Why shoot bracketed exposures for HDR instead of adjusting a single RAW?
Asked 8/29/2013
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For HDR, many photographers use auto exposure bracketing, but RAW files also let you raise or lower exposure in post. What is the real advantage of shooting multiple bracketed exposures instead of taking one RAW and adjusting its exposure afterward? Specifically, when you change exposure on a RAW file, are you recovering real image data or just remapping the same recorded data? How is that different from capturing a separate frame with a longer or shorter exposure?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user11392. Source · Licensed CC BY-SA 4.0
user11392
12y ago
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A single RAW gives you flexibility, but only within the dynamic range the sensor captured in that one exposure. Adjusting exposure in RAW processing mainly remaps the recorded sensor data: you can often recover some highlights and lift shadows, but you are not creating new detail where the sensor clipped to pure white or where shadow data fell into noise.
Bracketed exposures can capture scene brightness beyond what one RAW can hold. A brighter frame records cleaner shadow detail, while a darker frame preserves highlights that would otherwise clip. Combining them increases usable dynamic range and can improve noise performance in the darkest and brightest regions.
So the key difference is this: changing exposure on one RAW redistributes a fixed capture; taking multiple exposures records different parts of the scene more optimally.
That said, modern cameras have strong RAW latitude, so a single RAW can be enough for many scenes. Bracketing helps most when the scene’s contrast exceeds the camera’s dynamic range. The tradeoff is that multiple exposures are harder with movement—people, leaves, or anything shifting between frames can create HDR merging problems.
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