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Proper exposure check when shooting ARRIRAW


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#1 Dmitry Savinov_38080

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Posted 05 August 2015 - 06:23 PM

Hi everyone!
I am going to shoot my first non-film whidescreen theatrical film using ALEXA XT and ARRIRAW
I am familiar with ARRIRAW quite enough 'cause I used to shoot a lot of TV spots with it and always used REC709 output for monitoring and False Color instrument to check exposure 'cause the final target was always TV Rec709 or similar gamma and ARRIRAW was an instrument to achieve cleaner and "over-resolutionized" image for better VFX workflow and so on
Now it is a completely different story,I will apply kind of look for on-set monitoring but there is no assurance that it should be exactly the same image that we will approve finally in post in terms of contrast,gamma etc
Can I still use False Color as my main exposure meter in this case or it will be better to use that old good friend as my Sekonic Multimeter is and to treat ARRIRAW as a 800 ASA film stock?
Thanx for your answer in advance

Edited by Dmitry Savinov_38080, 05 August 2015 - 06:38 PM.

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#2 David Mullen ASC

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Posted 05 August 2015 - 07:21 PM

If your important exposure information falls within Rec.709 then you know you have it in raw... so if you are comfortable with using Rec.709 false colors, then I don't see why it wouldn't work in this case, unless by recording ARRIRAW, you can't use false colors for the Rec.709 monitor-out.  Are you saying that you are using an external LUT box on set, or a monitor that allows LUT's to be loaded, so your camera has to send out Log-C from the monitor-out?

 

You can use your meter of course but you won't be monitoring raw, it's got to be either a flat Log-C image or something with a Rec.709-type gamma applied.


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#3 Dmitry Savinov_38080

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Posted 05 August 2015 - 07:33 PM

I hope the production is able to rent a state-of-art on-set monitor which applies LUTs and even works in DCI P3 space
I understand that Rec709 is much "tighter" than pure 12bit RAW and proper exposure in 709 color space made by eye and FalseColor should work for DCI for sure but would I loose some lattitude at the same time? And there could be some extreme lighting situation and I would not use the complete Alexa's dynamic range watching Rec709 image that does not represent it?
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#4 Albion Hockney

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Posted 06 August 2015 - 12:33 PM

Yea I always wonder this, people who just work with Rec 709 image. Because there is a lot more there rec 709 is very crunchy and high contrast....so if you just look at Rec 709 seems you might wind up with a kinda flater/less dynamic image then you could have or atleast be afraid to take some risks

 

I don't think you need a state of the art monitor for it to take LUT's ...if you can afford an arri raw pipeline for a feature this shouldn't be even close to a problem.


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#5 David Mullen ASC

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Posted 06 August 2015 - 04:05 PM

You can't "lose" dynamic range by working within Rec.709 but recording raw, the only question is whether there is a disadvantage in lighting for the narrower range in terms of making boringly flat images.

For the most part, no. For one thing, your display gamma, whether P3, a film print made from a digital negative, or Rec.709, isn't going to display most of that dynamic range anyway unless you are making an HDR version, so you'll be color-correcting to bring log into the narrower display contrast. Second, most of that extra range outside of the middle 11 stops is used for the extreme ends of exposure, the points where things are plunging to black or burning out to white. Having the wider range of the Alexa in raw or log mainly means that you'll be able to feather how things clip or crush more gracefully, but generally you aren't putting important information at those extreme ends.

Third, nothing is stopping you from toggling to a log version on set to see the actual range being recorded, if you think that might affect some decisions on set. Often you do that to check the clip point, for example, if daytime curtain sheers look too burned out on the monitor, you can check the log version to see what detail you will actually get to work with in post. But it is more dangerous to check log for shadow detail because just because you can see it in log, doesn't mean it is usable being so close to the noise floor, you still want to err on having more detail there from lighting rather than hoping you can just dig it all out of the bottom.
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#6 Dmitry Savinov_38080

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Posted 06 August 2015 - 04:58 PM

Thanks a lot David for very useful info!
Anyway they told me I would be able to check my daily footage via Barco 2K on screen in native ARRIRAW with my LUTs embodied anytime there could be any doubt so it will be the best way to double-check my exposure
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#7 Stuart Brereton

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Posted 06 August 2015 - 11:41 PM

Yea I always wonder this, people who just work with Rec 709 image. Because there is a lot more there rec 709 is very crunchy and high contrast....so if you just look at Rec 709 seems you might wind up with a kinda flater/less dynamic image then you could have or atleast be afraid to take some risks

Most people work with a 709 image because that is what all television ends up being displayed in. Even if you are shooting for a cinema release in DCP or Print, you are going to be losing a similar amount of dynamic range. It makes sense to monitor in the least forgiving format, so that you can light the way you want, knowing that there is more information there, should you need it.


