Explain Gamma

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Explain Gamma

ben egbert
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p.1 #1 · Explain Gamma

I keep seeing gamma used. The only place I find gamma in Photoshop is one of the sliders under the exposure layer.

It has exposure, offset and gamma correction. Is this where you change gamma? It appears to be interactive with exposure. I can darken an image with the gamma slider or the exposure slider but I suspect they are doing it in a different way.

How and when do you use it. If the gamma you use is someplace else in Photoshop, where?

Jan 02, 2013 at 08:35 PM

Eyeball
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p.1 #2 · Explain Gamma

Do you use Curves, Ben? Gamma is basically using Curves and pushing up or down on the mid-point of the curve. Positive gamma will darken mid-tones and increase contrast in the upper mid-tones and highlights at the expense of contrast in the shadows. Negative gamma will lighten mid-tones and will increase contrast in the lower mid-tones and shadows at the expense of contrast in the highlights.

Personally, I normally use Curves for these types of adjustments since I can be more precise with Curves and the Curves/Histogram display gives me a little more visual feedback about what is going on. The blessing/curse aspect of Photoshop though is there are always a hundred ways to get to the same result.

Exposure moves the right-hand end-point of the curve (the white-point).
Offset moves the left-hand end-point of the curve (the black-point).
Since Exposure and Offset move the end-points, they can cause clipping when moved too much in the wrong direction.
If you play with the Exposure, Offset, and Gamma sliders while watching the histogram, you will see pretty clearly what is happening.

Here is a pretty good tutorial on Cambridge regarding Gamma, although it is not Photoshop specific and gets into issues regarding monitor gamma and raw-linear vs. raw-with gamma curve applied.

http://www.cambridgeincolour.com/tutorials/gamma-correction.htm

Jan 02, 2013 at 08:55 PM

RustyBug

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p.1 #3 · Explain Gamma

+1 @ Dennis

Exposure tweaks from the highlights
Offset tweaks from the shadows
Gamma hits them all

It is similar to levels with the middle slider being gamma also. While it does "darken" things, it mainly expands/contracts the relationship between lights vs. darks via the formula of Output Value = Input Value ^ gamma value.

IIRC ... it functions based on numeric value range of 0-1 rather than 0-255 (although conversion math exists) Such that with an input value of .5 with a gamma of 1.00 would be .5^1=.5 ... i.e. in=out.

Change the gamma to 2.0 for the same .5 input value and it would be .5^2 = .25 Change the gamma to 2.0 for a .9 input value and it would become .9^2 = .81

Thus (on a scale of 0-1) .5 would be 50% (roughly 127 @ 255 * .5) and .25 would be 25% (roughly 64 @ 255 * .25). .9 * 255 = 230, .81 * 255 = 206. So, while our 127 input value darkens by 63, our 230 value only darkens by 24, hence the non-linear adjustment via the exponential calculation.

Correspondingly, if our input value was .5 and gamma of .5 applied @ .5^.5 =.71 (roughly 255 * 71% = 180).

Curves, levels and gamma can serve the same function, but if you'll notice (easy to watch in levels) the endpoint adjustments are additive/subtractive in linear intervals. Gamma is exponential.

We mostly use the 0-255 scale but it corresponds to the 0-1 scale ... which you might also notice if you ever go into 32 bit mode it changes to 0-1 scale during certain functions.

NOTE: different modes apply gamma inversely where 2.2 in one mode might have the same effect as .45 in a different mode. I think it is linked to how sometimes PS goes from dark to light vs. light to dark in different modes (LAB being the base iirc)

Well ... you asked.

Jan 02, 2013 at 09:47 PM

ben egbert
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p.1 #4 · Explain Gamma

Hmm, now I have to ask, why do we say gamma when we mean to adjust curves or levels?

Here is what got me going on this. People here often speak of adjusting gamma and I ran into this article where somebody boosted gamma by 3. I wanted to know what they actually did.

