Influence of ISO on shadow and highlights in RAW recordings

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jwleon

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Influence of ISO on shadow and highlights in RAW recordings

PostMon Aug 20, 2018 8:07 am

Hello Folks!

I spent the last two hours looking into something that has caught my interest for quite a while and I was not able to figure it out on my own. So, if there are any ressources I have missed that answer my question, kindly point me to them and I will delete this thread :D

The topic is ISO in RAW recordings on the URSA Mini Pro 4.6K. I learned that the camera is ISO invariant, meaning, to my understanding, that the sensor-data is recorded to RAW at native ISO of 800 and the ISO-value is just meta-data.

Somewhere along the articles and posts I read when learning how to use the camera, I read that the ISO is not only sensor amplification, but also influences the dynamic range in the dark and bright areas. Someone said that this even goes so far that using ISO 1600 in daylight is not unrealistic because it gives more dynamic range in the high tones and therefore can prevent highlights, which are obviously there in daylight, from losing information. This is an information I could not validate from any official resource, but seen from a technical point of view it definitely makes sense to me.

IF it is like this and the ISO value influences the stops of dynamic range above and below mid-grey, I am curious if this also applies to RAW recordings and, if yes, if it is also applied by the RAW-processing software later or if this is not "invariant", meaning it directly applies to the sensor and the recorded data.

The topic is a bit difficult to describe or phrase, if you have not completely understood the process (and are not fluent in English ;-) ). Just let me know if you have questions about what I mean.

Thanks in advance and best greetings!
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Denny Smith

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostMon Aug 20, 2018 5:36 pm

When recording Raw, ISO is only MetaData. When recording film log, ProRes or DNxHD, then ISO changes the exposure curve above and below mid grey, as you described. While ISO is often analog gain control on many cameras, it does not add or reduce sensor gain on BM cameras, sensor gain is fixed or an auto dual gain system optimized for the camera’s native ISO, so on the Pocket camera and a Ursa Mini 4.6/Pro Cameras, this is 800.
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Robert Niessner

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostTue Aug 21, 2018 12:49 am

I will answer Jonathan in German, as this is his and my native language and he wrote he has difficulties to understand this topic in English.

Jonathan, sämtliche Blackmagic Kameras (mit Ausnahme der kommenden Pocket 4k) sind ISO invariant. Das bedeutet für RAW-Aufnahmen, dass Änderungen des ISO-Wertes ein reines Setzen der Metadaten sind. Das Beschneiden der Zeichnung in den Lichtern wird durch den ISO-Wert nicht beeinflusst - egal ob ISO 200 oder ISO 1600. Bei ProRes-Aufnahmen ändert der ISO-Wert lediglich das Aussehen der angelegten Gammakurve, hat aber ebenfalls keinen Einfluss auf den Beschnitt der Lichter.
Du kannst in RAW völlig verlustfrei jederzeit in der Nachbearbeitung aus einer ISO 200 Aufnahme eine ISO 1600 Aufnahme und vice versa machen. In ProRes geht das bis zu einem gewissen Grad ebenfalls.

Die Basis-ISO der UM46k liegt bei ISO 800. Ich persönlich nutze bei Außenaufnahmen meistens ISO 400, bei starken Kontrasten ISO 200 und bei Innenaufnahmen ISO 800, sowie bei Schwachlicht auch mal ISO 1600. Das ist aber nur, um in der Nachbearbeitung leichter loslegen zu können bzw. um beim Drehen den Gesamteindruck des Bildes beurteilen zu können. Es wird durch das Ändern der angelegten Gammakurve lediglich die Verteilung der Werte über und unter Mittelgrau verschoben.

Du verlierst keine Zeichnung, solange Du nicht den Sensor per Blende/Belichtungszeit/ND unter- oder überbelichtest - die ISO-Werte haben keinen Einfluss darauf (im Gegensatz zu Kameras mit ISO-varianten Sensoren - sprich mit einer analogen Vorverstärkungsstufe).
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Sean van Berlo

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostTue Aug 21, 2018 6:24 am

Great answer Robert. My understanding of German goes far enough to read it but not to comfortably answer in it, so I'll keep it English. You mentioned you prefer iso 200-400 for high contrast and outside scenes, while I always understood it to be desirable to shoot around 800 to see into the highlights in this scenario. Why do you choose for the former? Thanks!
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Robert Niessner

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostTue Aug 21, 2018 10:23 am

Sean, I've setup some simple FX curve presets in Premiere Pro which I can slap onto my footage and get an instant nice starting point for final tweaking. For that ISO 800 often looks too bright, while ISO 400 gives me a nice middle ground. Seeing into highlights would only work, if I did underexpose on purpose compared to ISO 400.

On the other hand I did an outside interview shot (ProRes) where I had to put the talent into the shadows (and tried to brighten up his face with a 90W Dedo-LED) and had the background in direct sunlight (a train which could not be moved somewhere else). I shot that at ISO 1600. - After some tweaking in post it looked surprisingly nice - even without using power windows.
Saying "Thx for help!" is not a crime.
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Robert Niessner
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Rakesh Malik

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostWed Aug 22, 2018 11:46 pm

Different cinematographers develop their own techniques for different cameras. Even for Alexas. A lot of the decision comes down to preference in terms of how you work; some cinematographers like to down rate their cameras' ISO to 400 from a "native" 800 (for example) so that they are looking at an image that appears slightly underexposed, then light for that. The result is a one stop over exposure, but with extra detail reserved for the shadows, and due to the bit of extra light required to compensate for the underexposure, less noise.

There's a very good article about ISO invariance here:
http://www.red.com/learn/red-101/iso-speed-revisited

While it SOUNDS like it's Red specific, it's not; what it describes is equally applicable to BMD cameras, and it includes pictures to illustrate what happens when you change ISO (in this article you'll see FLUT used, just think of it as ISO that's adjustable in smaller increments, and you'll be fine).
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jwleon

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Re: Influence of ISO on shadow and highlights in RAW recordi

PostFri Aug 24, 2018 6:45 am

I guess that were the most detailed, professional and helpful answers I received so far in any forum I have ever been a member ;-) Thanks a lot for those information - even though it is as I thought it is, but simply could not confirm it from my own tests.

By the way, @Robert, I tried your approach of using ISO 400 and 200 for a outdoor night scene yesterday - it was possible, since the scene was well lit from several natural light sources and our own lights. I see what you mean and it definitely worked out. In addition, you can better remove noise if you light your scene for lower ISOs, I guess...

Thank you all and best greetings,
Jonathan

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