Today’s Question: Can you clarify what is meant by “tone mapping” for HDR images? Why is it an option when using HDR Pro in Photoshop but not with Camera Raw [or Lightroom Classic]?
Tim’s Quick Answer: The term “tone mapping” refers to taking a high dynamic range (HDR) image and converting the pixel date to the tonal and color range of a “normal” 16-bit per channel image.
More Detail: A high dynamic range (HDR) image contains a much wider range of tonal values than a “normal” photograph. The way this is accomplished is by taking the image data from multiple bracketed exposures and blending them into a 32-bit per channel image.
For context, an 8-bit per channel (such as a JPEG capture) can contain up to almost 16.8 million possible color values. A 16-bit per channel image (the maximum for a raw capture, though not all cameras support this high a bit depth) can contain more than 281 trillion possible color values. An image in the 32-bit per channel bit depth can contain color values numbering more than 79 with 27 zeroes, which happens to be more than 79 octillion possible color values.
Of course, the incomprehensibly large number of possible color values for a 32-bit per channel image don’t mean very much in the context of a normal photographic workflow. That’s especially true considering that normal human vision is estimated to be close to matching 8-bit per channel color.
But 32-bit per channel images aren’t intended to provide extreme possibilities in terms of color and tonal values available. Rather, they’re meant to contain huge amounts of image data for special purposes. To put such an image to meaningful use in a photographic workflow it needs to be converted to 16-bit per channel mode, and in the context of converting an HDR image to a “normal” image the term for that process is tone mapping.
So, you can think of tone mapping as the process of taking the incredibly huge range of color and tonal values in a 32-bit per channel HDR image so they fit into a “normal” 16-bit per channel image. This is a process that can be handled directly by software that enables you to create an HDR image in the first place, making it somewhat “invisible” to the user. For example, if you assemble an HDR image in Camera Raw as explained in a previous Ask Tim Grey answer, you are using Camera Raw to perform the tone mapping step to translate the 32-bit per channel HDR data to a 16-bit per channel image.
