Understanding ISO with the RED ONE

[Dj Joofa]

I guess the biggest risk with digital gain is posterization, due to only using a smaller part of the code values. How do you fight against that? Dithering maybe?

Dither can try to conceal posterization to some extent. However, it is always good to add dither before digitization and not afterwards. Audio engineers have used this fact for a long time. Basically, dithering before digitization helps reduce the perception of harmonic distortion due to quantization in a better way than adding it after digitization. And, this is also a side benefit of analog gain that it already provides a "dither-like" effect before digitization.

 

[Graeme Nattress]

With digital gain, you're always using all the code values, but you're placing greater image significance to the darker ones. With Analogue gain you're spreading the important part of the image over all code values, but loose highlights.

Joofa, on my Canan I found for 1 stop, I could like either analogue or digital gain. For more than one stop, analogue looked better. But with RED a can do a few stops digital without it looking bad in the way that the Canon looked bad. I don't have analogue gain control on the RED to do a direct comparison though.

Graeme

 

[Miguel "Macgregor" De Olaso]

Fact #1: RED has no shoulder/rolloff into the highlights because it's a linear sensor. It clips abruptly. To achieve the same look as film we have to underexpose and then apply a curve that imitates film's behavior in the highlights. Underexposing pushes the signal towards the shadows, where most of the noise lives, and then it'll be lifted by the film curve. So this technique will be limited by how much noise we can accept to lift.

Fact #2: ISO in the Film world equals sensitivity/coarseness of the stock. In the SLR world it's the analog gain applied to the sensor data before it's digitised. In the RED world it's the metadata that's used for the interpretation of the raw sensor data, untouched. This interpretation affects the live monitoring and can be changed to any other interpretation in post.


#1 + #2 = Eureka! We can actually use RED's ISO control for something useful! [1] We can apply the film-look-resulting underexposing strategy on set AND not have to look at a dark image!

That's all there is to it!

Now what Macgregor tests provide us is an empirical data point that gives us an idea of how much we can underexpose and not sacrifice too much image quality. It looks like ISO 500 is an acceptable point, and at ISO800 the road starts to get bumpy... but, by all means, DO YOUR OWN TESTS and determine the amount of noise you're prepared to accept in your circumstances (daylight/tungsten, etc).
[/COLOR][/SIZE]

That was a great summary of the whole thread!

 

[Jason Sinclair]

Oh please. What a mess.

Supernovafilms and Reactor88 I have to ask you to check your facts before you post. When someone shows a disagreement with you, revise your logic before you make multiple wrong posts. Please don't be stubborn.





And now let's go back to Macgregor's second post (emphasis mine):


If you are basing your analysis on the shots from iso.wmv, these are different shots indeed. You can even check by eye that the background movements are not the same.

I understand that this is a complex subject and it's easy to confuse some things if you don't pay a lot of attention. Sometimes you really have to read the same things twice just to be sure of what we're really talking about. So please check your facts before you start pontificating/theorizing and worst of all... confusing the rest of the people even more. Don't add to the noise.


Actually, the idea is not that complicated. Macgregor has tested empirically what Stu had theorized about.

Fact #1: RED has no shoulder/rolloff into the highlights because it's a linear sensor. It clips abruptly. To achieve the same look as film we have to underexpose and then apply a curve that imitates film's behavior in the highlights. Underexposing pushes the signal towards the shadows, where most of the noise lives, and then it'll be lifted by the film curve. So this technique will be limited by how much noise we can accept to lift.

Fact #2: ISO in the Film world equals sensitivity/coarseness of the stock. In the SLR world it's the analog gain applied to the sensor data before it's digitised. In the RED world it's the metadata that's used for the interpretation of the raw sensor data, untouched. This interpretation affects the live monitoring and can be changed to any other interpretation in post.


#1 + #2 = Eureka! We can actually use RED's ISO control for something useful! [1] We can apply the film-look-resulting underexposing strategy on set AND not have to look at a dark image!

That's all there is to it!

Now what Macgregor tests provide us is an empirical data point that gives us an idea of how much we can underexpose and not sacrifice too much image quality. It looks like ISO 500 is an acceptable point, and at ISO800 the road starts to get bumpy... but, by all means, DO YOUR OWN TESTS and determine the amount of noise you're prepared to accept in your circumstances (daylight/tungsten, etc).

I hope that's clear and short enough
A lot of this info is bound to change with the release of build 16, with significant changes to dynamic range, signal processing, live monitoring... we can only wait!



[1]It's just an annotation, it doesn't touch the signal, you're always ISO320. In that sense, IMHO the DP has lost a tool for effectively controlling his exposure, and I think that analog gain gives the DP more flexibility. Graeme has talked about analog vs digital gain in fxguide's red centre podcast #4, and he recognised that analog gain has its strong points. I hope RED changes gears and the Mysterium X gives us that opportunity. But that's just me, and it's a subject for another thread.

I feel like a complete and utter idiot. Thanks for clarifying that and sorry for pointing anyone in the wrong direction.

 

[donatello b]

bump ... good info here !!

 

[Spike Baumann]

Yep but how does it fare with the new builds?

 

[Miguel "Macgregor" De Olaso]

Here you go:
http://www.reduser.net/forum/showthread.php?t=17756&highlight=macgregor

 

[Spike Baumann]

Cool. Thanks.