filmic v4 on the way

[spid (Sami)]

@spid

(Also, I am not sure that I see why exposure is automatically applied with +1 EV specifically. That seems to assume that middle grey falls at a specific raw level below sensor saturation (3.5 stops, right?), but there is no reason for that to be the case, and using in-camera metering, that’s going to vary a lot from camera to camera. Apparently, some Panasonic cameras even have negative highlight headroom, meaning that if you expose for a 18 % reflectance card, then 100 % is already beyond sensor saturation.)

Since that message, I have come across some data that seems to support the +1 EV.

First, some background: ISO 12232:2019 defines various ways to compute an ISO value. Out of those, CIPA mandates the use of either SOS (Standard Output Sensitivity) or REI (Recommended Exposure Index), which are computed as \frac{10\,\text{lx}\cdot\text{s}}{H} where H, for an average scene, is a mid-tone exposure. More specifically, for SOS on a camera that produces 8-bit output, H is the exposure required to produce an image signal level of 118 (so middle gray in sRGB), and for REI, H is the arithmetic mean exposure recommended by the camera provider.

Now, there is another, saturation-based method, which DxOMark happily applies to RAW files (even though ISO values are not supposed to be reported for RAW files) and then calls “measured ISO”. While highly misleading and very frequently misinterpreted, it is still useful information. Saturation-based ISO speed is computed as \frac{78\,\text{lx}\cdot\text{s}}{H} where H is the exposure that saturates the output. (Processed output according to the standard, RAW output in the case of DxOMark.) That value of 78 was chosen so that, for a camera where saturation-based ISO speed coincides with an exposure index (SOS or REI), there is approximately 41% (½ EV) of headroom above 100% reflectance. (We can indeed verify that 1.41 \cdot \frac{100}{18} \cdot 10\,\text{lx}\cdot\text{s} \simeq 78\,\text{lx}\cdot\text{s}.)

Now, cameras tend to deviate from this coincidence, generally in the direction of having more highlight headroom (except for a few Panasonic cameras as I mentioned previously). An analysis of DxOMark’s data by Bill Claff suggests that at least as of 2014, the deviation was heavily centered around 0.33 EV of additional headroom relative to the ISO setting (on top of the ½ EV accounted for by ISO speed), for a total of 0.83 EV of headroom above 100 %.

That means that for a shot metered by an average camera (as of 2014), to get 100 % reflectance at 100 % in the RAW and 18 % reflectance at 18 %, we need to apply +0.83 EV to the RAW values.