My latest obsession is trying to match reflectance values with IRE values to create a "Zone System for video." This is both easy to do and impossible, so click through if you'd like to see how far I've gotten…
First, if you don't know what the Zone System is, take a few minutes and learn how it works.
I recently had a discussion with Charles Poynton, guru of all things HD, and he gave me a very helpful equation to convert reflectance values into IRE:
100 * ((normalized reflectance)^(gamma) * 1.099 - 0.099)
"Normalized" is a mathematical turn-of-phrase I learned from Adam Wilt, which means that you take whatever value you're dealing with and convert its range to 0..1, so that the new value falls in between. For example, 18% gray--the photographic "standard" (there is still some debate on this subject)--would become 0.18 when normalized, because its normal range is 0..100 and we've compressed that to 0..1.
If we plug that value into the above equation and use the standard Rec 709 value for gamma of .45, we get:
100 * ((.18)^(.45) * 1.099 - 0.099) = 40.9 IRE
I round it off to 40 IRE for my own exposures.
According to Mr. Poynton--if I properly understand his email and the related pages in his book "Digital Video and HD"--the simple equation that is traditionally used to map reflectance values:
100 * (normalized reflectance)^(gamma)
…doesn't work because the Rec 709 power function, which creates the gamma "curve", doesn't kick in until the signal hits around 10 IRE. Below 10 IRE the signal is linear, not a power function. This is, apparently, meant to reduce the amount of noise in the signal as the toe of a power function rolls off gently into noise, which increases visible noise near black. By making this part of the curve a straight line the signal plows into the noise floor at a much steeper angle, which costs a little bit of shadow detail near black but makes the noise floor much less obvious.
If you don't account for this linear portion of the curve and use the second, simpler equation then gray lands at 45 IRE. If you use the first equation, which incorporates * 1.099 - 0.099 to offset the linear portion of the curve, then 18% gray lands at ~40 IRE.
Mr. Poynton also says this value will not result in 18% gray when viewed on a studio monitor because there's been some tweaking in the monitor to accommodate viewing the image at low luminance. For me, though, it's close enough.
Here's the mathematically correct (as best I know, given my poor math skills) table of Zone System values for video according to the literal Rec 709 spec:
The shades of gray are rough estimates only.
The trick, of course, is that virtually no camera operates at Rec 709 spec anymore; they operate WITHIN it. That's an important distinction. The Rec 709 spec was only ever designed to contain about six stops of brightness, and modern cameras capture at least ten stops and often more. They do some interesting things to cram all that information in there, and we'll look at that a bit more in my next article.
The bottom line is that this chart shows how the worst camera in the world will reproduce brightness values, or how a good camera that's dumbed down by using the Rec 709 gamma option will reproduce brightness values. (My advice: never use the built-in Rec 709 gamma curve. Never!)
To answer my own question, yes: cinematographers should be able to light a set with a meter, before the camera arrives, and get fairly good results. Knowing this very basic Zone System is the first step. The next step, of knowing which values to focus on regardless of the camera used, is the subject of my next article.
The more I learn about cinematography and video the less I seem to know, as everything I've ever learned seems to be backed up by a "but…". Still, I love learning about the hidden side of things I take for granted, and there's a lot in video that I've taken for granted over the years. It's one thing to know that something works, but it's quite another to know why. Many people are happy with the former. The geeks among us ask the questions that take us to the latter.
Both groups can do great work, but the geeks tend to be handy because, when something goes wrong--and, in production, something always does--we can fix it.
Art Adams | Director of Photography | 12/20/2012 | www.artadamsdp.com