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Author Topic: Exposure Slider and Histograms  (Read 7359 times)
RobertBoire
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« on: December 29, 2011, 11:08:27 PM »
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Hi,

Some of the posts in a recent thread on ETTR got me to puzzling over how the Exposure slider works in general in LR. The general idea being that ETTR increases the amount of light received at the sensor and that  the "over-exposure" that occurs (assuming the highlights are not blown) can be corrected in post by decreasing the exposure with the exposure slider.

Now if each adjustment of +/- 1 Ev in post is equivalent to +/- one stop on the camera, I would have thought that adjusting the exposure slider should simply shift the entire histogram left or right - ie shift the values from one level to another without changing the relative values of the peaks themselves. This is clearly not what happens. Adjusting the exposure changes not only the position but the shape of the histogram as well - even when there is room at the high and low ends so that clipping is not an issue.

What am I missing?

And it the exposure slider is adjusting...exposure.. then what is the brightness slider adjusting?

Thanks
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Bryan Conner
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« Reply #1 on: December 30, 2011, 05:31:39 AM »
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According to "Real World Camera Raw with Adobe Photoshop CS5", the brightness slider basically "lets you redistribute the midtone values without clipping the highlights or shadows." 

I highly recommend purchasing this book for the version of Photoshop that you are working with.  I have purchased one each time I purchased Photoshop beginning with CS.  It is cornucopia of information for Camera Raw users.
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NikoJorj
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« Reply #2 on: December 30, 2011, 08:46:36 AM »
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What am I missing?
The fact that the histogram is gamma encoded maybe, with a sRG gamma that differs from what I'd like to think as the log2 nature of an exposure shift?
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Nicolas from Grenoble
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Guillermo Luijk
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« Reply #3 on: December 30, 2011, 10:36:46 AM »
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A log histogram (i.e. an histogram in EV divisions) actually shifts left/right with exposure. These are real log histograms from RAW files obtained at 1EV intervals:




Unfortunately LR, ACR or any camera's histograms are not logarithmic, so you can simply not expect the previous behaviour from them. It's just a matter of how the X-axis scale is chosen, or in other words a matter on the low degree of intelligence of the developers for not including such a trivial option in their software (this excludes our forum member Eric Chan).

Regards
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RobertBoire
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« Reply #4 on: December 30, 2011, 12:03:34 PM »
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Thanks Guillermo, very interesting.

So presumably shifting the exposure slider is equivalent to adjusting f-stop (speed, ISO) in the camera itself.

A question though..

I noticed that red and green separate as the EV is decrease.. though their relationship to each other and the blue stay about the same. Why?

Also what are the patches on the right?

Would you mind telling me how you produced this histogram?
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Guillermo Luijk
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« Reply #5 on: December 30, 2011, 12:14:32 PM »
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So presumably shifting the exposure slider is equivalent to adjusting f-stop (speed, ISO) in the camera itself.

A question though..

I noticed that red and green separate as the EV is decrease.. though their relationship to each other and the blue stay about the same. Why?

Also what are the patches on the right?

Would you mind telling me how you produced this histogram?

Yes, thanks to sensor linearity, the only difference between changing exposure in the RAW developer and in the camera is SNR (DOF and motion blur considerations aside). Pushing exposure through aperture/shutter, or even through ISO improves SNR, while pushing exposure in software doesn't.

These histograms are RAW histograms obtained from a series of shots against a white wall, that is why they are so narrow (very low dynamic range scene). The G channel is the winner because it usually is. Green photosites receive more light for two reasons: in real life there is more power in that area of the spectrum, and camera colour filters are weaker for the G channel. The patches on the right indicate the colour of the image without white balance applied, that is why it it greenish.

This would be a more typical log histogram, and the effect of altering exposure in the camera:



It corresponds to this image:



Regards
« Last Edit: December 30, 2011, 12:20:57 PM by Guillermo Luijk » Logged

RobertBoire
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« Reply #6 on: December 30, 2011, 01:10:11 PM »
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RAW histograms obtained from a series of shots against a white wall


Now I am not sure I got it...

Did you change the exposure in camera or is it a single shot with exposure changed in post processing?

I tried a crude version of my own by taking a shot against a wall and then changing exposure in LR. The colors never separated. Would this be because LR does not have a log axis?

Regards
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Guillermo Luijk
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« Reply #7 on: December 30, 2011, 01:50:31 PM »
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Now I am not sure I got it...

Did you change the exposure in camera or is it a single shot with exposure changed in post processing?

I tried a crude version of my own by taking a shot against a wall and then changing exposure in LR. The colors never separated. Would this be because LR does not have a log axis?

Sorry, I didn't give you enough information. I did a series of shots, changing exposure (shutter). The RAW files obtained were developed in a very special way, which can be considered RAW in terms of calculating the RAW log histogram:
  • No white balance applied (-r 1 1 1 1): this means each RGB channel keeps its captured relative exposure.
  • No output colour profile conversion (-o 0): this means the RGB channels do not get mixed on each other (in a regular RAW development, each of the three R'G'B' output channels is a function of all three RGB captured input channels).

To do this forget about using any commercial RAW developer, since they do not allow any of the two things above, all of them apply a white balance and convert to some output colour profile. I did it with DCRAW. The command to do such thing is:

dcraw -v -r 1 1 1 1 -o 0 -4 -T file.cr2

The resulting TIFF's were input into Histogrammar to obtain the log histograms.

