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Thread started 08 May 2012 (Tuesday) 14:54
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Red blown out number in LR

 
weegee
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May 08, 2012 14:54 |  #1

I've seen some comment that 240 on the red scale is about the limit of being blown out. LR uses a different method, percentages. Does anyone know the equivalent. I did some simple math 240/255 to get 94%, but I'm not a tech head when it comes to this, so not sure if it's as simple as that.


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René ­ Damkot
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May 12, 2012 06:57 |  #2

LR uses MelissaRGB internally, so unless you are softproofing in LR4, the clipping warning will be baed on that.

http://www.getcolorman​aged.com …management/clip​warninglr/ (external link)


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tzalman
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May 12, 2012 08:39 |  #3

René Damkot wrote in post #14420069 (external link)
LR uses MelissaRGB internally, so unless you are softproofing in LR4, the clipping warning will be baed on that.

http://www.getcolorman​aged.com …management/clip​warninglr/ (external link)

And if you are softproofing the numbers will change to 0-255 values. 250 @ sRGB us a better white point than 240.


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Wilt
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May 12, 2012 09:08 |  #4

René Damkot wrote in post #14420069 (external link)
LR uses MelissaRGB internally, so unless you are softproofing in LR4, the clipping warning will be baed on that.

http://www.getcolorman​aged.com …management/clip​warninglr/ (external link)

"Lightroom uses MelissaRGB internally (ProPhoto RGB with sRGB Tone Response Curve)."...head HURTS! :cry:

Can someone turn that statement into something that a 6th grader can understand?!


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tzalman
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May 12, 2012 11:25 |  #5

Wilt wrote in post #14420417 (external link)
"Lightroom uses MelissaRGB internally (ProPhoto RGB with sRGB Tone Response Curve)."...head HURTS! :cry:

Can someone turn that statement into something that a 6th grader can understand?!

In order to retain the maximum color range the camera can capture, LR does all its editing in the widest color space, ProPhoto RGB. However, working color space profiles usually incorporate a tone curve for gamma correction. ProPhoto has a gamma 1.8 curve. But one of the advantages of Raw processing is that the editing can be done with the tonal values still in the linear relationships in which the sensor captured them. So Adobe created a space called Melissa RGB with ProPhoto's gamut but without the tone curve.

However, there is one little problem. A linear image is very dark and flat. If that image were displayed a lot of people would be very upset. So LR applies a tone curve to the display data sent to the monitor, just before converting it to your monitor space. The histogram is also based on that gamma corrected data. The tone curve that is used is borrowed from sRGB which uses a curve that is sort of gamma 2.2 but not exactly (in the darkest shadows it is linear, 1.0, and at the top it is 2.4, but most of its length has the 2.2 curve.) So we get ProPhoto primaries and the sRGB TRC.


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May 12, 2012 11:33 |  #6

Thank you, Elie! ( am nevertheless glad that my reading comprehension is better than a 6th grader!)


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weegee
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May 13, 2012 02:10 |  #7

hmmmm. Maybe I asked the wrong question then.

Is there a way to tell you are over exposed to blown out levels in LR3? Can you read one of the RGB percentage values or anything else to give you an indication?


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tzalman
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May 13, 2012 06:14 |  #8

weegee wrote in post #14423673 (external link)
hmmmm. Maybe I asked the wrong question then.

Is there a way to tell you are over exposed to blown out levels in LR3? Can you read one of the RGB percentage values or anything else to give you an indication?

Not exactly, only approximately. As said above, LR works in a wide gamut space. When you export you will most likely export to a medium (Adobe RGB) or narrow (sRGB) space. Think of it like a ball-park. The same hit will drop in for a single in ProPhoto Stadium, be off the wall for a double in Adobe Stadium and out of the park in sRGB. (Except in our game home runs are a bad thing and doubles are undesirable.) A tone that is 240 in LR might become 248 or 253 in sRGB, depending on the color. The problem is there are no simple formulas for determining how much the shift will be because the distances from the (grey) center of each space to the red, green or blue primaries that define the limits of the gamuts are very different. In the illustration below the difference between ProPhoto RGB and sRGB in the green direction is huge and in the red direction sRGB and Adobe RGB are virtually identical and the difference from ProPhoto RGB is smaller. That means that Red 240 (94%) in LR might be Red 245 in sRGB, although Green 240 would be clipped in sRGB.

That is one off the reasons why softproofing in LR4 is so welcome. In LR3 the best advice is to leave some head-room above your white point, how much exactly is impossible to say. I always used to set the white point at around 96% and if it was important to check the histogram of the rendered image in a pixel editor and if necessary do a levels adjustment.


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weegee
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May 13, 2012 11:11 |  #9

ah ha. OK, now I understand.

So with the soft proofing on LR4 as you convert it to SRGB or Adobe RGB, you can then read the RGB values on the 255 scale and make sure you're capping at 240?

I guess the problem is like you said you don't know what the new numbers will be after the conversion, so you might have to go back and forth a couple times.


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Red blown out number in LR
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