Single-Image HDR — Ask Tim Grey
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Today’s Question: I’ve got a single image that has a pretty broad range of tonal values. Is there a way to create an HDR image from a single photo? I tried creating additional versions of the original (-2, -1, +1, +2 stops) and then combining them, but that didn’t work. Do you have any suggestions of what would work?
Tim’s Quick Answer: Put simply, you can’t create a true high dynamic range (HDR) image from a single exposure. You can, however, use the tone-mapping feature of most HDR processing software to apply adjustments to that single image.
More Detail: An HDR image involves combining multiple exposures into a single file, blending all of the information from the multiple images into a single processed image with a higher range of tonal values. This is the reason that an initial HDR image assembled from multiple 16-bit per channel captures will generally be a 32-bit per channel HDR image.
That 32-bit per channel image then needs to be tone-mapped to a “normal” tonal range represented by a 16-bit per channel image. This tone-mapping step can be applied to any image, even if it is not a true HDR image.
Some software tools for processing HDR images allow you to use multiple images processed from a single capture, using different exposure adjustments for each copy of the image. This would require that the RAW captures be processed and saved in another image format such as TIFF, however. If you simply made multiple copies of the same RAW capture with different adjustments saved in metadata, the HDR software would not be “fooled”, since the underlying RAW captures would all contain the same capture data.
So, there is no real reason to process the same RAW capture with multiple exposure adjustment variations. Instead you can simply open the original RAW capture in the HDR software, using the tone-mapping features to create the interpretation you prefer. Put simply, applying adjustments to create multiple interpretations of a single capture does provide any additional data compared to the single original RAW capture. To truly leverage the benefits of HDR imaging, you need to capture multiple bracketed exposures in the camera to begin with.