Knowing that the Arri Alexa has 15 or so stops of dynamic range is nice, but it's hardly useful information unless you know how those stops are arranged at different ISO settings. Fortunately I've done the hard work for you.
Or rather I should say that Adam Wilt and I did the hard work, as we both traveled to Chater Camera in Berkeley recently with a DSC Labs DX-1 17-stop dynamic range chart intent on discovering Alexa's hidden secrets. (The biggest secret was "Why is this Alexa just sitting here?" as we'd managed to luck upon the only day during a multiweek period when it was actually available for testing.)
Dynamic range charts are a lot of fun if you want to play games like "My camera's dynamic range is bigger than yours!" but they don't tell us how to exploit dynamic range in our work. My goal in these tests is to find how that dynamic range is distributed, which is much more useful to me as a cinematographer.
The only solid exposure reference that an HD camera offers is the point where the sensor clips, and the DSC 17-stop chart is designed with that in mind: by exposing the chart such that the first chip just hits white clip we can count how many chips are visible until they blend into the black background. As each chip is one stop, counting the visible chips to the right of white (don't count the first one for this test) gives us a number for the camera's full dynamic range in lens stops.
When using a camera "film style" we don't set the exposure based on clipping, but with an eye to exposing the image properly as a whole. My preference is to use the Zone System and expose certain objects in the scene so that they reproduce with a specific tonality. That can only be done by knowing how a camera or film stock responds to different reflective values of light and using that knowledge in concert with a spot meter. The Zone System hinges around 18% gray, and as there isn't a high dynamic range chart in existence that is set up to measure exposure in relationship to that middle tone, I set out to make one.
I set the Alexa to record ProRes 4444 to SxS cards in "legal" mode. I set the Alexa's ISO at Arri's recommended "sweet spot" of 800 and, while watching a waveform monitor, I set the exposure on the DSC Chart so that the first chip just flattened out into clipping. Then I looked at the waveform monitor and determined which step fell closest to 18% gray (generally 42% to 45% on a waveform), and marked that chip by putting a thin strip of black tape vertically through it. The tape creates a "notch" in the waveform trace and allows us to watch what happens to that middle gray reference as we change the camera's ISO settings.
Adam and I only tested four ISO settings-1600, 800, 400 and 200-because the chart is calibrated in one stop increments. (We tried a Stouffer wedge chart with 1/3 stop increments on a prior occasion and beyond a certain point the steps became too fine to count in the shadows.) We shot the chart at each ISO setting in both LogC and Rec 709 mode, and then did an exposure test simply to measure overall dynamic range. This resulted in two sets of results: one that showed how many total stops were visible below white clip on the chart, and another that showed how those stops were distributed above and below 18% gray on the waveform monitor. The first tells us how much range the camera has overall at a specific ISO, and the other tells us how much exposure latitude we have before we hit white clip or the noise floor (black).
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