Brightness vs Exposure - What's the Difference?

[Panopeeper]

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

Have you seen this thread?
http://forums.adobe.com/message/1209993

[digitaldog]

Have you seen this thread?
http://forums.adobe.com/message/1209993

No. Thanks. These are also 5DMII shots.

[bjanes]

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.

Regards

Guillermo,

ACR definitely uses a baseline exposure. One can determine the BaselineExposure by converting the raw file to DNG and reading the exif data. The pertinent extract from the EXIF is shown below. The BaselineExposure is discussed in the DNG specification (Link to PDF).

I have found that if I use the BaselineExposure offset and set the ACR tone curve to linear, I get a fairly good representation of the raw file. For example here are the 10 steps in the image I showed previously. The table shows the ACR linear value for sRGB, the 14 bit raw value, the normalized raw value, and the normalized raw value converted to gamma 2.2 (=normalized raw ^[1/2.2] * 255). There is good agreement.

[attachment=15313:Table.gif]


Processing file D:\DataFiles\Nikon\D3\LowISO\Gabor\Jun092008_LowIso_0005.dng
Byte order: Least Significant Byte First


       33432 Copyright                      ASCII     Public Domain
       50708 UniqueCameraModel              ASCII     Nikon D3
       50721 ColorMatrix1                   SRATIONAL       9336/10000           -3405/10000              14/10000           -7320/10000          

 14779/10000      
                                                             2763/10000            -914/10000            1171/10000            8248/10000      
       50722 ColorMatrix2                   SRATIONAL       8139/10000           -2171/10000            -663/10000           -8747/10000          

 16541/10000      
                                                             2295/10000           -1925/10000            2008/10000            8093/10000      
       50727 AnalogBalance                  RATIONAL     1000000/1000000       1000000/1000000       1000000/1000000    
       50728 AsShotNeutral                  RATIONAL      503937/1000000       1000000/1000000        677249/1000000    
       50730 BaselineExposure               SRATIONAL        -50/100        
       50731 BaselineNoise                  RATIONAL          60/100        
       50732 BaselineSharpness              RATIONAL         100/100        
       50734 LinearResponseLimit            RATIONAL         100/100        
       50736 LensInfo                       RATIONAL         280/10               1050/10                 35/10                 45/10        
       50739 ShadowScale                    RATIONAL           1/1          
       50781 RawDataUniqueID                BYTE      

[Guillermo Luijk]

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.

I wouldn't call it issue, but just highlight strategy. When a single channel is clipped in the RAW file, no matter if the other two remain intact, ACR forces neutral (R=G=B ) highlights. It recovers texture, but always in B&W.

This image displays highlight recovery in an area with RGB channels clipped in the RAW data at different locations.
Consider the position of the red cross: only the G channel was blown there but the final image was gray in tha area after highlight recovery.

ACR definitely uses a baseline exposure.

Thanks Bill, I didn't know about that. However even if that Baseline exposure were 0, you would get blown information because of WB, so an exposure correction would still be needed. To have the RAW developed without any exposure correction at all, is a fuzzy idea since WB means exposure correction. The only way to have your RAW files developed making sure no exposure correction is introduced, is to apply UniWB (1.0 multipliers) when developing, but this is a useless idea.

Regards

[digitaldog]

I wouldn't call it issue, but just highlight strategy. When a single channel is clipped in the RAW file, no matter if the other two remain intact, ACR forces neutral (R=G=B ) highlights. It recovers texture, but always in B&W.

Well whatever the strategy, I'll take that one built into Raw Developer. In the captures above, as well as other's I've produced, RD's strategy is always visually preferable.

[Guillermo Luijk]

Well whatever the strategy, I'll take that one built into Raw Developer. In the captures above, as well as other's I've produced, RD's strategy is always visually preferable.

I just feel curious Andrew, could you apply Raw Developer to my RAW file in the area I showed? and any in the thread is invited to try his own developer too.

Here is the link for download: RAW2.

Regards

[digitaldog]

I just feel curious Andrew, could you apply Raw Developer to my RAW file in the area I showed? and any in the thread is invited to try his own developer too.

