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BLACK GAMMA

Gamma camera settings black master pedestal settings

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#1 Stefano Stroppa

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Posted 09 April 2015 - 10:51 AM

I'm a little confused on how to play with the gamma setting for the lower part of the curve. 

 

what's the difference between 

BLACK LEVEL and BLACK GAMMA and MASTER PED in camera settings? 

 

what the relationship with noise? 

 

if for instance I want to change the setting of my camera so as to see more into the shadows (without taking into account any color correction in post)

 

I set (just as an example) 

black level at -2

master ped at +2 so as to lower the contrast

black gamma (that alters the whole shape of the gamma curve in its toe) at +2 to brighten the image

 

am I right? 

 

my question is: how does noise react to this new setting?


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#2 David Mullen ASC

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Posted 09 April 2015 - 11:11 AM

Black gamma is sort of the low-end version of knee manipulation, you are playing with the gamma curve in just the shadow region, changing how contrasty the shadows look.

 

Anything that is lifting up detail in the shadows is also increasing noise in the shadows so you'd probably want to use a mildly lower overall ISO rating to compensate, though if you overdo that, you'll have clipping problems in the highlights.  But that also depends on how much you reset the blacks in post back down to black.

 

I believe the Master Pedestal sets where the black gamma curve begins (the shadow version of the Knee Point), whereas Black Level (or Set-Up) sets where black begins.


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#3 Stefano Stroppa

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Posted 10 April 2015 - 04:31 AM

Thank you Mr.Mullen! 

 

What I don't understand the difference between Master Pedestal and Black Level, I feel like it's almost the same thing: if Master Pedestal works the same way as the knee point, just in the shadows, and Black Level sets where black begins, if I look into a waveform and play with both these settings what the difference? 

 

If I make the black level higher I'll see more shadow details, but it's not the same as having an high Master Pedestal, to say, more than 7.5IRE, so to see more into the shadows?


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#4 David Mullen ASC

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Posted 10 April 2015 - 10:16 AM

If you use an 11-step grey scale, the type with two rows, the second one running the reverse direction, it creates a stepped X-shape on a waveform, so it is much easier to see how the gamma, pedestal, and black level work on the image as you change them.


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#5 Jeremy Cavanagh

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Posted 10 April 2015 - 01:44 PM

If you use an 11-step grey scale, the type with two rows, the second one running the reverse direction, it creates a stepped X-shape on a waveform, so it is much easier to see how the gamma, pedestal, and black level work on the image as you change them.

Exactly the sort of chart that has been used to line up studio television cameras for years checking, blacks, gamma, knee levels and also flares.

 

The question about the difference between master pedestal/black and black level might be answered this way: black level may refer to the black level of an individual channel i.e. Red, Blue or Green while master pedestal or master black refers to changing the level of the blacks of the three camera channels together. However, again, that's for television cameras, I don't have enough experience with digital cameras used for film to know if the same language or descriptions have crossed the devide.


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#6 John E Clark

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Posted 10 April 2015 - 02:25 PM

Exactly the sort of chart that has been used to line up studio television cameras for years checking, blacks, gamma, knee levels and also flares.

 

The question about the difference between master pedestal/black and black level might be answered this way: black level may refer to the black level of an individual channel i.e. Red, Blue or Green while master pedestal or master black refers to changing the level of the blacks of the three camera channels together. However, again, that's for television cameras, I don't have enough experience with digital cameras used for film to know if the same language or descriptions have crossed the devide.

 

For me the use of 'master pedestal' is somewhat of a hold over from Analog TV. In the various signalling methods, of which NTSC/RS-170 is the one I'm most familiar with (and have been trying to forget for years...), Black on the TV screen was determined by 'no electrons hitting the phosphorous'... and so the 'black pedestal' would set the analog voltage for that level. (For example, in US NTSC, the 'pedestal' was 7.5% IRE, while the 'blanking' level was 0 IRE, and the sync pulse was -40 IRE. But NTSC-J had 0% IRE for both 'black' and 'blanking'.).

 

All of that 'disappears' with Digital, although, when creating media for TV display, one had to account for these sorts of things.

 

In film there may be a sort of analogous parameter, namely Film Base + Fog, which sets the minimum density of unexposed film, which will set the 'black' level of the negative relative to creating the 'positive' image.


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