There’s a maxim in medicine: “If you hear hoofbeats, think horses, not zebras.” It means look for the most common maladies first. I’ve always liked that rephrasing of Occam’s Razor.
What’s that got to do with today’s topic, which is using the zebra viewfinder display option in the Sony alpha 7R Mark II (a7RII to its friends)? Nothing, really. I guess I was just free associating.
The a7RII zebra option shows areas in the live image in the finder that have greater than a certain exposure as black and white diagonally striped areas. The least sensitive is labeled “100+” in the menu (Gear->1->first entry).
I set the setting effect to “on”, the zebra sensitivity to “100+” and pointed the camera at a white target that was lit with a 5500 degree Kelvin light. I adjusted the exposure so that I could just see the zebras. I looked at the histogram. The top part of the populated area was just under 10% of the way from the left to the right side. That’s a bit under a stop.
Thus, setting the zebras to 100+ and adjusting the exposure so you can’t see them will give you an exposure that’s even less optimum (in ETTR terms) than looking at the histogram. It’s a conservative way to avoid clipping, though.
I don’t know for sure, but I assume that the zebras are derived from the preview image in much the same way that the live luminance histogram is, and that they may give you the wrong answer for highly chromatic subjects or infrared-modified cameras. If that’s true the zebras could be made less conservative (in the sense that they tend to err on the side of avoiding clipping) by the techniques used to make the live histogram more closely represent the real raw histogram, but they’ll always be more conservative than the live histogram.