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#8 Robin R Probyn

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Posted 07 August 2015 - 04:22 AM

The F55/5 has a very handy feature in the assign button menu.. High/Low exposure .. press once and it gives you the high exposure and again for the low.  in the VF with any MLUT you are using.. 

 

I don't think many people want to operate or pull focus in LOG either..  Re 709 MLUT is the way to go..


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#9 Albion Hockney

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Posted 07 August 2015 - 09:35 AM

Most people work with a 709 image because that is what all television ends up being displayed in. Even if you are shooting for a cinema release in DCP or Print, you are going to be losing a similar amount of dynamic range. It makes sense to monitor in the least forgiving format, so that you can light the way you want, knowing that there is more information there, should you need it.

 

this seems like a misconception to me because when you do a color grade the colorist starts with the log and then brings it back to legal. so you are compressing down the log that has all the dynamic range. the rec 709 you see on set not at all what you end up with.


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#10 Stuart Brereton

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Posted 07 August 2015 - 10:38 AM

 

 the rec 709 you see on set not at all what you end up with.

Of course it is. Why would you color time your material to look wildly different to how it did on set, unless it was for a special look? I light and expose my images as I want them to look in their delivery format, knowing that Log C or S-Log or whatever gives me wiggle room to deal with blown highlights, or crushed shadows if i need to.

 

There is zero point in lighting and exposing to the full extremes of a Log image because you are going to lose all that additional information the moment you convert to a display color space.


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#11 David Mullen ASC

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Posted 07 August 2015 - 11:06 AM

The trouble with viewing only log on the set and lighting for that range is that you'd be trying to make the image look "correct" for log in terms of the whites and blacks.  In particular, since "white" on a grey scale falls around 65 IRE in something like ARRI Log-C, your tendency would be to try and get your whites up to around 80 or 90 IRE just to look better on a monitor.

 

Think of it like shooting film in the old days -- the negative has more dynamic range than the projection-contrast print made from it, but when you tested the film stocks before beginning production, you viewed prints, you didn't project the negative.

 

So in your tests, you might have found that in the print, you had about five stops over and five stops under before things burned out to white or fell off into black, so about 11-stops displayed for a negative that holds about 14 to 15 stops.  

 

So this is very similar to a Rec.709 gamma applied to a log gamma signal.  The Rec.709 version is the "print" and the log version is the "negative".  You light for the print -- what can be displayed -- but you color-correct from the wider range of the negative, which gives you flexibility to bring things up or down.

 

The only caveat is that on set using a Rec.709 or similar LUT to convert the log signal, you don't generally have access (or time to use) tools like knee compression, luminance keys, and Power Windows which allow you to pull detail out of the extreme ends of log when necessary, such as a hot lampshade or curtain sheers or glare off of a sidewalk.  Generally you just switch to viewing log to see if that stuff is getting recorded, then switch back to your Rec.709-ish LUT.

 

The trouble with lighting for the full range of log is that it can't be displayed that way later, not if you want normal blacks and whites on screen, so you may be more gutsy if lighting from looking at a log image, but as soon as you set your blacks to normal black in the final color-correction, you'll be shocked by the shadow detail that gets buried, and if you try to lift them up so that they can be seen, you end up with milky and noisy blacks on display.  So it is better to think of the information at the extreme ends of log as just being there to make sure your shadows and highlights can roll off more gracefully into black or white rather than abruptly, i.e. more like film less like video.

 

When I used to shoot PanaLog on the Genesis, I sometimes would view log on the set -- with PanaLog, it's a very mild log curve, you just have to remember that white is around 70 IRE and black around 10 IRE, so it's not horrible to view, but you naturally tend to overexpose when viewing PanaLog only because of the dingy whites.  But with ARRI Log-C, it is even worse, it's more like 65 IRE for white and 15 IRE for black, which is what is necessary to fit 14.5 stops of DR between 0 and 100.

 

C-Log in the Canon C300 is similar to PanaLog because it doesn't have the DR of an Alexa, so you can get away with just viewing C-Log on a set monitor if you had to.


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#12 Albion Hockney

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Posted 07 August 2015 - 12:05 PM

Thanks for explanation David. The film neg analogy makes sense.

I think the misconception and you explained it is that the information is not there or will be gone when you go to color grade. It won't be gone it will be buried a bit, and yes if you lift it up and use it it will be noisy/milky ....but often times aesthetically I like using that space. It of course depends on the project.

 

 

That said I shoot mostly on the red in Red Gamma 3 and never even bothered to ask this are the Red Gammas Rec 709 legal? 


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#13 Dmitry Savinov_38080

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Posted 07 August 2015 - 05:01 PM

The point is that photochemical process had a bit different print film stocks in terms of contrast (and of DR as result)...)

Thanx everybody,now I am completely convinced in Rec.709 monitoring with switching sometimes to LogC to be sure with selected exposure,as I do usually on TV jobs
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