Posted 30 October 2015 - 12:07 PM
Generally I've found that 70 IRE is the lowest level I could set a zebra, at least it was 10 years ago when I last used a broadcast video camera. 70 IRE in Rec.709 is a light grey similar to a pale Caucasian face in full exposure lighting like in a newsroom, in an interview. It's also about what a cement sidewalk reads when exposed normally. So I found it useful when setting exposures on the run, I'd stop down just so the 70 IRE zebras disappeared on a face or only appeared on the hotter spots of the face, or I'd use a sidewalk as a starting point in setting exposure. However many operators can't stand 70 IRE zebras since they appear over many highlights in the shot. As a DP/operator, I was happy for the exposure information.
If you set two zebras, one at 70 and the other at 100 IRE, then you'd find that when in Rec.709 and pointing the camera at an 11-step grey scale, you'd get zebras in the grey patch one-stop over 18% grey and another over the white patch, which can be useful info though I find two zebras too confusing to look at.
Trouble today is that if the camera is recording log where white itself is at 70 IRE or even 65 IRE, you can't set a zebra to appear low enough for skin tone exposure so it would only be useful for setting normal whites (5-stops over 18% grey), not super-whites. So I think zebras only work for run-and-gun exposure setting when viewing Rec.709.