How to find the correct exposure for a new LUT.

[alisterchapman]

Different LUTs may have different brightness ranges compared to other LUTs or commonly used gammas such as Rec-709. As an example where normal Rec-709 might be exposed so that a white card is around 90%, the s709 LUT is correctly exposed when the same white card is at 83%. So, it is important to know what the expected brightness level for each LUT that you use is so that you can expose it correctly. 

 

If you have a new LUT that you haven’t used before. How do you find the correct exposure level for that LUT?

 

If your camera has a waveform display as the FS7/FX6/FX9 do this is easy. It's a little trickier with the FX3 and FX30, but still possible.

 

Set the camera up in the CineEI mode.

 

Start by turning on the cameras waveform then turning OFF the LUT. If you don't have a waveform monitor set the zebra level to 61% (On the FS3 and FX30 use the C1 custom zebra setting to set the zebra level to 61% and the range to +/-2% - the narrower range makes the zebra indication more precise).

Now with the LUT OFF you are measuring the S-Log3 and you should expose the S-Log3 so that a 90% reflectivity white card is at 61% and middle grey card at 41%. If you don'r have a proper grey card a piece of white paper can be used but in this case it should be exposed a touch brighter at around 63-64%.

 

Now turn ON the LUT for the SDI and HDMI (the waveform measures the signal on the SDI/HDMI what the waveform is measuring is indicated directly above the waveform display), check that the EI is set to 800 to match the base 800 ISO (if you camera has a different base ISO set the EI to match this different base ISO).

 

Now look at the waveform and wherever white and middle grey are, that is the correct level for the selected LUT. Skin tones will typically be half way between white and middle grey. So, if with the LUT ON white ends up at 80% and middle grey at 45% then skin tones will be approx 62%. 

 

On the FX6 and FX9 If you are unsure of where 61% or 41% are then go to the Zebra settings and set Zebra 1 to 41% and Zebra 2 to 61%, this will add orange lines to the waveform at 41% and 61%.

 

You can also then experiment with the levels of the zebra values and the orange lines , moving them up and down up to confirm the levels you are seeing on the waveform when you measure the LUT. 

 

On the FX3 and FX30 you will need to move the zebra level up and down to discover the point where the zebras appear over the white card with the LUT on. This can be done using the C1 custom zebra setting and you will be able to move the zebra level up and down until it appears over the white card while viewing the live image on the LCD.