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Author Topic: Brightness vs Exposure - What's the Difference?  (Read 20420 times)
pflower
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« on: July 09, 2009, 01:29:34 PM »
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Idle thought but ...

Any raw files generated by a digital camera are automatically imported by LR with a Brightness of +50. ( I am sure there is a way to disable that but that is not my question)   Sometimes this is fine as far as I am concerned, sometimes the Brightness needs to be reduced.  Depends upon the picture.

But what exactly does "Brightness" do?  If I reduce Brightness to zero but then increase Exposure I can't get precisely the same effect.  

So does anyone know what "Brightness" does as opposed to increasing exposure?
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Andrew Fee
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« Reply #1 on: July 09, 2009, 01:54:05 PM »
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Exposure sets the white clipping point, brightness moves the midtones and tends to avoid clipping.

Personally, I tend to set exposure/blacks, put brightness/contrast to zero and use the tone curve to adjust the image.
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pflower
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« Reply #2 on: July 09, 2009, 05:11:26 PM »
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Quote from: Andrew Fee
Exposure sets the white clipping point, brightness moves the midtones and tends to avoid clipping.

Personally, I tend to set exposure/blacks, put brightness/contrast to zero and use the tone curve to adjust the image.

Thanks for that.  Actually I do exactly the same thing, but I just wondered.


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Guillermo Luijk
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« Reply #3 on: July 09, 2009, 05:34:39 PM »
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Exposure and Brightness are the same for the shadows, a linear scaling of the RGB channels. After that, Brightness starts to preserve the highlights by gradually compressing/expanding the high tones, while exposure linearly blows information in exactly the same way as the camera would do. Brightness never blows information, even if ACR's clipping warning activates; that happens just because the warning works with a threshold.

All this is much better seen by reverse-engineering the effect of the Brightness and Exposure sliders of ACR in terms of curves:

Exposure


Brightness


By plotting them together it's easy to see that in the shadows and low mid tones:
 * Exp +1EV equals Brightness +50
 * Exp +2EV equals Brightness +100
 * Exp +3EV equals Brightness +150
...



Both controls are additive, so Exposure +4EV plus Brightness +150 will lift the shadows by 7 stops.

For more info on these and other adjustments, both in ACR and PS: HOW IMAGE LEVELS ARE AFFECTED BY ACR AND PS. I was recently told that the brightness control in PS was lately (CS4, possibly CS3 too) redesigned, probably getting closer to ACR's brightness.

Regards
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Panopeeper
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« Reply #4 on: July 09, 2009, 07:44:04 PM »
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Once I asked about the exact relationships between the adjustments on the related Adobe forum. The answers were from useless blathering to plain nonsense; so I created a template to observe the effects. Now I redid it a bit better.

The raw file http://www.panopeeper.com/Download/SampleF...tments_Grey.dng contains a graduated gray image (generated, not shot). If one loads this in ACR and leaves all adjustments as they are, then one gets a graduated grey image with a nice histogram, except in the black corner; I have not managed to smoothen it out.

Now, pick five-six sample points under each other, from the very top to the very botton and do different adjustments. The sample changes and the histogram show the effect.

ADDED: the color space must be sRGB
« Last Edit: July 09, 2009, 09:13:06 PM by Panopeeper » Logged

Gabor
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« Reply #5 on: July 09, 2009, 08:07:47 PM »
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Quote from: GLuijk
Exposure and Brightness are the same for the shadows, a linear scaling of the RGB channels. After that, Brightness starts to preserve the highlights by gradually compressing/expanding the high tones, while exposure linearly blows information in exactly the same way as the camera would do. Brightness never blows information, even if ACR's clipping warning activates; that happens just because the warning works with a threshold.

All this is much better seen by reverse-engineering the effect of the Brightness and Exposure sliders of ACR in terms of curves:

Guillermo,

An excellent demonstration--the best I've seen. Do you have a similar analysis for the recovery slider?

Bill
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madmanchan
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« Reply #6 on: July 09, 2009, 10:27:00 PM »
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Nice work, Guillermo.

Another distinction: negative Exposure values will also invoke CR/LR's highlight recovery. Not true of Brightness; there is no connection between Brightness and CR/LR's highlight recovery logic.

