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Understanding the Difference between Log & Gamma

 
Author desmondqford
ZRO
#1 | Posted: 7 Sep 2012 06:22 
I've been attending a really interesting DIT workshop / lecture series. Last weeks class was on the benefits of Log Encoding and Gamma Curves. Of course, Light Illusion / Steve Shaw was referenced more than a few times. Many of the concepts are difficult to understand though, so let me know if I got the gist of it...

What I cam away with is that camera sensors see light very differently than humans. So much so that we must manipulate how the camera captures and/or outputs the light or resulting image so as to make it better for humans to see.

Gamma curves are basically designed to compensate for low bit depth and low dynamic range. Gamma gives use more information where we want it most: in the midtones.

As for Log, it's designed to redistribute light evenly so as to maximize the areas most important to human vision, such as midtones. Log actually adds dynamic range, which gives us more latitude to work with in post.

So both gamma curves and log encoding are ways to get more out of our cameras today, but they're not necessary for high end cameras such as an Alexa, Red or F65 because they already have high dynamic range, high bit depth, high resolution and little to no compression.

Please correct em if I'm wrong on any of this. I'd be grateful if anyone could add to or shed some light onto these concepts, why we have gamma curves and log encoding, etc.


dezzy

Author Steve

INF
Male
#2 | Posted: 12 Sep 2012 09:27 
Interesting no one has commented on this...

Am I the only one who will answer???
Steve Shaw
Mob Boss at Light Illusion

Author desmondqford
ZRO
#3 | Posted: 13 Sep 2012 03:02 | Edited by: desmondqford 
You're always the only one who answers... prolly cause you're the only one who knows the answer! LOL

In other news, I attended another DIT course tonight on Luts and Color Management, and wouldn't you know, your Lut primer was used as the textbook.

You should start teaching classes, tutorials / webinars, I'd be the first to sign up.

Author Steve

INF
Male
#4 | Posted: 13 Sep 2012 13:27 
desmondqford:
What I cam away with is that camera sensors see light very differently than humans. So much so that we must manipulate how the camera captures and/or outputs the light or resulting image so as to make it better for humans to see.

All camera sensors see light in Linear space.
This Linear Light needs to be remapped for a number of reasons, mainly to do with better use of the available bit depth.
A linear image needs a lot more bits to maintain the same level of image granularity.
See the Light Illusion white papers - DI Guide, Scene-to-Screen - where this is explained in more detail.

desmondqford:
Gamma curves are basically designed to compensate for low bit depth and low dynamic range. Gamma gives use more information where we want it most: in the midtones.

Nope, gamma curves are to enable a TV to show an image correctly. This is based on old CRT technology and video cameras that were pre-CCD sensors. Using Gamma Curves for other reasons is a later development, and aims to help get the most from a camera, but means the images will need grading before delivery.

desmondqford:
As for Log, it's designed to redistribute light evenly so as to maximize the areas most important to human vision, such as midtones. Log actually adds dynamic range, which gives us more latitude to work with in post.

Nope - again the Light Illusion white papers are the best to read. LOG is a form of visually lossless compression to enable the use of less bits for a given scene dynamic range. LOG doesn't increase Dyncmic Range - nothing can, as that is fixed by the camera sensor.

desmondqford:
So both gamma curves and log encoding are ways to get more out of our cameras today, but they're not necessary for high end cameras such as an Alexa, Red or F65 because they already have high dynamic range, high bit depth, high resolution and little to no compression.

Nope - See above. And many top end cameras do indeed use Gamma Curves and/or LOG Encoding...

Steve
Steve Shaw
Mob Boss at Light Illusion

Author desmondqford
ZRO
#5 | Posted: 14 Sep 2012 21:36 | Edited by: desmondqford 
Thanks for clearing that up for me, Steve. One thing though...

The instructor of the course demonstrated the difference between log and standard mode by using a Chroma du Monde chart and waveform monitor (Leader 5750, I think). Once he switched to log mode, via a Sony F3, we could see that there was in fact another stop in the highlights on the waveform. It went from 11 stops to 12 stops, if I remember correctly.

How is that possible?

Thanks again for your detailed response.

dezzy

Author Steve

INF
Male
#6 | Posted: 14 Sep 2012 22:27 
Simple, because a Rec709 profile artificially limits the cameras real capabilities to make the image look nice on a normal TV.

With no profile the camera would display all it can (what the sensor sees basically) but that would not look good on a normal TV.

But, that would be Linear Light, so not easy to work with, so a non-standard gamma curve or a LOG profile just redistributes the image data to make it easier to work with

Steve
Steve Shaw
Mob Boss at Light Illusion

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 Understanding the Difference between Log & Gamma

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