Regards.
« Last Edit: December 30, 2011, 01:54:45 PM by Guillermo Luijk » Logged

Alan Goldhammer
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« Reply #8 on: December 30, 2011, 03:21:49 PM »
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Green photosites receive more light for two reasons: in real life there is more power in that area of the spectrum, and camera colour filters are weaker for the G channel.
Not unless the laws of physics have changed since I took light and optics in college (admittedly a lot of years ago now).  The shorter the wavelength of the light, the more energy it has, so you decrease the light's energy as you move from violet at the short end of the spectrum to red at the long end of the spectrum.  Green is pretty much right in the middle (550nm).  Maybe you are using 'power' in a different way than I think about it.

Alan
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Guillermo Luijk
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« Reply #9 on: December 30, 2011, 03:56:59 PM »
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The light spectrum depends on the scene, and the max peak could not be around 550nm. But the combination of the sensor colour filters plus the entire light spectrum shape usually ends in the G channel accounting more photons than the R, and the R channel more than the B. Even in shots of a blue sky the G channel gets higher exposure than the B channel in most cameras and situations.

This is the RAW histogram:



from this scene:



The image has strictly blues and reds, but G got higher exposure than R and B.

Regards
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Alan Goldhammer
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« Reply #10 on: December 30, 2011, 05:10:20 PM »
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What you are seeing in the example you post is spectral overlap which is well documented.  It has nothing to do with "power" of green (unless of course you are taking a picture of Green Lantern).  I'm not trying to be difficult on this just making the point that it is not related to the energy of the photon which is inversely proportional to the wavelength of the light (UV stronger than infrared).

Edit Added - see THIS for the spectral overlap of the Leica M-8 sensor
« Last Edit: December 30, 2011, 05:17:26 PM by Alan Goldhammer » Logged

Guillermo Luijk
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« Reply #11 on: December 30, 2011, 06:14:42 PM »
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Alan, forget about the word 'power' or 'energy', it's just photon count. Just wanted to point that the G channel is the one that accounts for more photons in most real world scenes (producing higher RAW encoded levels), and this is after the combination of the spectral profile of light in usual situations plus the colour filters of the Bayer sensor.

Regards
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RobertBoire
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« Reply #12 on: December 30, 2011, 07:54:34 PM »
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The recorded intensity of the green channel may be higher, but I do not think this answers the original question.  Which was why did the red and green channels separate from the blue in the first set of histograms posted by Guillermo? Has this got something to do with turning off the WB?

BTW thanks for the details?

R
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deejjjaaaa
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« Reply #13 on: December 31, 2011, 12:45:58 AM »
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To do this forget about using any commercial RAW developer, since they do not allow any of the two things above, all of them apply a white balance and convert to some output colour profile. I

iridient raw developer is a commercial and does (it has an option to set wb using per channel multipliers, albeit does not allow different G1 and G2 multipliers, and has an option to disable conversion to some output color profile).
« Last Edit: December 31, 2011, 12:48:56 AM by deejjjaaaa » Logged
Alan Goldhammer
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« Reply #14 on: December 31, 2011, 06:59:07 AM »
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Alan, forget about the word 'power' or 'energy', it's just photon count. Just wanted to point that the G channel is the one that accounts for more photons in most real world scenes (producing higher RAW encoded levels), and this is after the combination of the spectral profile of light in usual situations plus the colour filters of the Bayer sensor.

Regards

On that we can agree! Cheesy  Thanks for the continuing interesting posts on all of this.  It would be great if camera makers could simplify everything and just record raw 'real' histograms in the camera.  Maximizing SNR will be easier to accomplish.
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Guillermo Luijk
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« Reply #15 on: December 31, 2011, 07:05:49 AM »
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why did the red and green channels separate from the blue in the first set of histograms posted by Guillermo? Has this got something to do with turning off the WB?

It's simply because the B channel is the one that collected less photons in my home's wall. In those histograms you see what the sensor saw, and the B photosites received less photons. After white balance the 3 channels would get perfectly aligned but I deliberately didn't apply WB.

This is a good example to explain why the B channel is said to be noisier than the other two. Actually it is not, what happens in most real life situations (like in my home's wall, under tungsten lighting) is that the B channel records much lower exposure than G and R, making its SNR worse (because given an ISO, SNR only depends on RAW exposure). But at the same level of exposure, all three channels have symmetrical SNR.

A way to obtain UniWB (WB cancellation) for a digital camera, is to find which precise colour produces the same RAW exposure on all three channels. It should be a colour similar to this:



Shooting at that colour without applying WB on my Canon 350D, I obtained this RAW histogram (nearly perfectly aligned channels):



Regards
« Last Edit: December 31, 2011, 07:13:15 AM by Guillermo Luijk » Logged

RobertBoire
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« Reply #16 on: December 31, 2011, 11:25:46 AM »
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Thanks. All very interesting. I will need to spend some time on the applications you provided links to.


 
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bjanes
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« Reply #17 on: January 01, 2012, 08:32:50 AM »
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The resulting TIFF's were input into Histogrammar to obtain the log histograms.

Guillermo,

I used Histogrammar ver 1.2 yesterday and it worked fine, but this morning I got a message that the program had expired and I should download a new copy. I downloaded the latest copy and got the same message. Please advise.

I am posting this message publicly since others are undoubtedly interested in using the program. Those who do find the program useful should make a donation (I have). Thanks to Guillermo for his contributions to this forum and wishes for a happy and productive new year.

Bill
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Guillermo Luijk
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« Reply #18 on: January 01, 2012, 09:14:12 AM »
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OK, I'll upload a new build ASAP. The old one will work if you set any date before 2012.
I thought I had already sent you a never-expiring copy. I will.
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bjanes
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« Reply #19 on: January 01, 2012, 09:18:30 AM »
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OK, I'll upload a new build ASAP. The old one will work if you set any date before 2012.
I thought I had already sent you a never-expiring copy. I will.


Thanks,

Bill
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