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Whenever I read / hear about HDR (High Dynamic Range) photography, someone usually says something along the lines of "Actually, you're talking about tone mapping, not HDR."

Please can someone explain what Tone Mapping is, and how it relates to HDR?

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    \$\begingroup\$ LOL, a question that creates a question that creates a question. Funny how that works out sometimes... \$\endgroup\$
    – PearsonArtPhoto
    Commented Jan 25, 2011 at 12:58

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An HDR image has a high dynamic range, which means a very large ratio between the brightest and darkest parts of the image. An HDR image on a normal (low dynamic range) monitor will actually look very flat:

This is because that huge range of brightnesses has to be compressed to fit into a much smaller range of brightnesses. This results in an overall lack of contrast, hence the flatness.

This image has a split personality, the skies are very bright and the subject much dimmer, if we could use all the monitor's brightness range for the sky, it would look pretty good:

But we'd totally lose our subject. Likewise if you used all the monitors brightness range for the subject it would also look good, but we'd totally lose the sky:

It would be great if we could combine them in some way, or carefully ration out the brightness range we have to work with so we make most use of it. This is where tonemapping comes in.

What tonemapping does is instead of mapping the whole image into the monitor's brightness range in one go, it adjusts the contrast locally so that each region of the image uses the whole range for maximum contrast (there's a bit more going on here, it depends on the tonemapping algorithm used). Here is the same image tonemapped:

The reason that tonemapping is not HDR is that you can tonemap a single low dynamic range image in order to make it more contrasty. The result looks similar but with much more noise in the shadows:

Disclaimer: for the love of God don't do this to your images!

This is a demonstration of the fact that dynamic range and noise are opposites, in fact dynamic range is usually defined in terms of the noise floor of an image. This is because there is a point at which any tonal differences in an image get lost in noise, so this defines the darkest thing you can image (which in turn with the brightest thing, defined by the point at which the signal clips) determines the dynamic range.

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    \$\begingroup\$ Best explanation of how tonemapping works I've read. \$\endgroup\$
    – gerikson
    Commented Jan 25, 2011 at 9:41
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    \$\begingroup\$ Someone needs a sensor clean :( Good example of bad tone mapping, by the way - I've always wondered why people destroy an image's dynamic range like that :) The resulting image could be termed "Low Dynamic Range". \$\endgroup\$
    – thomasrutter
    Commented Jan 25, 2011 at 9:45
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    \$\begingroup\$ @thomasrutter Nothing the clone tool can't handle! But yes its time for a clean, been shooting a lot of low light stuff so big aperture, this is the first time I've stopped down past f/5.6 for a while. I was just commenting on another thread how I change lenses whenever I want to and don't lay awake at night worrying about dust, this at least proves I practice what I preach! \$\endgroup\$
    – Matt Grum
    Commented Jan 25, 2011 at 11:12
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    \$\begingroup\$ I'd like to add than in my opinion, you can many times get more natural looking results my doing the tone mapping manually, as in this example: i.imgur.com/LHyEp.jpg (Or doing it in camera using a reflector or flash.) \$\endgroup\$
    – che
    Commented Jan 25, 2011 at 11:23
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    \$\begingroup\$ @che I totally agree, in the few occasions I've needed to extend the dynamic range I've done it by hand (see photo.stackexchange.com/questions/3394/…) I could do with updating the answer by discussing different tonemapping methods. I also have a version of that same image which I did with flash! \$\endgroup\$
    – Matt Grum
    Commented Jan 25, 2011 at 12:23
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I've tried to explain tone-mapping as well as I can in this video:

http://mulita.com/blog/?p=6021

It's free, just for the asking. Comments are welcome.

George

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