If using a LUT to judge the exposure of a camera shooting log or raw it’s really important that you fully understand how that LUT works.
When a LUT is created it will expect a specific input range and convert that input range to a very specific output range. If you change the input range then the output will range will be different and it may not be correct. As an example a LUT designed and created for use with S-Log2 should not be used with S-Log3 material as the the higher middle grey level used by S-Log3 would mean that the mid range of the LUT’s output would be much brighter than it should be.
Another consideration comes when you start offsetting your exposure levels, perhaps to achieve a brighter log exposure so that after grading the footage will have less noise.
Lets look at a version of Sony’s 709(800) LUT designed to be used with S-Log3 for a moment. This LUT expects middle grey to come in at 41% and it will output middle grey at 43%. It will expect a white card to be at 61% and it will output that same shade of white at a little over 85%. Anything on the S-Log3 side brighter than 61% (white) is considered a highlight and the LUT will compress the highlight range (almost 4 stops) into the output range between 85% and 109% resulting in flat looking highlights. This is all perfectly fine if you expose at the levels suggested by Sony. But what happens if you do expose brighter and try to use the same LUT either in camera or in post production?
Well if you expose 1.5 stops brighter on the log side middle grey becomes around 54% and white becomes around 74%. Skin tones which sit half way between middle grey and white will be around 64% on the LUT’s input. That’s going to cause a problem! The LUT considers anything brighter than 61% on it’s input to be a highlight and it will compresses anything brighter than 61%. As a result on the output of your LUT your skin tones will not only be bright, but they will be compressed and flat looking. This makes them hard to grade. This is why if you are shooting a bit brighter it is much, much easier to grade your footage if your LUT’s have offsets to allow for this over exposure.
If the camera has an EI mode (like the FS7, F5, F55 etc) the EI mode offsets the LUT’s input so you don’t see this problem in camera but there are other problems you can encounter if you are not careful like unintentional over exposure when using the Sony LC709 series of LUTs.
Sony’s 709(800) LUT closely matches the gamma of most normal monitors and viewfinders, so 709(800) will deliver the correct contrast ie. contrast that matches the scene you are shooting plus it will give conventional TV brightness levels when viewed on standard monitors or viewfinders.
If you use any of the LC709 LUT’s you will have a miss-match between the LUT’s gamma and the monitors gamma so the images will show lower contrast and the levels will be lower than conventional TV levels when exposed correctly. LC709 stands for low contrast gamma with 709 color primaries, it is not 709 gamma!
Sony’s LC709 Type A LUT is very popular as it mimics the way an Arri Alexa might look. That’s fine but you also need to be aware that the correct exposure levels for this non-standard LC gamma are middle grey at around 41% and white at 70%.
An easy trap to fall into is to set the camera to a low EI to gain a brighter log exposure and then to use one of the LC709 LUT’s and try to eyeball the exposure. Because the LC709 LUT’s are darker and flatter it’s harder to eyeball the exposure and often people will expose them as you would regular 709. This then results in a double over exposure. Bright because of the intentional use of the lower EI but even brighter because the LUT has been exposed at or close to conventional 709 brightness. If you were to mistakenly expose the LC709TypeA LUT with skin tones at 70%, white at 90% etc then that will add almost 2 stops to the log exposure on top of any EI offset.
Above middle grey with 709(800) a 1 stop exposure change results in an a 20% change in brightness, with LC709TypeA the same exposure change only gives a just over 10% change, as a result over or under exposure is much less obvious and harder to measure or judge by eye with LC709. The cameras default zebra settings for example have a 10% window. So with LC709 you could easily be a whole stop out, while with 709(800) only half a stop.
Personally when shooting I don’t really care too much about how the image looks in terms of brightness and contrast. I’m more interested in using the built in LUT’s to ensure my exposure is where I want it to be. So for exposure assessment I prefer to use the LUT that is going to show the biggest change when my exposure is not where it should be. For the “look” I will feed a separate monitor and apply any stylised looks there. To understand how my highlights and shadows, above and below the LUT’s range are being captured I use the Hi/Low Key function.
