ISO and then realize later that my highlights are clipped that I thought were safe in camera.

If 500 is the magic number does that mean that 2000 ISO give you 2 stops of headroom?

Thanks!
 
It's EI, not ISO

EI 2000 gives you about 6.2 stops of highlight latitude. Generally overexpose 1-1.5 stop to get cleaner shadow (meaning shooting around EI 800)
 
white, with 90% reflectance, in a REC709 color space, is about 2.4 stops over 18% middle gray. For something like Slog3, middle gray is 41 IRE and white is 61 IRE at EI2000. 2.4 stops over is 62 and 83 IRE. The remaining headroom is no longer falling on the linear part of the gamma curve, so "headroom" is really a very small and logarithmically diminishing space. As you drop the EI further than 2.4 stops, you are fundamentally crushing the hilite detail. I'm not so sure you're really gonna get a very pleasing image above 2.5 stops overexposed. Headroom becomes a rather ambiguous term.
 
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If I understand correctly, you are asking at which point the clipping point of REC 709 LUT (used for exposure monitoring, likely in conjunction with zebras) and clipping point of underlaying S-Log 3 signal are the same (and your fear is that at certain EI the underlaying S-Log 3 may actually clip before the REC 709 LUT shows clipping). This article by Alister Chapman talks about it specifically and how REC 709 is better as an exposure guide than LOOKS (in example LC709A). http://www.xdcam-user.com/2014/12/ultimate-guide-for-cine-ei-on-the-sony-pxw-fs7/ In either case, of course, the higher the EI rating the greater the highlights headroom .
 
broughtonfilm said:
If I understand correctly, you are asking at which point the clipping point of REC 709 LUT (used for exposure monitoring, likely in conjunction with zebras) and clipping point of underlaying S-Log 3 signal are the same (and your fear is that at certain EI the underlaying S-Log 3 may actually clip before the REC 709 LUT shows clipping). This article by Alister Chapman talks about it specifically and how REC 709 is better as an exposure guide than LOOKS (in example LC709A). http://www.xdcam-user.com/2014/12/ultimate-guide-for-cine-ei-on-the-sony-pxw-fs7/ In either case, of course, the higher the EI rating the greater the highlights headroom .
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Correct, that's a good way of phrasing my questions. It seem like the mr. chapman had the same idea to set the EI presets to 500, 1000 and 2000. A lot of what he wrote is over my head at this point, however, it seem that if I use the 709 LUT and shoot at 500 EI I should be ok and don't really have to worry about cliping highlights if they don't appear to be visually clipping in camera (not by using zebras).
 
egproductions said:
Sounds like I'm not going anything crazy to my highlights by exposing for 500 EI since that's only 2 stops overexposed.
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Unfortunately, things aren't really so cut and dried when using slog. A lot of the choices to be made really depend on the dynamic range of the scene and where you want the details to fall. Raising the EI pushes middle gray down, crushing shadows and opening the hilite detail. Lowering the EI pushes middle gray up, crushing the hilites and opening the shadows. Keep a wary eye on your WFM to see if you're close to clipping. Be aware that you can clip a single color channel, RGB, even if your luma WFM looks OK. Try it out on scenes of varying dynamic range, bring it into your post workflow, grade it and judge for yourself. If it works, and you like it, do it.
 
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