Log and Raw Don’t have highlight a highlight roll off.

This just keeps coming up over and over. Almost all log gamma curves and the majority of raw recording formats don’t have a highlight roll-off. Any roll off that you might see is probably in the LUT’s that you are using.

The whole point of log and raw is to capture as much information about the scene that you are shooing as you can. Log normally achieves this by recording every stop above middle grey with a constant amount of data, so even the very brightest stop has the same amount of recording data as the ones below it – there is no roll off.

In conventional limited range recordings such as Rec-709, hypergamma, cinegamma etc, highlight roll-offs work by reducing the contrast in the highlights to make the amount of data needed to record the very brightest stops much smaller than used for the rest of the image. This allows 2 or 3 stops to be squeezed into a very small recording range, keeping most of the recording data available for a nice bright high contrast image. The reduction in contrast in the extreme highlights helps hide any highlight handling problems and makes it appear as though the sensors clipping point is reach in a more pleasing soft manner.

But you don’t want this in a log or raw recording as it makes grading much harder as the footage will contain different contrast ranges, each needing it’s own grading adjustments. Also by reducing contrast in the highlights you are reducing the data. It would be very difficult to un-pick a highlight roll off and if you did want to expand the data back out you will get issues such as banding.

S-log-levels Log and Raw Don't have highlight a highlight roll off.
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey and there is no highlight roll-off. Note that current sensor only go to +6 stops over middle grey so S-Log2 and S-Log record to different peak levels.

S-Log2 and S-Log3 like almost all log gammas have no highlight roll-off. The only roll off is from middle grey and down. So if you underexpose you will start to roll away the data in your scenes mid range and that’s not good. Expose for the mid range, this is the most important part of any image. If your highlights are a bit clipped don’t worry about this too much. In post production you can add a roll off in the grade that will make any clipped highlights roll away gently. Adding a bit of highlight diffusion in post will also nicely mask any clipped highlights and make them look natural.

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3 thoughts on “Log and Raw Don’t have highlight a highlight roll off.”

  1. Alister, in the past you’ve preached avoiding overexposing S-log too much because of compression in skin tone highlights (i.e. only over expose a couple stops to reduce shadow noise).

    Where does that compression you warned us of come from since, as you say, there’s no highlight rolloff?

    Thanks.

  2. 1: The light in a scene doubles every time you go up a stop, so each extra stop in the scene has double the range of the previous one. Meanwhile log records every stop above middle grey with the same amount of data. So RELATIVE TO THE SCENE as you go up and up log records with less and less data. (a standard gamma or hypergamma reduces the recording data down to zero at or just before the cameras clip point. Log continues to record the same amount of data, but the range in the scene is increasing).

    2: In the past, before we had wonderful tools like LUT Calc, Colour Managed Workflows, ACES etc most grading involved applying a LUT designed for the cameras base exposure. The LUT would almost always introduce a highlight roll off that would start 1 or 2 stops above skin tones. So if you over exposed you would put skin tones into the highlight roll off of the LUT and things would not look good. We didn’t have linear workflows like ACES and as a result it was difficult to correctly deal with log that was exposed extra bright, it was difficult to make skin tones look good.

    Much has changed since then. We now have the proper tools to recover data in the correct manner from brightly exposed log. We have exposure compensated LUT’s that will re-expand the brightly exposed log back to levels and ranges similar to the scene. We have colour managed workflows that convert the log to linear, again restoring the the real world range for highlights so we can tweak and adjust them more easily. So many of the problems that used to occur from exposing too bright are no longer issues.

    But, the way log records hasn’t changed. So relative to the scene there is still less data in the brighter stops. So you don’t want to expose too bright but at the same time a bit of extra light on the sensor will often improve the signal to noise ratio and give a cleaner image. So there is a balancing act between noise and highlight data. With an FS5 and FS7 I choose to shoot at around +1.5 stops. I might go to +2 for a scene with large shadow areas and limited highlights. + 3 is almost always too bright and often introduces problems with highlight recovery and possibly skin tones.

    How brightly you shoot varies from camera to camera. Sony’s Venice for example has very little noise so I don’t find I need to shoot bright with Venice.

  3. Thank you for the thorough response. The explanation that the aforementioned compression is a result of the LUT clears it all up for me. You’re very right about dealing with overexposure being much simpler nowadays. One of my favorite features of Resolve 15 has to be how easy it is to just right click a node and set it to operate linearly so a quick exposure offset can be made via gain. Too few people seem to realize how easy it is. Maybe I’ll make a video out of it.

    Thanks again Alister. Buying you another beverage.

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