A way of recording camera images in which each doubling of scene brightness takes about the same step of signal, so a very wide brightness range fits in the file; the footage looks flat until it is converted for a display.
A camera maker's logarithmic transfer curve applied to scene-linear sensor data. ARRI describes Log C as scene-based, with the signal rising by a fixed amount for each stop of exposure, and Sony's guide puts an 18 per cent grey at 41 per cent of the signal in S-Log3. Each maker's curve is different and needs its own transform to a display standard.
Log footage is not a low-contrast 'look'. It is an intermediate that has not yet been turned into a picture, and adding contrast and saturation by hand is not the same as applying the transform it was designed for.
A log clip opened without its transform shows grey shadows, grey highlights and weak colour, because mid-grey sits near 40 per cent of the signal instead of where a display expects it.
It is why material straight from a cinema camera looks wrong on a monitor, and why the first question about a flat-looking clip is which curve it was recorded in.
A log curve is described by where it puts known scene values. Sony's guide states that S-Log3 records an 18 per cent reflectance grey at 41 per cent of the signal, and ARRI describes Log C as adding a fixed amount of signal for each stop of exposure. The reason for the design shows in arithmetic. In a 10-bit linear file the brightest stop takes about 512 of the 1,024 code values and the tenth stop down about one; sharing the codes equally across 14 stops would give each about 73. Real curves have a toe near black and do not share the codes exactly equally.
A clip recorded in one maker's log is converted with another maker's transform.
Each curve places mid-grey and the stops differently, so the wrong transform gives wrong contrast and shifted colour that then has to be graded out. The fix is the correct transform, not a correction on top.
The crew judges exposure from the monitor while it shows the image through a show LUT.
A show LUT can be built to display the picture darker than the recording; on one series it showed the image a stop down. Exposure has to be judged knowing what the table does, or from the log signal itself.