HDR versus Tone Mapping

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Today’s Question: How would you differentiate HDR from tone mapping?

Tim’s Quick Answer: Tone-mapping is actually one of the steps involved in creating an HDR (high dynamic range) image, but it is also possible to use tone-mapping adjustments on a non-HDR capture to produce a creative effect for the photo. So in general I would say that HDR is a process for assembling multiple exposures into a single image, and tone-mapping is a process for applying adjustments to a photo, including for HDR images.

More Detail: And HDR image is a high dynamic range image that is generally assembled from multiple individual exposures. When you assemble multiple exposures into the initial HDR image, the result is generally a 32-bit per channel image (or sometimes a 16-bit floating point image. In both cases the result is an image that contains a much greater range of tonal values than can be presented in a “normal” 8-bit or 16-bit per channel image.

Once you’ve created the initial HDR image, the next step is to perform a tone-mapping adjustment. This step essentially takes the extreme range of tonal values in the initial HDR image and maps them to a “normal” range of tonal values, generally producing a 16-bit (or 8-bit) per channel image as the final result.

Often you will perform the tone-mapping adjustment as a step in the overall process of creating the final HDR image, but this step can also be performed separately. For example, you could send a set of photos to Photoshop in order to make use of HDR Pro to create a 32-bit per channel HDR image, and then you could use the Develop module in Lightroom to apply the tone-mapping to that HDR image.

It is also possible to apply the same tone-mapping adjustments to a non-HDR image. You won’t magically create a greater dynamic range in the image, but you can create the impression of greater dynamic range by darkening down the highlights and brightening up the shadows, for example.

So, if you want to create an image that contains the full range of tonal values in a scene that contains too great a range to be captured in a single image, you’ll need to produce a “true” HDR image by capturing multiple exposures at different exposure settings. Regardless of whether you’ve captured a “true” HDR image or a single capture, you can still generally apply tone-mapping to the image to apply various adjustments.

But again, it is important to keep in mind that the only way to expand the true dynamic range of an image is to assemble multiple captures into an HDR result. Just because you apply tone-mapping to a photo doesn’t mean you have automatically expanded the dynamic range represented in the image.