Here is the link for download: RAW2.

Regards

At least on my end, that link provides a lot of garbage text. By all means, feel free to upload to my pubic iDisk:

My public iDisk:

thedigitaldog

Name (lower case) public
Password (lower case) public

Public folder Password is "public" (note the first letter is NOT capitalized).

To go there via a web browser, use this URL:

http://idisk.mac.com/thedigitaldog-Public

[Guillermo Luijk]

During the weekends I am in the coast and have not enough DSL speed for heavy up/downloading, I'll upload it next Monday.
Anyway I didn't have any notice of that RAW file not working OK, anyone else tried?

[digitaldog]

I got it (using different browser).

What's interesting so far is, when using RD, turning on clipping overlay, I can fully remove it by altering its exposure to -2 stops, +28 for highlight recovery. In ACR, no adjustment removes clipping overlay! That said, I don't see the magenta effect on your image.

The question then becomes, do I attempt to make both converters roughly produce the same rendering appearance?

[Guillermo Luijk]

What's interesting so far is, when using RD, turning on clipping overlay, I can fully remove it by altering its exposure to -2 stops, +28 for highlight recovery. In ACR, no adjustment removes clipping overlay! That said, I don't see the magenta effect on your image.

The question then becomes, do I attempt to make both converters roughly produce the same rendering appearance?

If with 'clipping overlay' you mean the clipping warning in the RAW developer, I think it's just because of the different way both warnings works, not because one RAW developer performed better than the other. ACR is known to indicate blown areas when all three channels are RAW clipped no matter what controls are adjusted, giving up to recover any info on those areas because it's impossible. No idea how Raw developer works, but I am pretty sure it will not recover anything in the windows facing outside.

Magenta effect? what do you mean? as far as I know, everytime a magenta cast appears in the highlights is a software problem: the RAW developer is considering a saturation point for the RAW data higher than the real one. That produces a magenta cast where pure white should appear because of a lack of G values due to the white balance:

Wrong sat. point:


Correct sat. point:


There are considerations about the magenta issue in the DCRAW Tutorial (see 'SENSOR SATURATION LEVEL'), and in the article about the Fuji Super CCD (see 'TONO MAGENTA EN LAS ALTAS LUCES').  


Could you please post the best image you manage to get from Raw Developer? I would like to see if it performs some colour interpolation.

Regards

[digitaldog]

Here's a quick and dirty comparison. What I tried to do was get the table to have the same degree of tonality in both but can't. In ACR/LR, I'm still not getting all the detail so to speak. I'm also seeing issues with smoothness albeit, without the magenta issues I saw on my 5DMII. It appears that ACR/LR boast saturation which I don't have a problem with. But look at the plants in the window and the beam of wood on the wall. I vastly prefer what I see in RD. Here's a JPEG but what I'll do is pop both in a folder in my iDisk. Again, this isn't super valid science because I'm sure I could spend more time with both products attempting to get a closer rendering match. There's a lot of other settings (sharpness, etc) that could be tweaked. But initially what I see is RD produces smoother, more "film like" rendering, and it appears to be able to pull out more highlight detail.

[digitaldog]

If with 'clipping overlay' you mean the clipping warning in the RAW developer, I think it's just because of the different way both warnings works, not because one RAW developer performed better than the other.

Agreed. They just play differently. That said, I seem to be able to pull more detail out of RD.

Magenta effect? what do you mean?

Sorry. Its that color issue mentioned on the forum link which I should not have adopted. But in my cement shot (and other's I've produced), its some kind of color and smoothness issue. Often magenta.

[Guillermo Luijk]

Yes, it also seems to me that RD recovers more detail. I wouldn't dare to say, but I have a feeling that perhaps RD recovers more texture in areas where just one channel remained unclipped. Anyway it could just be a difference in contrast, i.e. the info is in the ACR image as well, but needs further processing. EDIT: on the left RD clearly obtained something while ACR seemed to produce pure white.

If you have some RAW file that clearly produces magenta to you, I'd like to analyse it.

Regards.

[digitaldog]

If you have some RAW file that clearly produces magenta to you, I'd like to analyse it.