If you are someone that creates your own LUT’s an important consideration is to ensure that if you are shooting test shots, then grading these test shots to produce a LUT it’s really, really important that the test shots are very accurately exposed.
You have 2 choices here. You can either expose at the levels recommended by Sony and then use EI to add any offsets or you can offset the exposure in camera and not use EI but instead rely on the offset that will end up in the LUT. What is never a good idea is to add an EI offset to a LUT that was also offset.
9 thoughts on “Using LUT’s for exposure – choosing the right LUT.”
OK. This post has now confused the heck out of me. I have commonly been using the LC709TypeA LUT shooting Cine EI – S-Gamut3.Cine/Slog3 and rating at 800EI, as in keeping with your recommended past workflow for low noise in shadows. Then judging exposure with the LUT image and waveform. But this seems to tell me that I have been overexposing by 2 stops? So what is a correct baseline then? Just use the same LUT but rate camera at 2000EI native so WYSIWYG in the viewfinder? I’m not sure I read an answer here. Thanks.
It’s a common mistake. The only LUT that gives true 709 levels where you expose white at 85-90% is 709(800).
Sony separated the LUT’s into different categories. LUT’s (for exposure) and Looks (for creative “looks”). LC709TypeA is a Look, designed to make the pictures look different from vanilla 709. How do you make something look different? You use different brightness and color levels. LC709 provides a film like look, so it uses film like brightness levels which are much lower than vanilla Rec-709.
Every single Look, every single LUT, needs to be exposed at the levels that that LUT or Look is designed for, there isn’t “one level fits all” so it’s vital that you understand what those levels are if you want to establish a correct baseline exposure. The correct baseline exposure for LC709Type A is white at 72% and middle grey at 41%, quite a bit darker than normal 709. Skin tones should be around 55%
To establish the correct exposure for any LUT use a middle grey + white card to measure and set the correct log exposure at the base ISO. Then turn on the LUT and see where middle grey + white fall. This is the base exposure for that LUT.
I often come across people at workshops that tell me they have never found it necessary to use a low EI. Then they tell me they use the LC709 TypeA LUT and expose it with white at 80-90% and skin tones at 70%. So in fact they are actually shooting at the equivalent of +1 to 2 stops.
I posted below after you answered. I think I’ve been doing it correctly based on what you said. The confusion also is that since I shoot with the 709(800) LUT then use the LC709 TypeA LUT as a grade start, I have never seen that I’ve overexposed. It has always looked correct. Sorry, I doubled down on confusion and I was doing it correctly all the while. Thanks!
OK, I’ve confused myself perhaps, but please correct me if I’m wrong. I actually shoot with LUT P1:709(800%) and initially grade with the LC709TypeA LUT. But I guess I now believe that rating the camera anything other than 2000EI will be incorrect in the viewfinder.
If you use 709(800) with the camera set to the native EI (2000EI on a FS7 or F5) and expose white at 90% then the S-log3 will be recorded with white at 61%, middle grey 41%. This is the base exposure.
Then in post if you apply the LC709 Type A Look to the above base exposure footage white will be 72% and middle grey 41% – This is still “correct” but will be darker and flatter to give the “look”.
Using 709(800) again, if you set the camera to 1000 EI and expose the LUT exactly the same (90% white) now the S-Log3 will be recorded with middle grey at 49% and white at 69% due to the offset that the camera applies to the LUT that makes the LUT 1 stop darker. The recording is now +1 stop brighter.
Thank you for this super interesting post! Can you recommend LUT s for + 1,5 stops for slog 2? Where can I find those LUTs ? Thank you Chris
Thanks a lot, It would be so so useful if you can explain what really “look profile” is? And difference between “look” and “lut”.
First off thank you for all your amazing posts, tutorials and generally sharing your expertise, incredibly helpful and it’s much appreciated..
I have one slightly longwinded question regarding the high/low feature vs waveform – using the Rec 709 (800) LUT on the viewfinder, but turned off on SDI2 2, I can use the zebras at 90% to expose for 90% white on the LUT while eyeballing the waveform to make sure the underlying SLOG3 isn’t blowing out the highlights. Would you condone this method or do you consider the high/low function to be more accurate because of the small size of the waveform? Or is it just horses for courses?