I tried to see how my 1DS-mk3 does in this test but not knowing how to adjust gamma by 3 I was stuck.

I need to study your math.

http://www.petapixel.com/2012/12/13/canon-6d-and-5dmk3-noise-comparison-for-high-iso-long-exposures/

Jan 02, 2013 at 10:42 PM

ben egbert
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p.1 #5 · Explain Gamma

Eyeball wrote:
Do you use Curves, Ben? Gamma is basically using Curves and pushing up or down on the mid-point of the curve. Positive gamma will darken mid-tones and increase contrast in the upper mid-tones and highlights at the expense of contrast in the shadows. Negative gamma will lighten mid-tones and will increase contrast in the lower mid-tones and shadows at the expense of contrast in the highlights.

Personally, I normally use Curves for these types of adjustments since I can be more precise with Curves and the Curves/Histogram display gives me a little more visual feedback about what is going on. The blessing/curse aspect of Photoshop though is there are always a hundred ways to get to the same result.

Jan 02, 2013 at 11:16 PM

Eyeball
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p.1 #6 · Explain Gamma

Well, in this case he was just trying to explain in an easy and replicatable manner how he boosted the brightness of the image (and the contrast of those dark tones) to better expose the noise. He either used the Gamma Adjustment setting of the Exposure Adjustment or he moved the mid-point in Levels. As Rusty pointed out, they are the same and use the same numeric values.

Gamma is used pretty commonly in the photographic and video worlds but it is not so commonly used among Photoshop users as a term. I suspect most Levels users don't think of changing the Gamma when they move that mid-point but that is what they are doing. I also suspect that Exposure is not a very commonly used adjustment in Photoshop these days - more for legacy support than anything else. Aunti Pode may be an exception.

Jan 02, 2013 at 11:23 PM

RustyBug

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p.1 #7 · Explain Gamma

ben egbert wrote:
Hmm, now I have to ask, why do we say gamma when we mean to adjust curves or levels?

Well, gamma & curves aren't quite the same thing.

I'm guessing that since gamma goes back to exponential profiles being applied to linear input at the "global" level for display/film purposes, the distinction of "gamma" retains global application ... whereas for curves, it is selective application of exponential adjustment. Gamma is applying a "single" uniform curve ... Curves has the capability of applying "multiple" (plural), non-uniform curves ... thus a distinction in nomenclature. Levels ... is likely a pseudo-repackage of Gamma into a 0-255 scale to coincide with typical 8 bit RGB values to make it more "user friendly" to follow.

But, this is all just a guess.

As there are several ways to do things in PS ... some of them show up duplicated in different places / modes / modules. But in building a database/software, you can't always have things named the same even if they do the same thing, depending on how it is structured.

Jan 03, 2013 at 01:28 AM

ben egbert
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p.1 #8 · Explain Gamma

Eyeball wrote:
Well, in this case he was just trying to explain in an easy and replicatable manner how he boosted the brightness of the image (and the contrast of those dark tones) to better expose the noise. He either used the Gamma Adjustment setting of the Exposure Adjustment or he moved the mid-point in Levels. As Rusty pointed out, they are the same and use the same numeric values.

Gamma is used pretty commonly in the photographic and video worlds but it is not so commonly used among Photoshop users as a term. I suspect most Levels users don't think of changing the Gamma when they move that mid-point but that is what they are doing. I also suspect that Exposure is not a very commonly used adjustment in Photoshop these days - more for legacy support than anything else. Aunti Pode may be an exception.

So how to I apply a gamma of 3? I have my test images but I still need to boost gamma to 3 to see what my images look like compared to the test shots. I am thinking about getting a 6D because I like to do long exposure high ISO but I want to make sure I would be getting an improvement.

Jan 03, 2013 at 02:40 AM

ben egbert
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p.1 #9 · Explain Gamma

RustyBug wrote:
ben egbert wrote:
Hmm, now I have to ask, why do we say gamma when we mean to adjust curves or levels?

