What does the under/overexposure warning mean in image editing software?

Question

We all know exposure warnings that a digital camera offers: If shows which parts are so bright or dark that there's no more distinction towards even brighter or darker areas possible. Meaning that those areas can't show any more detail. And that's a limitation of the recording feature of the hardware, i.e. of the sensitivity range of the sensor.

That's all clear.

Now, what puzzles me is that Lightroom keeps creating such warnings when I change an image's exposure. E.g. like this: I open the "Histogram" and "Basic" panes, turn on the overexposure warning in the histogram (click on the top right rectangle) and then move the Exposure slider to the right. That soon leads to areas becoming totally white, and the overexposure warning kicking in.

This makes no sense to me, as here those are just numbers now, and there's no limit to the dynamic range of the numbers, right? I would expect that the editing software can work with endless dynamic range, and only when I apply this to a output medium, i.e a printer, monitor or whatever, I need to choose the available range of that medium, and then either clip the range or compress it.

But here, Lightroom appears to tell me right away that I'm outside of some range, but which range is that, exactly?

Background: Let's imagine I have a display medium that can handle a wider EV range than what the camera had. Now imagine I have a picture that contains the sun, all overexposed, due to the camera's limits. I would now want to use an image editing software to manually put more "detail" into this img area of the sun, raising the brightness there for better effect, because my out medium could handle that additional range. How would I accomplish this, if a tool such as Lightroom apparently already would clip my range at that point?

Answer 1

There is going to be a limit to the numbers you can use, and absolute white is as far as you can go -- it doesn't matter whether that's a 255,255,255 value in an 8-bit image or 65535,65535,65535 in a 16-bit image (or the corresponding values in a 32-bit image). Once you've hit black at one end and white at the other, you've exhausted the available dynamic range, no matter what the output device is.

Screens, backlit transparecies and projectors can never get darker than "off" or brighter than "all the way on". Paper can never get brighter than the paper itself, nor darker than the darkest ink you can put on it. A high dynamic range might change the distance between black and white (as in a screen with a very high contrast ratio), or it may mean a less visible, smoother transition between levels, but black will still be black and white will still be white. So even though the original bits in the RAW file might still be available for Lightroom (or a Photoshop smart object or similar) to calculate with, it's the output values that matter as far as clipping goes. White and black are hard clipping limits, so the overexposure and underexposure clipping warnings are telling you something you ought to know.

Answer 2

You need to interpret those exposure warnings as this:

"If you export an LDR image with the current settings, these are the areas where you have lost detail that exists in the original unedited picture".

This is very useful, because you can move sliders and immediately see the effect your change will have in a final exported LDR image.

Also note that Lightroom will certainly not have infinite dynamic range available to it. Computers are pretty bad at representing infinite, pictures are typically represented in the computer (Lightroom or otherwise) in one of just a few formats:

• 8 bits per pixel: standard for LDR pictures. The range for each pixel value is 0-255.
• 16 bits per pixel: standard for HDR pictures. The range for each pixel component is 0-65535. If you shoot raw, this is the most likely representation, even though raw files are typically 10 or 12 bits per pixel only.
• 32-bit floating point: each pixel is a floating point number. In this mode typically the a pixel component value range from 0.0 (black) to 1.0 (white) and out of range values can be represented easily. While this may seem like a great format, floating point numbers also suffer from the problem that not all possible values can be represented, for any desired number you have to find the closest value that can be represented in this form, like with integers. Operations in floating point format are slower than with integers.
• 64-bit floating point: same as above, but more accurate and slower to process.

Note that for all of the above, a color picture requires three value to represent a pixel, the red, green and blue components. So in reality, the 8 bits per pixel format uses 24 bits to represent a color picture. A grayscale picture has only one component per picture.

Answer 3

I know it's lame to answer my own question, especially if others have also tried to provide an answer. Yet, the best explanation I've found just now is on this web page: photozone.de - Brightness & Exposure.

It reveals to me that I've had a misconception about the "Exposure" function in editing software.

One thing that Stan Rogers helped me to understand is that we're always only looking at a relative brightness range from 0 to 1.

What I expected "Exposure" to do is to shift all mid-gray values towards the extreme end (black or white), in a way that would compress the lowest or highest values proportionally to their position. I.e, that when I'd increase exposure, a mid-gray tone (0.5) would be increased more (e.g. by 0.2) than a tone that's already very bright (e.g. 0.9 only by 0.03). That way, especially if using high-precision numbers internally (i.e. floating point numbers), there'd be practically no loss in information.

However, it turns out that changing the Exposure is a direct imitation of what the camera does, i.e. it simply shifts the values of 0 to 1 by a constant offset, and that can lead to clipping if the shift exceeds the borders.

I was simply not expecting that a tool would offer a control that would change the information in such a way. In my opinion, this kind of Exposure control that unavoidably leads to clipping is hardly ever useful, whereas a "brightness" control as I've explained above, which would lighten everything up/down proportional to their current brightness is something that would alwmost always be the preferred operation because it avoids loss of detail better.

Yet, Lightroom and all other tools I know of all offer the Exposure control prominently instead of the - IMO - much better brightness control, thereby effectively misguiding the casual photographer into using tools that aren't giving him the best results.

Or do I (still) miss something here?