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Schewe
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« Reply #7 on: July 09, 2009, 11:55:03 PM »
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Quote from: GLuijk
I was recently told that the brightness control in PS was lately (CS4, possibly CS3 too) redesigned, probably getting closer to ACR's brightness.

The Brightness and Contrast adjustment was indeed changed in CS4. Both sliders were changed to behave more logically (not sure if its the same logic as ACR cause ACR is doing things in linear). CS4's adjustment has a "Legacy" checkbox in case you liked the old logic. Don't matter much to me cause I only ever used B&C on channels not images.
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Panopeeper
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« Reply #8 on: July 09, 2009, 11:59:57 PM »
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Quote from: madmanchan
Another distinction: negative Exposure values will also invoke CR/LR's highlight recovery
What does this mean in clear-text regarding an image, which does not have clipped raw pixels? What about accurate ETTR and pulling back the "exposure" in the raw conversion?
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Gabor
bjanes
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« Reply #9 on: July 10, 2009, 06:25:27 AM »
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Quote from: madmanchan
Nice work, Guillermo.

Another distinction: negative Exposure values will also invoke CR/LR's highlight recovery. Not true of Brightness; there is no connection between Brightness and CR/LR's highlight recovery logic.

Eric,

Now that you have brought up the topic of highlight recovery, can you tell us when one should use the ACR recovery slider and negative exposure for images with burned out highlights? My own take is that one should use negative exposure if the whole image is too light from overexposure and the highlights are also burnt. In this case a linear correction is used. Recovery is best used for high dynamic range subjects where the overall exposure is correct, but the highlights are clipped because they exceed the dynamic range of the camera. In this latter case, the correction is non-linear, affecting mainly the highlights.

Bill
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madmanchan
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« Reply #10 on: July 10, 2009, 09:03:19 AM »
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Quote from: Panopeeper
What does this mean in clear-text regarding an image, which does not have clipped raw pixels? What about accurate ETTR and pulling back the "exposure" in the raw conversion?

Gabor, if the image has no clipped raw pixels, then from CR/LR's point of view, there is no highlight recovery to perform. Therefore, using negative Exposure compensation in CR/LR does not perform highlight recovery.
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madmanchan
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« Reply #11 on: July 10, 2009, 09:09:19 AM »
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Quote from: bjanes
Now that you have brought up the topic of highlight recovery, can you tell us when one should use the ACR recovery slider and negative exposure for images with burned out highlights? My own take is that one should use negative exposure if the whole image is too light from overexposure and the highlights are also burnt. In this case a linear correction is used. Recovery is best used for high dynamic range subjects where the overall exposure is correct, but the highlights are clipped because they exceed the dynamic range of the camera. In this latter case, the correction is non-linear, affecting mainly the highlights.

Bill, yes, that is how I would describe the distinction, too.  As you know, in earlier versions of CR, there was only the Exposure slider, but no Recovery slider. Thus the only way to invoke highlight recovery was to use negative values for Exposure ... which, clearly, darkens the image overall. Hence, using Exposure purely for highlight recovery purposes has a very large side effect, which is that all pixels become darker, even ones in the shadows. This is a good thing if reducing overall exposure is one of your desired goals as well. However, if you like the midtones and shadows where they are already, then using negative Exposure will make them too dark. In the past, increasing Brightness was the way to bring them back.

Using the newer Recovery slider is rather like decreasing Exposure and increasing Brightness. When you increase Brightness, you cause the highlights to compress. This means that highlights will have less contrast. This is clearly indicated in Guillermo's Brightness plots.
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Guillermo Luijk
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« Reply #12 on: July 10, 2009, 04:13:10 PM »
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Quote from: bjanes
Guillermo,

An excellent demonstration--the best I've seen. Do you have a similar analysis for the recovery slider?

Thanks Bill. I haven't that analysis since I am still using CS2 and a very old version of ACR (yes, I am so lazy to update the endlessly changing software).

