Photoshop Tips&Tricks: HDR

This is a quick and dirty introduction to creating HDR (High Dynamic Range) images with Adobe Photoshop. This works in both CS2 and CS3 versions.

HDR is "new" and HDR is cool. It can help to resolve many exposure challenges that are presented when photographing various objects and scenes. One of the most common examples of this a scene that frames both: indoor and outdoor scenario. Most commonly, indoor portion of the photograph will come out underexposed with detail flat and dark while outside will be overexposed with highlights blown out of proportion. Even when photographing landscapes, tricky lighting make it very difficult to find a proper exposure balance for all the elements in the shot. HDR can help. The technique is most effective when applied to scenes with relatively still objects.

By taking a series of bracketed exposures (in other words 3 or more photographs each taken at different exposure value), the resulting images each will contain different level of detail located in various areas that are properly exposed. By then combining these, the resulting image will encapsulate all of the detail that would be impossible to obtain in one shot.

So, here's how to get started. First and foremost, it will make it very easy to achieve best results if all photographs are taken when camera is mounted on a stable surface or a tripod. This will save a great deal of processing power because processing alignment of the images for merging will be minimized (in some cases alignment fails, so it should not be relied upon) . Next, take a series of shots with different exposure values varied at 1 stop each. Three is the minimum requirement to process the images in Photoshop, but depending on the lightning challenges there can be as many as reasonably needed (usually around 6 or so in most cases).

Photoshop produces best results with images in RAW format, so using them is encouraged. Once you have the series of the images with variant exposures, HDR process is as follows:

1. Create 32-bit HDR image file
- select Open -> Automate -> Merge to HDR
- load set of source photographs for processing
- check "Attempt to Automatically Align Source Images" to ensure proper alignment
- click 'Ok' to initialize HDR processing
- HDR tool then displays a preview file with resulting histogram from all images. It may look very dark or very bright, but do not be alarmed. HDR tool will also display the "White Point" slider preset at the most optimal point according to the algorithm. It is not always reliable as some highlight detail can be lost with the automated setting. Usually shifting the slider towards maximum value fixes this problem.
- Click 'Ok' to generate 32-bit HDR image file

2. Convert HDR to 16-bit (or 8-bit) LDR (Low Dynamic Range)
- Select Image -> Mode -> 16 (or 8) -bit Channel
- Once file has been converted to LDR, HDR Conversion tool will allow to adjust the image for digital processing. This is the final step in creating desired image. Four options are available via this tool:
  • Exposure and Gamma
  • Highlight Compression
  • Equalize Histogram
  • Local Adaptation
Each one of these settings will allow you to adjust image's contrast and brightness to achieve the desired effect. Most flexible is "Local Adaptation" as it uses data based on pixel-per-pixel range to adjust values rather than image information in whole. Experiment with various Radius (range affected based on pixel location) and Threshold to achieve the desired results. In most cases, HDR created images require some additional processing to adjust saturation levels and reduce noise.

Below is an example of an image created via HDR tool in Photoshop.

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