Well, gamma & curves aren't quite the same thing.

I'm guessing that since gamma goes back to exponential profiles being applied to linear input at the "global" level for display/film purposes, the distinction of "gamma" retains global application ... whereas for curves, it is selective application of exponential adjustment. Gamma is applying a "single" uniform curve ... Curves has the capability of applying "multiple" (plural), non-uniform curves ... thus a distinction in nomenclature. Levels ... is likely a pseudo-repackage of Gamma into a 0-255 scale to coincide with typical 8 bit RGB values to make it more "user friendly" to follow.

But, this is all just a guess.

As there are several ways to do things in PS ... some of them show up duplicated in different places / modes / modules. But in building a database/software, you can't always have things named the same even if they do the same thing, depending on how it is structured.

For the sake of normal images, I probably know what I need here. I often bump midtones, I just never think of it as gamma. But I still don't know how to apply an exact 3 gamma boost.

Jan 03, 2013 at 02:41 AM

Eyeball
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p.1 #10 · Explain Gamma

Just go to Exposure Adjustment and move the Gamma Correction slider until it shows "3".

Alternatively, use a Levels adjustment and move the mid-point slider until the mid-point numeric read-out shows "3".

Jan 03, 2013 at 03:04 AM

RustyBug

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p.1 #11 · Explain Gamma

or .33 (1/3) depending on which mode you're in.

Jan 03, 2013 at 03:11 AM

ben egbert
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p.1 #12 · Explain Gamma

Eyeball wrote:
Just go to Exposure Adjustment and move the Gamma Correction slider until it shows "3".

Alternatively, use a Levels adjustment and move the mid-point slider until the mid-point numeric read-out shows "3".

Thanks just tried it. I get somewhat different results between the two methods, but either one give terrible results compared to the 6D shown in the article. I suspect there is a typo in the article.

If I had a 6D, I would simply compare results between cameras. Even the before gamma results look a lot worse.

Jan 03, 2013 at 03:12 AM

Eyeball
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p.1 #13 · Explain Gamma

You're right. For Gamma > 1, the two methods give different results. For Gamma <= 1, they are the same. It seems that the Exposure Adjustment Gamma setting moves the black point. I don't think it should do that. I wonder if that's a bug.

I suspect he used the Levels adjustment for his comparison. He doesn't explain very well what, if anything, he did in DPP before bringing the images into PS. I think you need very dark (almost lenscap-on dark) images to show what he is showing.

These noise comparison tests are always tricky. For one thing, real ISO can differ from indicated ISO among camera models. If a manufacturer is "cheating" with a real ISO that is less than indicated, the noise results can be impacted. There is also a relationship to dynamic range and the way the manufacturer is artificially setting the noise/black floor. I can, for example, raise the Blacks slider in Lightroom to eliminate most, if not all, shadow noise. The question is how much good detail did I lose in that process?

Jan 03, 2013 at 03:46 AM

AuntiPode
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p.1 #14 · Explain Gamma

FWIW, I mention gamma often because the Gamma slider on exposure adjustment is something I use frequently. I usually tweak the exposure and gamma together because they interact. There are other ways to skin the cat, but I like the exposure panel controls more than some folks.

Jan 03, 2013 at 08:13 AM

ben egbert
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p.1 #15 · Explain Gamma

I had you in mind Karen when I asked this question. I can see the two sliders ought to be worked together.

When you lighten an image with the exposure slider, do you then tweak back midtone using gamma? Or the other way around?

Jan 03, 2013 at 03:05 PM

ben egbert
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p.1 #16 · Explain Gamma

Eyeball wrote:
You're right. For Gamma > 1, the two methods give different results. For Gamma <= 1, they are the same. It seems that the Exposure Adjustment Gamma setting moves the black point. I don't think it should do that. I wonder if that's a bug.