But as Eric indicates in some of the previous posts, highlight recovery (activated over blown areas when exposure is set to negative values, or by the recovery sliders in modern versions of ACR), cannot be properly modelled in terms of curves since it is not such a simple operation, but 'something else'. Highlight recovery consists of some kind of replicating information from the non-blown channel/s in an area into the clipped ones, usually in neutral (R = G = B ) colours to preserve texture in a conservative non-coloured fashion. DCRAW's highlight recovery for instance tries to go further imitating the surrounding colour, but not always succeeds.

ACR left, DCRAW right



BTW Eric, can you give an explanation to the slightly unexpected behaviour in the deep shadows of the Exp/Brightness curves? thery are not 100% straight lines near 0. Has this some relation to the sRGB gamma curve?

Regards.
« Last Edit: July 10, 2009, 05:15:58 PM by GLuijk » Logged

Colorwave
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« Reply #13 on: July 10, 2009, 10:23:09 PM »
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Quote from: GLuijk
(yes, I am so lazy to update the endlessly changing software).
LOL, Guillermo.  You certainly don't fit my interpretation of the word lazy.  Your work and insight is very much appreciated, as is the rest of the brain trust that has already weighed in here in Eric, Jeff, Gabor, etc.  This is quite a resource for the inquiring mind.
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Peter_DL
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« Reply #14 on: July 11, 2009, 04:24:00 AM »
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Quote from: madmanchan
Using the newer Recovery slider is rather like decreasing Exposure and increasing Brightness. When you increase Brightness, you cause the highlights to compress. This means that highlights will have less contrast. This is clearly indicated in Guillermo's Brightness plots.
Another consequence of compression (beneath the non-linear section of such Brightness curve) is a loss of color saturation.
Unfortunately, the Recovery slider i.e. the combination of Recovery + Exposure do not seem to be free of this effect at all.


For the following test, the Red patch of a ColorChecker capture was driven close to clipping by means of the Exposure slider:
Exposure 2.4, Recovery 0, Brighness 0, Contrast 0, Curve Tab 0, Profile: base matrix.
HSB (pRGB)= 13/55/98 measured in Photoshop

Recovery was set to 25, then Exposure was raised again close to clipping:
Exposure 3.3, Recovery 25, Brighness 0, Contrast 0, Curve Tab 0, Profile: base matrix.
HSB (pRGB)= 12/40/98

Now Recovery was dropped, Exposure was reset and Brightness was raised (based on a mid gray for reference) to an overall similar output:
Exposure 2.4, Recovery 0, Brighness 40, Contrast 0, Curve Tab 0, Profile: base matrix.
HSB (pRGB)= 11/41/99

You may note that there’s a comparable major drop in highlight saturation.


Finally, I’m not using Recovery a lot, just a bit after Exposure, Brightness + Contrast and the Curve Tab are already set.
It’s still among my #1 reasons to edit in Photoshop (see true recovery).

Peter
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bjanes
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« Reply #15 on: July 11, 2009, 09:28:31 AM »
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Quote from: GLuijk
Thanks Bill. I haven't that analysis since I am still using CS2 and a very old version of ACR (yes, I am so lazy to update the endlessly changing software).

But as Eric indicates in some of the previous posts, highlight recovery (activated over blown areas when exposure is set to negative values, or by the recovery sliders in modern versions of ACR), cannot be properly modelled in terms of curves since it is not such a simple operation, but 'something else'. Highlight recovery consists of some kind of replicating information from the non-blown channel/s in an area into the clipped ones, usually in neutral (R = G = B ) colours to preserve texture in a conservative non-coloured fashion. DCRAW's highlight recovery for instance tries to go further imitating the surrounding colour, but not always succeeds.

Guillermo,

Since you have not upgraded PS, I gather that ACR is not your main raw converter. IMHO, the improvements in ACR are alone are worth the upgrade price if you make much use of ACR.

As you indicate, highlight recovery with color data is complex, but I've done a few simple tests with a monochrome image using the green channel. These are not as exhaustive as your tests, but do give some useful data. I photographed a step wedge that was overexposed and rendered in ACR into sRGB using an exposure value of -0.5 EV to account for the BaselineExposure that ACR uses for my camera (personally, I do not like this "feature" of ACR). The top 3 steps (each 0.1 EV) are blown.

