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FORUMS Post Processing, Marketing & Presenting Photos RAW, Post Processing & Printing 
Thread started 25 May 2012 (Friday) 21:02
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Gamma Correction

 
Glenafton
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May 25, 2012 21:02 |  #1

Can someone explain to me in simple English what is "Gamma Correction" I looked it up on the net and was bombarded with mathematical equations and symbology that left me in a spin. I ask because the post processing software that I use (ACDSee pro 5) has a "Use gamma correction" and I do not know if I should or not.

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Skaperen
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May 26, 2012 02:00 |  #2

When electron beams of various levels hit the phosphors of a CRT display, the light intensity is not a linear relationship. It has an inverse power curve. Roughly speaking, as the voltage intensity goes up, the light level does not go up as much. The relationship is known mathematically. This is known as a gamma curve.

Gamma CORRECTION involves taking levels intended for light output, and producing the appropriate voltage to get this.

When computers were using CRT displays, they just output numbers and that became voltages. Correcting the gamma curve got pushed back to making image data itself.

To complicate things more (how can the world be interesting without people making it more complicated), different CRTs, and different computer products, have different gamma curves that need to be corrected in different ways. Since the gamma correction is integrated into the image file, the wrong gamma correction is often used.

With pixel value numbers rating from 0 to 255, you might think 127 to 128 is half-bright. But that's not true. Somewhere between 192 and 208 is about half bright. That's a gamma corrected pixel level.


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Glenafton
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May 26, 2012 03:31 as a reply to  @ Skaperen's post |  #3

Skaperen,
Thank you for taking the time to explain to me. As I am still confused I think that I will leave well enough alone
Glenafton



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tzalman
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May 26, 2012 04:27 |  #4

Maybe this will be more helpful; a less electronics oriented and a more photographic explanation, oversimplified but understandable:

A camera sensor absorbs light photons and outputs electrons. The relationship is linear - twice as many photons = twice as many electrons. Then the camera translates the number of electrons into digital numbers that will be used as the basis for the image. But there is a little problem - our eyes-brain combination does not see the world linearly like the camera does. Our vision is more sensitive to the shadows (because that's where the danger lurks), that is, our brain brightens up the shadows and that is the way we expect to see the world. If we see a linear photo straight out off the sensor, it looks very dark to us, so when going from the Raw stage to the jpg stage then camera's processor automatically alters the tonal relationships. It has been discovered that the very same gamma curve that characterizes CRT tones is also pretty close to what our brain does to input from the eyes, so that curve is used. For the sake of uniformity, it is today usually the curve for gamma = 2.2.

Below is an example of a linear photo (the way the camera sees the world) and the same photo after gamma correction. There is also a graph showing a gamma 2.2 curve. Notice that the darker values are pulled up (brightened) and stretched rather extremely while the highlights are compressed.


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PhotosGuy
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May 26, 2012 09:01 |  #5

Glenafton wrote in post #14486698 (external link)
As I am still confused I think that I will leave well enough alone
Glenafton

Why? Go ahead & play with it. As long as you don't save the changes to the original image, you aren't going to hurt anything.


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May 26, 2012 10:08 |  #6

DPP has a fun little function in its Raw tab -- you can click a little option to see the Linear image (without the Gamma Correction). Try it and you may be shocked to see what the sensor actually collected!

In the previous versions of Lightroom you could get a similar effect by pulling Brightness all the way back. In LR 4 I'm not sure what the best way of getting things "Linear" is. But, I prefer the correction:)!


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Glenafton
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May 26, 2012 19:28 |  #7

Gentlemen,
Thank you for explaining what was to me a very clouded issue. It is somewhat clearer especially when you look at the comparison photographs. I will (Perhaps) have a play with the settings but as I am an old b****r and still afraid that I will damage things if I alter them too much
Glenafton



Camera: 5d Mkll, EOS 700d, EOS 550d, EOS 400d dedicated IR. Lenses: EF 24-70 f2.8 L. EF 70-300 f4-5.6 L IS. EF 50mm Prime f 1.4 EF 100MM F2.8 l Macro IS USM EF-S 17-85 f4-5.6 IS USM and a picture of a cheese sandwich

  
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tonylong
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May 26, 2012 20:09 |  #8

Glenafton wrote in post #14489295 (external link)
Gentlemen,
Thank you for explaining what was to me a very clouded issue. It is somewhat clearer especially when you look at the comparison photographs. I will (Perhaps) have a play with the settings but as I am an old b****r and still afraid that I will damage things if I alter them too much
Glenafton

You can't "damage" a Raw file/photo with your Raw processing software. that's part of the "power of Raw" -- you can tweak your settings all over the place and the underlying data does not change. It's called "non-destructive editing". You can process a Raw, convert/save/export to an output image file, and move on/walk away from it. You can revisit it an hour, a day, a year, 5 years later, and if you wish you can "revert" to the original settings or open it in the latest and greatest new Raw processing software, and start from the beginning, and nothing has been lost in that original Raw data!


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Skaperen
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May 26, 2012 23:49 as a reply to  @ tonylong's post |  #9

Yes, we do see "better" in shadows. But if the image reproductions is truly faithful, we should see "better" in those shadows, too. The problem is the reproduction is not truly faithful for many reasons. The gamma curve is one aspect of the failure to reproduce faithfully. Gamma correction is applied to compensate for that well known (to the technical people) error.

But for reasons of history, they way it gets applied is not convenient. It was the display device with the error in television (e.g. the CRT). But the correction was being applied at the source (camera control circuits). That means the signals and their numeric values in between do not have a linear relationship to the display levels. This started in television systems and persisted into computer systems when they first used televisions as display devices. Image files then became part of that "signal" that was not linear, as well as digital interconnection like DVI.

To make things worse, the CRTs did not all have the same curve, and different compensations were, and still are, used.

Fortunately, RAW files store the linear values. Now if only we could push linear image files further to the destination. I know PNG supports this (set gamma attribute to 1.0 when storing linear pixels). Maybe TIFF can do this, too.

But we still have a world of BMP, GIF, JPEG, PNG, and TIFF based images with non-linear gamma corrected values that were meant for correcting the effects of a CRT, which fewer and fewer people are using. LCD displays operate with different characteristics than a gamma curve. They are not linear, either, but the gamma correction does not apply. They have to do a double conversion to undo the gamma correction, then apply a correction for their own differences.


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Tiberius
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May 27, 2012 04:31 |  #10

I may be wrong and/or oversimplifying things, but I always thought that the difference between adjusting brightness and adjusting gamma was that adjusting brightness was just adding or subtracting to the brightness value of each pixel by an equal amount, whereas gamma was just altering the midtone and leaving the brightness values for highlights and shadows alone.


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tzalman
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May 27, 2012 06:52 |  #11

The old, bad Brightness in early editions of PS was as you say, a linear and equal change to each pixel, including the black and white points, #1 below. Of course it would have also been possible to have a linear but not equal brightening like #2 in which lighter pixels are brightened more. Somewhere along the line that PS Brightness was changed to one which is non-linear and preserves the white and black points, as in #3, which is equivalent to pulling the middle arrow to the left in the Levels tool. It is also possible to have curves like #4 or #5 that brighten more in the highlights or the shadows, respectively, while still protecting the end ponts. The gamma curve illustrated above is like #5 but more extreme. Its shape is determined by the equation n1 = n^1/gamma, where gamma is 2.2.


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May 27, 2012 14:16 |  #12

Probably more than everything you wanted to know about gamma:

http://www.poynton.com​/GammaFAQ.html (external link)

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