I suspect he used the Levels adjustment for his comparison. He doesn't explain very well what, if anything, he did in DPP before bringing the images into PS. I think you need very dark (almost lenscap-on dark) images to show what he is showing.

These noise comparison tests are always tricky. For one thing, real ISO can differ from indicated ISO among camera models. If a manufacturer is "cheating" with a real ISO that is less than indicated, the noise results can be impacted. There is also a relationship to dynamic range and the way the manufacturer is artificially setting the noise/black floor. I can, for example, raise the Blacks slider in Lightroom to eliminate most, if not all, shadow noise. The question is how much good detail did I lose in that process?

The right way to do this test would be to test both cameras myself. My big deal now is a decision between the 6D which seems to be the high ISO long exposure king in the Canon camp or go for a 5dm3. Or just stay with my 1ds3 and wait for the high MP camera in a couple years.

I am on the cusp of quitting so I am not sure I could last 2 years. The 6D would possibly open up night photography and give me something different to do. But I am not sure I trust this test.

Jan 03, 2013 at 03:09 PM

RustyBug

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p.1 #17 · Explain Gamma

Ben,

Rather than looking at new bodies ... have you considered the use of a tracking device?

It would open up more opportunities for you, mitigate some technical problem areas ... and not be prone to "obsolescence" with subsequent technological advances of future generation camera and chasing the holy grail. Sort of in the "right tool" for the job realm rather than trying to fit the square peg into the round hole.

I think a tracking device would open up a whole new world for you.

Edited on Jan 03, 2013 at 04:46 PM · View previous versions

Jan 03, 2013 at 04:41 PM

ben egbert
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p.1 #18 · Explain Gamma

I am not doing astro work but moonlight shots which can freeze the stars in 25 seconds UWA down to 10 seconds at longer FL. Its a contest between time ISO and aperature. The sweet spot would be f4, ISO1600 and less than 20 seconds depending on focal length.

I am more interested in the landscape itself with the stars just a secondary element. I have to use a lot of NR now and it looks ok at web size but does not print.

I also need an upgraded live view for manual focusing.

Edited on Jan 03, 2013 at 04:52 PM · View previous versions

Jan 03, 2013 at 04:45 PM

RustyBug

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p.1 #19 · Explain Gamma

With a tracking device ... the "less than 20 seconds depending on focal length" is a limiting barrier than gets removed from the equation. This can translate into different ISO / aperture options.

Just "food for thought".

Jan 03, 2013 at 04:51 PM

ben egbert
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p.1 #20 · Explain Gamma

But how do I get the landscape if I am tracking? All the stationary stuff would move. I still need a long exposure to get enough light for the landscape portion. I suppose I could track the sky and do a long exposure of the land and then clone in the sky.

But I consider cloned in elements a back up plan not a primary plan.

My plan is not that hard. I am about one stop of ISO performance away from it now. The real problem is not just ISO, its amp glow. The older cameras had a lot more of it. This does not show up so much in testing just for ISO because the high ISO shots are typically much faster shutter speeds.

Edit:

I just re-read the article. The 5D-m2 he has been using and which he shows in the test was astro converted already. I missed it. My 1DS-mk3 is not astro converted. One of the things they do is add a heat sink to the sensor to reduce amp glow. I was expecting my 1DS-mk3 to at least match the 5dm2 he tested. But sans astro conversion I have no reason to expect that.

It looks like the new gen cameras have addressed amp glow. The 5D3 is starting to look better. I am not looking for an astro camera, just one that can produce print size images at ISO1600 with long exposures.

I also have serious doubts about a high MP camera. It will probably be in the 1DX price range and I am through paying that much for a camera. 20MP is sufficient for the print sizes I can print.

Also, this probably should have been posted at the gear forum, but originally I was just interested in gamma.

Jan 03, 2013 at 04:57 PM

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