[attachment=15306:D3_Stouf_0003.png]

I then used exposure and recovery in an attempt to correct the overexposure. The results are shown graphically. As expected, exposure brings down all values by an equal value, whereas recovery affects the highlights and leaves step 10 alone.

[attachment=15307:Exposure.gif]

[attachment=15308:Recovery.gif]

Since a gamma curve has been applied by ACR, a log plot shows the relationships more accurately:

[attachment=15309:ExposureLog.gif]

[attachment=15310:RecoveryLog.gif]

Here is the raw histogram as shown by Rawnalize.

[attachment=15311:RawHistogram.gif]


Bill
« Last Edit: July 11, 2009, 09:36:10 AM by bjanes » Logged
madmanchan
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« Reply #16 on: July 11, 2009, 09:43:16 AM »
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Quote from: DPL
Another consequence of compression (beneath the non-linear section of such Brightness curve) is a loss of color saturation.
Unfortunately, the Recovery slider i.e. the combination of Recovery + Exposure do not seem to be free of this effect at all.

Yes, that is a known characteristic of Camera Raw's tone curve implementation: when the tone curve is applied, areas that become darker have increased saturation; areas that become lighter have decreased saturation.
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madmanchan
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« Reply #17 on: July 11, 2009, 09:47:26 AM »
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Quote from: GLuijk
BTW Eric, can you give an explanation to the slightly unexpected behaviour in the deep shadows of the Exp/Brightness curves? thery are not 100% straight lines near 0. Has this some relation to the sRGB gamma curve?

Guillermo, I am not sure, to be honest.

What was your Point Curve setting when you made these plots?
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Guillermo Luijk
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« Reply #18 on: July 11, 2009, 10:49:16 AM »
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Quote from: bjanes
Since you have not upgraded PS, I gather that ACR is not your main raw converter. IMHO, the improvements in ACR are alone are worth the upgrade price if you make much use of ACR.
You are probably right Bill, but the point is that I am not using any commercial RAW converter now. Time ago I found out that a totally neutral RAW development (like the one produced by DCRAW in 16-bit mode or ACR with _all_ settings to 0) is fine for me as a starting point. After that I just apply a couple of curves (some samples here), and believe me I don't miss anything else to fiddle with. I know this is a quite strange workflow, but it's OK for me. I am not a compulsive shooter so I can afford it.

Quote from: bjanes
I photographed a step wedge that was overexposed and rendered in ACR into sRGB using an exposure value of -0.5 EV to account for the BaselineExposure that ACR uses for my camera (personally, I do not like this "feature" of ACR).
I don't think there is any BaselineExposure in ACR. ACR simply can blow information when the exposure slider is set to 0 because of the white balance, which it seems to apply in a destructive way, i.e. with >=1 multipliers. To compensate for this overexposure due to WB, you need to add some negative exposure correction, not being this a problem at all. And it will not always be -0.5EV; some images may need -1EV, some others may be well served with -0.3EV depending on the WB applied and the particular exposure achieved in all three channels in the RAW data.


Eric my curve was set to totally linear for the reverse engineering tests. It seems as if ACR (and PS too) applies some extra lifting in the deep shadows when increasing the overall exposure, but no idea why.

Regards
« Last Edit: July 11, 2009, 11:24:48 AM by GLuijk » Logged

digitaldog
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« Reply #19 on: July 11, 2009, 12:03:25 PM »
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Quote from: GLuijk
BTW Eric, can you give an explanation to the slightly unexpected behaviour in the deep shadows of the Exp/Brightness curves? thery are not 100% straight lines near 0. Has this some relation to the sRGB gamma curve?


There's something "not quite right" with the ACR highlight recover too, which I think I'm seeing here and I've seen too (and reported to Eric). In the example below, using an ETTR capture, with both ACR and Raw Developer (awesome in terms of rendering quality), you can see that odd effect from using recovery. In the cement, it looks very similar the skin issue I see above.
« Last Edit: July 11, 2009, 12:04:11 PM by digitaldog » Logged

Andrew Rodney
Author “Color Management for Photographers”
http://digitaldog.net/
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