Lunch atop a (Springfield) skyscraper

Lunch atop a (Springfield) skyscraper
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For HDR, it's recommended to use auto exposure bracketing. However, an advantage of RAW is the ability to adjust exposure. Even Trey Ratcliff has said, "you can make a good HDR photo from a single RAW".

My question is, what's the advantage of shooting multiple exposures as opposed to taking one RAW and tweaking its exposure? (There must be an advantage, otherwise no one would bother with the significant limitations of AEB.)

I am looking for an explanation that discusses exactly what happens when one adjusts the exposure value of a RAW (is information lost?), and how that differs from what the camera records during a longer exposure.

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High Dynamic Range (HDR) Imaging is all about dynamic range. That is a way of describing the difference between the darkest and brightest parts of a scene we wish to photograph. Each stop is double the light of the previous one. A scene with, for example, 6 stops dynamic range has highlights that are 64 times as bright as the shadows.

Any JPEG, regardless of how it was produced, is limited to about 6 stops of dynamic range. Each color channel uses 8-bits, but about two of those bits are taken up by what is known as the noise floor. Most current DSLRs and other advanced digital cameras record either 12 or 14 bit RAW files with 10 to 12 stops of dynamic range at their base ISO sensitivity. What HDR (High Dynamic Range) Imaging attempts to do is squeeze as much dynamic range as possible into the 8-bits for each color channel in the JPEG standard.

As we mentioned above, most advanced digital cameras can record 10-12 stops of dynamic range in a single exposure. To get the same amount of information in a series of JPEG files you would need one exposure three stops darker and one exposure three stops brighter than a 'correct' exposure. Since there would be very little overlap between the darkest parts of the bright exposure and the brightest parts of the dark exposure we would also need to add a third exposure in the middle between the other two. This is where the -3, 0, +3 series of bracketed exposures to use in HDR processing comes from. So in most cases a properly exposed single RAW file can contain as much information as a -3, 0, +3 series of JPEGs.

But some scenes contain even more than the 10-12 stops of dynamic range a single RAW file can record. If we shoot a series of RAW files we can expand the total amount of information from the darkest to the brightest parts of the scene even more. By shooting a -3, 0, +3 series of RAW files, we can add an additional 6 stops of dynamic range for a total of 16-18 stops! That means that the brightest exposure records highlights that are 262,144 times as bright as the shadows in the darkest exposure. The more intermediate steps of exposure we have, the better the software we use to combine the images can do so smoothly.

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Can you elaborate on "most advanced digital cameras can record 10-12 stops of dynamic range in a single exposure"? How do we know this? Where is this information available for, say, a Canon 5D? This info would answer the question: how many stops can I increase/lower the exposure of a single RAW? – Philip Aug 29 '13 at 17:59
Read the spec sheet of any advanced digital camera. In general subtract 1-2 bits for the noise floor and whatever remains is the camera's dynamic range at base ISO. For instance, a camera that records 14-bit RAW files will have a DR of around 12 stops. If you need more precision than that, look at a technical review site such as that measures the 12-bit EOS 5D at 11.13 Ev at ISO 100. – Michael Clark Aug 29 '13 at 18:28
In addition to increasing/decreasing exposure, with a RAW file you can also adjust the light curve to compress or expand the distance between the brightest and darkest areas of the image. Depending on what software you are using for your RAW convertor you can also adjust the relationship of intermediate points to, for example, increase contrast between two tonal values that are very close. You can also decrease the contrast between two tonal values that are farther apart. – Michael Clark Aug 29 '13 at 18:46

Wider dynamic range. Modern cameras have pretty wide dynamic ranges, so what used to be solely the job of HDR can now be accomplished using a single RAW file, but you can still get more dynamic range by bracketing.

Say for example you have a camera with a 12.6 stop dynamic range. That's pretty wide, but if you do a +/- 3 stop exposure bracket, you now have a range of 18.6 stops since each of the + and - image overlap for 9.6 of the stops, but add another 3 on either end.

Effectively, the performance may even be a little bit better overall since the + image will likely have fewer noise problems in the darker parks of the dynamic range than the 0 image and the - image may have more information in the brighter portions. This depends on how broad the effective dynamic range is for your camera in those conditions is though.

A RAW file captures more information than is normally displayed in the JPEG preview. It captures information above the clipping point of the JPEG and data below the black point. Adjusting the exposure of the image slides the black and white point to use more a different portion of the information for generating the final image. HDR seeks to move the black point down and the white point up so that all the information fits in the JPEG.

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