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HDR (High Dynamic Range)

As the title would suggest, HDR stands for high dynamic range. Dynamic range refers to the range of brightness that exists in a photograph. This range is measure in F-stops. There are many discussions surrounding the dynamic range of black and white film vs. color film vs. digital sensors. Generally the accepted values are somewhere around 6 to 9 stops. As this discussion goes on, I will stick with 7.

Given the day to day photography that many of us do, 7 stops of dynamic range is more than enough to capture the images that you want. However, every so often, you will come across a situation where this amount of dynamic range is simply not enough.

In the past, the only way to accomplish this was to use a set of graduated neutral density filters to block out the part of the scene that is too bright. Now the scene can be exposed correctly without risking blown highlights or overly dark shadows. This method requires a set of high quality filters which can easily cost in excess of $500. On top of that they can only be used in situations where there is a distinct line separating the bright from the dark, such as a horizon or water surface.

In the digital sensor world, you can accomplish the same thing by taking multiple exposures of the same scene varying only the shutter speed. Changing aperture values will result in varying  depths of field in each image and should be avoided. The multiple exposures should vary by approximately 1 stop. Hence you would take a camera metered exposure, and then several underexposed and several overexposed, keeping the exposure compensation amount the same for all images. The more images you take the more dynamic range you will be able to capture, but more images will also require more work to process. Generally two under, two over and one metered are enough, but in scenes where the dynamic range is very large, more images might be necessary. While taking these images, shoot in manual mode and turn off auto-focus. (or use it to focus and then turn it off). If your camera has enough range in exposure bracketing for the scene then use it, but if it does not, then simply vary the shutter time by 1 stop increments. These images are then blended in Photoshop CS2 using the correctly exposed section of each image to form one final image that is correctly exposed.

Shooting these image in
RAW mode is generally better than jpeg. RAW images will need to be processed in Photoshop CS2 into PSD files, just remember to keep them in 16 bit mode. All of your RAW images should also be processed with the same settings in Adobe Camera Raw.

Once you have your images in a folder of your choosing, in Photoshop CS2 select File -> Automate -> Merge to HDR. Select the files that you wish to use to create your high dynamic range image. If you images where shot hand-held (very hard to do without VR) you can select the box in the lower left hand corner and Photoshop CS2 will attempt to align your images. Click OK. On computers with less than 2 gigabytes of RAM this can take some time, especially if you are merging 9 or more 100mb PSD files.

Once finished, a window will open showing the merged image with all the thumbnails used to create the HDR image on the left. Click ok.

Given that a computer screen is an 8 bit device, it is not capable of showing the entire dynamic range of the image and the result will not look like a finished photo. To display the image correctly, or print it, it has to be converted to 16 or 8 bit mode. Or you can save the image in its 32 bit mode and come back and work on it later.

To change the bit depth you click on Image -> Mode -> 16 bits/channel (or 8 bits/channel). A new dialog box opens up with several different options.
HDR dialog
These four modes are used to adjust brightness and contrast in the image.

This following is a direct quote from Adobe Help.
"Exposure and Gamma: Lets you manually adjust the brightness and contrast of the HDR image.

Highlight Compression: Compresses the highlight values in the HDR image so they fall within the luminance values range of the 8‑ or 16‑bpc image file. No further adjustments are necessary; this method is automatic. Click OK to convert the 32‑bpc image.

Equalize Histogram: Compresses the dynamic range of the HDR image while trying to preserve some contrast. No further adjustments are necessary; this method is automatic. Click OK to convert the 32‑bpc image.

Local Adaptation: Adjusts the tonality in the HDR image by calculating the amount of correction necessary for local brightness regions throughout the image."

Personally I prefer to use the Local Adaptation method as it gives you a toning curve and a histogram that you can use to adjust the image. Make the adjustments you wish and click ok. The resulting image can be edited and then displayed or printed.


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