Should be one on my iDisk in a folder called "For Drew" that produced the screen grabs above. There's two brackets, the one needing recovery is _MG_1247.CR2

[digitaldog]

EDIT: on the left RD clearly obtained something while ACR seemed to produce pure white.

Exactly and with very little effort. I can't get that data in ACR/LR no matter how I alter sliders or curves. And the area of the foliage outside the window with ACR looks tattered while the RD is smooth.

[Guillermo Luijk]

Exactly and with very little effort. I can't get that data in ACR/LR no matter how I alter sliders or curves. And the area of the foliage outside the window with ACR looks tattered while the RD is smooth.

I have extracted the RAW info, i.e. pure RAW data without demosaicing (the other image was an aproximation with neutral RAW development), taking only the R, G1 and B pixels of the Bayer matrix.

In that area where ACR displayed pure white, the R channel and only that channel, has some non blown info in the lines of the wooden floor. RD managed to take it into account and output it.

RAW R channel (RAW clipping blinking):


RAW G1 channel (RAW clipping blinking):


RAW B channel (RAW clipping blinking):

[Peter_DL]

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.

Understood regarding Brightness.
However, Recovery seems to do Luminosity recovery only,
even if the color information was/is in the Raw image (no real clipping) but just got affected e.g. by Brightness.

Peter

--

[bjanes]

I don't think there is any BaselineExposure in ACR.
Regards

Guillermo,

I converted the raw file you referenced earlier to DNG and looked at the EXIF. It is for a Canon EOS 350D DIGITAL, but there is no BaselineExposure tag. Perhaps it doesn't exist for the 350D.

Bill

[bjanes]

Understood regarding Brightness.
However, Recovery seems to do Luminosity recovery only,
even if the color information was/is in the Raw image (no real clipping) but just got affected e.g. by Brightness.
--

I looked at the behavior of recovery via negative exposure compensation in ACR using one of my overexposed step wedges and learned a bit about how it works. I selected an image that was overexposed by about 1 f/stop and split out the raw channels using the freeware astronomical program IRIS and the studied the resulting files in another freeware program ImageJ.

[attachment=15347:RawChannels.png]

In the green1 channel, the first four stops are blown out to white. Step 2 is intact in the red channel, as verified by the histogram. Step 1 is completely blown out in the blue channel, and step 2 is showing clipping in this channel.

I set the tone curve to linear in ACR and used a maximum negative exposure value of -4 EV, and the results are shown.

[attachment=15349:ACR_ScreenCap2.png]

In step 1, which is blown out in all channels, it is not possible to decrease the pixel value below 255. In step 2 there is valid date in the red channel and recovery is possible, but red=green=blue--i.e. the recovery is neutral as you and Guillermo mentioned. This behavior is also present in steps 3 and 4 which have valid data in the red and blue channels. Finally, in step 5 were all channels have intact data, color information appears. The white balance is slight off, and the RGB values are different.

When color information is lacking in one channel (in this the green channel), ACR has to guess, and using a neutral value is reasonable since extreme highlights often have little color. The Raw Developer program guesses that the color in areas with incomplete color information is similar to that in adjacent areas with intact channels.

This behavior can be useful in some instances, but it is better to expose so that channels are not blown. If the dynamic range exceeds that of the camera, two or more exposures can be made if the subject permits.

Bill

[bjanes]

Guillermo,

I converted the raw file you referenced earlier to DNG and looked at the EXIF. It is for a Canon EOS 350D DIGITAL, but there is no BaselineExposure tag. Perhaps it doesn't exist for the 350D.

Bill

Mea Culpa--I forgot to convert to DNG. The baselilne offset is +0.25 EV

50708 UniqueCameraModel              ASCII     Canon EOS 350D
50709 LocalizedCameraModel           ASCII     Canon EOS 350D Digital
50730 BaselineExposure               SRATIONAL         25/100        
50731 BaselineNoise                  RATIONAL         133/100        
50732 BaselineSharpness              RATIONAL         150/100        
50734 LinearResponseLimit            RATIONAL         100/100