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Author Topic: Exposure Slider and Histograms  (Read 7051 times)
meyerweb
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« Reply #20 on: January 02, 2012, 08:48:04 PM »
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See Bryan's comment, below, which is 100% correct.  There seems to be a lot of confusion over what the exposure slider is supposed to do, probably because Adobe chose a really poor name for it.  The exposure slider is not, and isn't supposed to be, the same as adjusting exposure in camera. It doesn't shift the all portions of the image up or down equally as adjusting the f-stop does (assuming there's no clipping of the image).

Hover your mouse pointer over the exposure slider while watching the histogram.  Notice that part of the histogram background turns a lighter gray?  THAT'S the portion of the image primarily affected by the exposure slider.

Now drag the Recovery slider to the right, and hover the mouse over that slider.  Notice that a different part of the background turns a lighter gray?  Do the same thing with the Fill and Blacks sliders.  Each slider primarily  affects a different part of the histogram, and the image.

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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Bryan Conner
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« Reply #21 on: January 02, 2012, 11:53:58 PM »
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I can not see the histogram (or the histogram background) change at all when I hover my mouse over any of the sliders in ACR 6.6 or ACR 4.6 (Windows 7 64 bit).  I tried this both before and after moving the sliders.
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Schewe
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« Reply #22 on: January 02, 2012, 11:57:04 PM »
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That only works in Lightroom...
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FranciscoDisilvestro
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« Reply #23 on: January 03, 2012, 05:40:19 AM »
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There seems to be a lot of confusion over what the exposure slider is supposed to do, probably because Adobe chose a really poor name for it.  The exposure slider is not, and isn't supposed to be, the same as adjusting exposure in camera. It doesn't shift the all portions of the image up or down equally as adjusting the f-stop does

There is no confusion, and the name is not poorly chosen. The exposure slider really does have the effect of "changing exposure" or basically linearly multiplying the channels by a factor. (It would not be possible to do ETTR if this was not true).

The best explanation I have come across is in this thread , where Guillermo Luijk shows the difference between exposure and brightness, and a few posts later, Eric Chan explains that when there are blown out areas, the exposure slider will invoke automatically highlight recovery
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Bryan Conner
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« Reply #24 on: January 03, 2012, 08:23:33 AM »
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Duh.....I guess that is the reason why the thread is in the Adobe Lightroom Q&A...LOL.  I did not pay attention to that before I made my statement.  Ha!  Thanks Jeff.

I don't know that I would ever use the feature if it were in ACR....but...it might make us ACR'ers feel like we are as important (smart) as the Lightroomers.  We like to play with the toys too!  Roll Eyes  Hint, hint Adobe.
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madmanchan
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« Reply #25 on: January 03, 2012, 10:31:45 AM »
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The Exposure slider in ACR and Lightroom is intentionally measured in stops (e.g., from -4 to +4 stops), and for the most part (*) it does correspond to the standard notion of photographic exposure.  For example, if you set Exposure to +1, this will double the exposure of the image.  The reason it may seem as if other non-linear stuff is going on, is because images are rendered in ACR/LR with tone curves -- e.g., for shaping the highlights and shadows.  For example, the default point curve of Medium Contrast is part of this tone shaping.  If you adjust exposure, then this affects how the values get mapped to the tone curve. 

While a log-base-2 histogram is useful for determining exposure levels, it is not particularly useful for judging rendered output for a color image once tone mapping is applied. Remember, the histogram in ACR/LR reflects the rendered output (output-referred image), not the original input image (scene-referred input).

Eric

(*) The reason I say "for the most part" is because of course there are limits.  For example, in the field if you were to reduce the exposure (e.g., by halving the exposure time), you can record more and more highlight detail.  But if you try to do this in a raw conversion software, of course that doesn't work.  In other words, reducing the Exposure slider can reveal a bit more highlight detail, but very soon you exhaust what was actually recorded in the image.
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RobertBoire
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« Reply #26 on: January 04, 2012, 10:52:38 AM »
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The Exposure slider in ACR and Lightroom is intentionally measured in stops (e.g., from -4 to +4 stops), and for the most part (*) it does correspond to the standard notion of photographic exposure.  For example, if you set Exposure to +1, this will double the exposure of the image.  The reason it may seem as if other non-linear stuff is going on, is because images are rendered in ACR/LR with tone curves -- e.g., for shaping the highlights and shadows.  For example, the default point curve of Medium Contrast is part of this tone shaping.  If you adjust exposure, then this affects how the values get mapped to the tone curve. 


Thanks for the clarification. Does this mean that if the tone curve is set to linear the exposure slider will correspond to manipulating in-camera exposure or are there other non-linear things (gamma?) going on.

R
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madmanchan
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« Reply #27 on: January 05, 2012, 07:58:49 AM »
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There are other non-linear things going on with ACR/Lr's Exposure because there are differences between real exposure (in the field) compared to digital exposure.  As mentioned earlier, in the field when you reduce exposure you can capture brighter and brighter details.  In post-capture digital exposure, you can't.  For example, decreasing exposure by 2 stops normally means all your image values get divided by 4.  This means what was previously pure white (e.g., 1) is now a gray (e.g., 0.25).  And in the field, you can now record new details above that level (e.g., in the range 0.25 to 1).  In post-processing, you can't.  There's nothing above 0.25.  So all you get are "gray" whites, which isn't photographically very useful.  ACR/Lr does non-linear stuff to keep whites white (instead of gray).
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bjanes
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« Reply #28 on: January 05, 2012, 08:22:20 AM »
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Thanks for the clarification. Does this mean that if the tone curve is set to linear the exposure slider will correspond to manipulating in-camera exposure or are there other non-linear things (gamma?) going on.

I'm not Eric Chan, but if one sets ACR's sliders to linear, the image will still be rendered into a color space with gamma encoding. With Histogrammar, one can take this into account by setting the gamma value appropriately. If one wants scene referred values, the method suggested by the ICC can be utilized, by converting from ProPhotoRGB to linear_rimm_rgb_ver4. Due to veiling glare, the image will still not be scene referred in the shadows.

Regards,

Bill
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Prof. Jewell
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« Reply #29 on: March 13, 2012, 10:18:59 PM »
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As of this date one still cannot install Histogrammar v1.2. The expired message still blocks startup.

Is there any way one can overcome this? ...Or?

Thanks,
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