Working with custom LUTs

[Thomas McNamara]

Hi everyone!

I’m curious about working with custom LUTs in camera during productions. I haven’t really dabbled into that too much yet but I’m hearing a lot of DPs who work this way for many reasons: exposure, nailing down a look early on and committing to it, making sure director/producers/clients have a nice image around the monitor that’s close to the final graded look, etc.

Right now, I do what most people do when shooting log on cinema cameras: I use the go to 709 LUT the camera has to offer (Arri 709 for Alexa, 709(800%) for Sony and so on), expose looking at that (as well as my exposure tools of course) and toggle the LUT on and off to double check the log every now and then to make sure I’m not loosing certain things.

That’s worked fine for me and I have no problems doing that at all, but I’m very interested (for commercial, film and music video work) in looking into developing a look or a series of looks beforehand and sticking to it throughout.

I just want to hear about common practices for creating these show LUTs and how each created LUT relates to exposure in camera. I’d also like to hear about common workflow practices structured around working this way, like what is the basis of the creation of a custom LUT for a project? Is it Raw footage from a pre-light for the project? Or just random test footage?

 

I’m curious to hear your answers!

[Robin R Probyn]

RE exposing with Monitor LUTs.. the Rec709 is the easy one ,as everyone knows the white/grey/skin tone levels .. and if its for TV thats the gamma that it goes out in anyway.. but using your own or other LUTs is fine also, as long as you know those levels for that LUT.. The Sony LC 709 (A) Look Luts are also quite popular.. the white level is lower the vanilla 709 (800) one..which catches people out some times.. but there again Log usually benefits from a bit of over exposure anyway..

 

FYI ..On the Sony F5/55 in assign menu only.. you can find Hi/Low .. this is also very handy for checking the underlying Log exposures quickly.. press once for what the high lights are doing.. press again shows the shadows .. one more press and it exists the mode..

[Thomas McNamara]

Thank you for your response Robin!

 

Part of my question has to do with what you brought up in your response: "but using your own or other LUTs is fine also, as long as you know those levels for that LUT".

 

How exactly can I be sure what the levels are for my created LUT? Say I create one in Resolve and I really give it quite a distinct look; say I really push the heck out of it or vice versa. Then I generate the LUT and load it into my camera. How can I be exactly sure what my tone levels should be exposed as? I know for a variety of LUTs because I've seen published numbers from camera companies about the tone values for those specific LUTs (like the LC 709A, 709 800, etc). But how can I know for my own custom LUTs with that degree of exactitude? Or is it just a matter of toggling it on and off to check the LOG values like I usually do?

[Phil Rhodes]

By defining a custom LUT, you are intrinsically defining what correct exposure is in terms of that LUT.

 

If it looks as you want it to coming through that LUT, it's correctly exposed. If it doesn't, it isn't. Generally your LUT will be designed to target a 709 display and you can waveform monitor that.

 

This, of course, raises the spectre of coming up with an extreme LUT which causes you to make extreme exposure decisions which cannot be made to look less extreme at a later date.

 

I am not a very big fan of overcomplicating this. Light for a decent 709 picture that's near what you want, fine. Use a custom LUT cautiously, to get people used to what you're going for or to assist in your exposure and lighting decisions, or to help the production design people get things looking as they should in context of the final intent, again, fine. But extreme looks can cause people to bake in decisions that are later regretted, especially as it is actually very difficult to come up with custom LUTs, especially extreme ones, which are adequately tested in a variety of conditions (overexposure, underexposure, inside, outside, backlight, soft light, high and low con scenes etc, etc) and which don't cause people to make very bad decisions. When you use a custom LUT you are essentially redesigning the camera system and second-guessing the manufacturer, which is fine, but something that should be entered into with a great degree of trepidation as you probably don't have the equipment, experience or time that the manufacturer had when they designed the camera. That's not to query your creative intent or to put the manufacturers on a pedestal - they make unfathomably stupid decisions all the time - but they will generally give you something that won't screw you over in an unanticipated situation.

 

That, and the process of on-set grading and assigning of custom LUTs and custom grades to shots in the hope they'll make it through into post is time-consuming, expensive, and fraught with error.

 

I'm not sure about it.

 

P

[Stuart Brereton]

I use a 'custom' LUT when shooting Sony Cameras. It's basically the LC709A LUT with a small gamma correction to increase the contrast in the mid tones, and a 10% desaturation. I often do the same thing when shooting with Arri Alexa.

 

I use a piece of software called LUTCalc. https://cameramanben.github.io/LUTCalc/

 

This allows you to use a standard manufacturer's LUT as a base and then add your own adjustments if you wish.

 

There are other ways of creating LUTs. You could take LOG material into Resolve, for instance, correct it to your chosen look, and then output that correction as a 3D LUT .cube file. That would give you a lot more control than something like LUTCalc, but you'd want to test the resulting LUT on a wide range of material to make sure it worked with everything.

 

As Phil says, working with a large number of LUTs can be time consuming and overly complex. I tend to stick to one or two per show, usually just DAY and NIGHT.

[Thomas McNamara]

Thanks a lot for the responses guys, really really helpful!

 

So I guess the idea is don't stray too far away from Rec 709 with your custom LUT should you choose to go that way, and if you do, rigorous testing is in order.