How to expose log video: the 18% gray target changes with the profile

18% gray targets vary by log profile

Manufacturers put 18% gray at different values in the encoded log signal, so start from the published value for the curve you chose.^1,2,3,4,5

The app's exposure tools measure two different things. The histogram counts RAW sensor values for each channel, before white balance and the log curve are applied. Zebras and false color show Rec.709 luma in IRE. A published value like the ones in the table describes the recorded log signal, so to check it, record a short test clip and measure it on the waveform or code-value display in your editing app.

The useful comparison is F-Log against F-Log2. Fujifilm puts the same 18% card at code 470 in F-Log and 400 in F-Log2. The card is the same in both cases, so the difference comes from the curve. Sony makes the same point another way: S-Log3 uses 41% output while S-Log2 uses 32%.^1,3,4

Stops are still useful after you know the anchor. Panasonic notes that a log curve changes signal by almost the same amount for each stop of exposure. That makes a stop a useful description of a deliberate move, not a replacement for the curve's own gray target.²

Set the midtone before you press record

Use a gray card as a known midtone rather than judging exposure from the flat log picture. Sony notes that S-Log is harder to monitor because its displayed contrast is lower than usual.⁶

The RAW histogram helps repeat an exposure

A gray card that fills a good part of the frame forms a distinct cluster on the histogram. Once the card records at the right value, putting that cluster in the same place, in similar light and with the same settings, repeats the exposure.

The histogram shows RAW sensor values before the log curve is applied, so switching between the app's seven log profiles does not move the cluster. The profile changes the recorded value, as the table shows. The log profiles and codecs page lists all seven.

Adjusting exposure from the 18% gray target

The manufacturer's mid-gray value is an anchor, and the scene still matters. ARRI describes exposure latitude as a split above and below 18% gray. Moving exposure changes how that finite range is divided between shadows and highlights.⁸

That is the decision behind advice to overexpose log. You may choose more exposure because shadow quality matters more, or less because a bright highlight matters more.⁸

If a face grades muddy, adding one or two stops will not automatically fix it. Bring the gray card to the published value for your profile first, then adjust for the shadows or highlights you want to keep.