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Dynamic Range vs Highlight Rolloff


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#1 Macks Fiiod

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Posted 03 July 2018 - 11:16 PM

Does "dynamic range stops in the highlights" and "highlight roll off" mean the same thing?

 

Or in theory, could a camera with 14 stops have worse highlight rolloff than a camera with 10?

 

Thanks.


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#2 aapo lettinen

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Posted 04 July 2018 - 12:54 AM

I think people normally mean the visual highlight gamma curve behaviour and saturation rolloff by the "highlight rolloff" term... the way the highlights clip rather than how many stops there is total. basically how the last 1 or 2 highest stops behave visually. this can usually be tweaked in grading as well so it is somewhat subjective. 

 

So yes the 14 stops camera can have "worse highlight rolloff" than a camera with 10 if the person thinks that the highlight clipping is not "smooth and nice looking" by his/her terms....people talk about "filmic video cameras" or "filmic digital cinema cameras" which usually means that the highlight rolloff is somewhat smooth looking and has a pleasant saturation transition as well (not like in for example pana af100 where one could have coloured clipping in highlights)


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#3 aapo lettinen

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Posted 04 July 2018 - 01:03 AM

for example, one can lower the highlight saturation smoothly by a curve in grading and smooth the highest part of gamma curve as well so that the clipping is not that hard looking and the highlights run lower and lower in saturation before they actually clip. this alters the "highlight rolloff" and makes the image "more filmic looking". or "more Alexa looking" if you will. 

One of the trick of the colorists here is to diffuse or blur the hard clipping highlights just a tiny bit so that they seem VISUALLY to be clipping softer and smoother, works very well most of the time. 

 

so it is kind of subjective and can be altered in post as well


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#4 David Hessel

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Posted 04 July 2018 - 09:25 AM

To add to what has already been said, most people find that film has a very pleasing highlight rolloff. With film in the mid tones the films response to light is almost linear meaning that an increase in scene brightness will have a similar increase in brightness of the final captured film. Has you move towards the highlights it takes larger and larger amounts of light to have an increase in visual brightness of the film.

 

For example in the middle ranges increasing the amount of light so that something in the real world appears twice as bright will also double the apparent brightness in the captured film. I believe that adding 4x more light will make the mid tones appear 2x as bright. As you move into the highlights it takes more light change to cause the same amount as visual change in the captured image. So as you get brighter it will take 8x as much light to make 2x visual difference, then 16x, then 32x, on and on until adding light no longer adds any increase in captured brightness. It is almost as if the highlights never actually clip its just no amount of additional light will have any visual effect.

 

With digital the sensors response to light is almost perfectly linear through out the range. They don't have the loss in sensitivity in the highlights that film has so the captured image just keeps getting bright and brighter until it suddenly hits the max and clips. In the final image you can see a sharp edge in highlights where it goes from non-clipped to clipped which looks artificial. Our eyes naturally behave like film.

 

Manufactures have built in processing to mitigate this effect and if you have a shot that has more captured DR than delivery you can adjust the rolloff in post to a degree, resolve has a tool for that. I would almost always go with less DR and better rolloff than more DR with harsh clipping.


Edited by David Hessel, 04 July 2018 - 09:26 AM.

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#5 Phil Rhodes

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Posted 04 July 2018 - 09:40 AM

I think part of this discussion is the reality that high dynamic range can be used in order to create nice highlight rolloff. That can be done by the manufacturer, in-camera, as is done with things like Alexa, or it can be done in grading, but ultimately the result is much the same. Highlight compression has always been a factor given limited-dynamic-range displays, which have long had far less range than the cameras. The thing is, it remains entirely relevant now as a way of making clipped areas - which are inevitable, even desirable, in order to give the viewer a contrast reference - more acceptable.

 

Film does this automatically. Electronic cameras can do this automatically, or they may be made to do it afterward.

 

So really they're separate but linked concepts. Reversal film has less dynamic range than most modern electronic cinema cameras but it still has subjectively "nice" highlights.


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#6 David Mullen ASC

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Posted 04 July 2018 - 10:09 AM

Yes, it helps to have more stops of exposure information if you are going to apply some form of knee compression to roll-off highlights more gradually.  This gets harder to do if your camera only can capture a narrow range to begin with because you're talking about working with the area above normal "white" on an 11-step chip chart, what are sometimes called "super whites".  So if your camera only records 10-stops and a white t-shirt in the scene is basically just below 100 IRE, there isn't more information to then compress into a gradual roll-off for things like hot lampshades and windows, objects that are a couple of stops brighter than the white t-shirt in the shot.


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