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Create a one stop darker LUT, possible ?


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#1 DavidKlaus

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Posted 02 April 2017 - 04:31 PM

Hi everyone.

This summer i'm gonna be shoiting a short film in 4k with the FS7. I will be recording the 4k signal on an odyssey 7q+ via the hdmi output. As far as i know, when you use the hdmi output on the FS7, you losse the ability to turn LUT's on or off separately on the different outputs.

The thing is i want to shoot in cine EI mode and to get cleaner images in s-log 3, i will set the exposure index at 1000 instead of the native 2000. Using a rec709 LUT, this will make my images darker and "force" me to overexpose by one stop. But the problem is, i can't use the LUT only on the viewfinder because of the limitation with the hdmi output.

So i was wondering, is it possible to create à LUT in davinci that will make my images one stop darker. I'm not sure how to do that. I would then load this LUT into the odyssey for monitoring.

Thanks very much for your answers. Hope i'm not too confusing.

Cheers
David
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#2 Satsuki Murashige

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Posted 02 April 2017 - 04:38 PM

How will you get cleaner images if you don't overexpose the sensor? 1000 ISO will only be cleaner than 2000 ISO if you adjust the exposure accordingly. I think in this case, the in-camera LUT (I'm assuming you're using the camera's mLUT function?) is working correctly.

 

*I see, you are talking about using the Odyssey's LUT function to monitor in Rec709. I think it would still be a mistake to do what you are saying. But you could certainly still do it in Resolve by making a custom grade and outputting a 3D .cube LUT.


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#3 DavidKlaus

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Posted 02 April 2017 - 05:01 PM

Hello. Thank you for your answer. Sorry. Maybe i did not explain right. Yes thats the idea. I'm going to overexpose the sensor by one stop and bring the signal down in post. But with the FS7, even if you rate it at 1000 iso and look at the result in the viewfinder, the images are still recorded at 2000 iso. Thats where the "1 stop darker LUT" comes in. I cant use the onboard LUT feature because of the 4k hdmi output. So i want to trick my eye and my scopes on the odyssey with a one stop darker LUT thats gonna force me to overexpose my image. But forget about the camera, i was just wondering if there was a standard method to create that kind of lut.
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#4 Frank Hegyi

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Posted 02 April 2017 - 07:02 PM

Check out LUT Calc, you can set up pretty much anything you want.

https://cameramanben...Calc/index.html


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#5 Satsuki Murashige

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Posted 02 April 2017 - 08:06 PM

I see what you are saying now, my mistake.

You could shoot a grey scale or chip chart with your 1000 ISO rating exposure, and bring that into Resolve. Apply your Rec709 LUT and adjust exposure back to normal. Then output that grade as a new .cube LUT, which should be readable in the Odessey. You would want to apply the exposure adjustments before the LUT so you are working off the Log image.
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#6 Adrian Sierkowski

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Posted 02 April 2017 - 08:39 PM

Wouldn't it just be easier to open up by 1 stop every time? Since you're looking at a LUT on the Odessey; you could just open up by 1 stop to compensate on your end.. not too big a deal. And set your meter to 1000 as opposed to 2000.


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#7 Robin R Probyn

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Posted 03 April 2017 - 01:17 AM

Hi David

 

Yes LUT Calc will do it probably the easiest..do you now about Alister Chapman's site.. XDCAM-User.com..  Im pretty sure he has some "off set" +1 and +2 REC709 MLUTS down load able from his site..  also alot of good info re all things Sony and EI mode shooting..   I often shoot 1 stop over in EI.. but do it as Adrian say,s.. just over expose it and live with the slightly "hot " looking monitor..  but if you have the Suites on set better to use the off set LUT to avoid  endlessly having to explain what you are doing..  or just turn the brightness down.. :)


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#8 Robin R Probyn

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Posted 03 April 2017 - 02:00 AM

In fact here is the link for under/over exp off set LUTs for Fs7 from his site.. 

 

http://www.xdcam-use...f5-and-pmw-f55/


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#9 DavidKlaus

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Posted 03 April 2017 - 02:32 AM

Thank you all for you precious answers ! 

 

I will have a look at LUT Calc, looks like a fantastic tool. In the end, i will probably do as Robin suggest. Overexposing by one stop is not too much of a deal. But i will probably try one or two of those LUT to do some tests and check if everything is right when i will be on set. I will also look at the LUT from xdcam-user. I know his website, it's a goldmine.

 

Thanks to you all guys, really appreciate it.


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#10 Robin R Probyn

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Posted 03 April 2017 - 03:11 AM

Its just if you want the off set LUT so you can have a VF and monitor that is the "right" exposure to look at.. if your running around and exposing off the MLUT in the VF is an obvious help.. 

 

I would also recommend Slog3.cine.. the subset of Slog3.. still big colour space .. larger than P3.. but alot easier in post.. 

 

Another small Gotcha .if post software doesn't read the Sony Metadata.. the rushes will just look over exposed to the editor/grader.. so if your not going to be there, good idea to let them know before hand.. or they just think you don't know how to expose properly.. or tell you you have a camera problem... :)


Edited by Robin R Probyn, 03 April 2017 - 03:16 AM.

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#11 DavidKlaus

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Posted 03 April 2017 - 03:53 AM

Yep i'll keep that in mind. As far as i know, the film will be edited on Premiere. Anyway, the footage is going to be recorded on the Odyssey. So the metadata is not being recorded. But the editor is a friend, i will let him know before he tells me i dont know how to use a camera  :D


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#12 Robin R Probyn

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Posted 03 April 2017 - 06:21 AM

I mentioned this before in another thread.. not sure if the Fs7 has this function.. on the F5/55 in the assign menu there is choice called High/Low... you can assign to any function button.. high gives you a quick look at the high end of curve.. very easy to just check how your high lights are holding.. and Low does the opposite.. I always have this on an assign button shooting Cine EI.. as the LUT will make it look like things are blown out.. where as in your under lying LOG they are usually not..  .. 

 

You will be losing one stop at the high end,by rating 1000 ISO.. but even then you have alot still up there..  


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