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Modifying Alexa Rec709 LUT for cleaner shadows

alexa mini LUT noise

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#1 Steve Conry

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Posted 07 August 2016 - 10:19 PM

Hey guys,

 

I wanted to see if I could get some opinions and perhaps guidance. I have recently switched over from Epic to Alexa.

 

I am loving the camera but noticing that when using the built in LUTs, the image puts so much of its DR toward protecting highlights and thus has a little more noise than i might like in some scenarios. I have read a lot about shooting 400 ISO and from what I can tell it costs about a stop overall. That said, I have been doing some tests where I am exposing hotter than my waveform would suggest (monitoring in rec 709 and commercial) but without clipping in LOG. I find that once corrected the highlights usually still look amazing but the noise is much more like what i am looking for. 

 

I am wondering if there is a way to take the ARRI rec 709 lut down a stop and load it into the camera so that I will naturally expose a bit thicker of an image with cleaner shadows, without gaining down to 400. Of course, also open to opinions on why this might be a flawed idea, or whether it is essentially the same thing as shooting 400 haha. I just find my taste leans toward less noise in mid and shadows, and with the alexa image holding highlights so nicely, i'd prefer to "expose to the right" a bit more without having my images coming in so hot in premiere when the LUT is applied.

 

Any thoughts/suggestions?

 

Thanks,

 

Steve


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#2 David Mullen ASC

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Posted 07 August 2016 - 10:52 PM

No, switching to 400 ISO doesn't reduce the dynamic range, it's still 14-stops more or less... you've just moved a stop of information off of the top end and given it to the shadows. See:

http://www.provideoc...u_need_to_know/

Alexa-Dynamic-Range-Spread.jpg

 

So if you really want to reduce noise, your best bet is to go lower than 800 ISO.

 

The only way to reduce noise in the shadows but keep the ISO at 800 and have your highlights look normal would be to crush the blacks, basically increase the contrast of the toe region. You could create an ARRI Look File to do that if you wanted so that your monitor image showed this look even if you were recording Log-C or Arriraw, and this might then force you to add more light to the shadows.


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#3 Steve Conry

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Posted 07 August 2016 - 11:34 PM

Thanks so much David. Really appreciate your help. I'll definitely do some tests with the lower ISOs, as it sound like this is the way to go.

 

If I were to go the LUT route, you would recommend just tweaking the knee or toe of the LUT rather than dropping the whole look by, say, a stop? Is there any benefit or drawback to this versus the ISO method?


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#4 David Mullen ASC

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Posted 08 August 2016 - 12:24 AM

Simpler just to use the lower ISO rating, and probably less artificial-looking than crushing the blacks. You just may have to adjust some of your brightest highlights now and then, knock them down if they are clipping too obviously.

 

You don't have to use 400 ISO, you could use 500 ISO, for example.


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#5 Phil Rhodes

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Posted 08 August 2016 - 03:39 AM

No, switching to 400 ISO doesn't reduce the dynamic range, it's still 14-stops more or less

 

Enquiring minds would love to know how they've done that.


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#6 David Mullen ASC

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Posted 08 August 2016 - 11:18 AM

Seems like they are just picking a different midpoint in the same range for different ISOs.
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#7 Phil Rhodes

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Posted 08 August 2016 - 11:29 AM

Yes, I guess I'm just being thrown off by the fact that they're not modifying, for instance, amplification at all. This is very rare and implies some interesting engineering decisions.


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#8 Steve Conry

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Posted 08 August 2016 - 02:01 PM

That makes sense. I think I started to think that lowering the ISO affected DR after reading this article: http://www.provideoc...how_you_use_it/

 

Towards the end it claims the Alexa has ~7 stops over and ~8 stops under at 800 and ~6/~8 at 400. Am I reading this wrong?


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#9 David Mullen ASC

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Posted 08 August 2016 - 04:07 PM

He says at the end that in Log-C there is +7 / -6 between white clip and textured blacks at 800 ISO and +6 / -7 for 400 ISO.  Same range.


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#10 Robin R Probyn

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Posted 08 August 2016 - 04:24 PM

AFAIK Sony Slog in Cine EI mode works the same way.. its just shifting the mid grey level up or down.. 


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#11 Phil Rhodes

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Posted 08 August 2016 - 05:18 PM

The point being that in a practical sense, altering the ISO does affect specifically headroom relative to a nominally ideal exposure, which I was making the mistake of conflating when I wrote that.

The lasting feeling for me is: why are crushed shadows considered less objectionable than blown highlights? They're artifacts of exactly the same limitations.
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#12 Stuart Brereton

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Posted 08 August 2016 - 05:39 PM

Perhaps because we expect to see less detail in shadows anyway, so crushed blacks are not as noticeable,


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#13 Steve Conry

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Posted 08 August 2016 - 06:05 PM

He says at the end that in Log-C there is +7 / -6 between white clip and textured blacks at 800 ISO and +6 / -7 for 400 ISO.  Same range.

 

 

Got it, must have missed that. Thanks again David!


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#14 David Mullen ASC

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Posted 08 August 2016 - 06:14 PM

Depends on how artificial the crushed blacks look... an audience doesn't necessarily know when a shadow was supposed to go to black, unless it is a day exterior scene where some detail in the shadows is expected.  Whereas if you can tell the whites are clipped, that's usually more artificial-looking.  Just depends though, again, on what the eye expects to see.  But we often assume a black shadow is intentional but a burned-out area of white where you'd normally expect to see detail to be a mistake.

 

It's also just conditioning, we're used to seeing film images with a long roll-off into overexposure but shadows that fall to black.

 

Also, the general rule is that the eye goes to the brighter objects in the frame, so we'd notice clipping sooner than a shadow that has gone black.


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