Tone your image using HDR

One of the best improvements in Adobe Photoshop CS5 is High Dynamic Range (HDR) processing. In its pure form, HDR allows users to merge three exposures — one each for shadow, midtone, and highlight detail — into one image with clear detail, from deep shadows to bright highlights. Toning with HDR allows you to adjust this now-single image to bring out more detail. It’s important to use an astroimage that has been stretched properly — that is, it has full detail at all levels of illumination without any clipping (see my November and December 2010 “Imaging the Cosmos” columns).

The “tadpoles” of IC 410 and nearby dark nebulae appear clearly enough in this image, which has already undergone stretching corrections to ensure all brightness levels show full detail. All Photos: Tony Hallas

The top image shows an apparently finished shot of IC 410’s “tadpoles” and neighboring dark nebulae. This look was possible by stretching the photo with “Curves” and resetting the black point via “Levels.” Careful attention is necessary to keep all of the detail. The image underneath is the result of “HDR Toning.” The process is complicated, so I’ll take this column and next month’s to cover it entirely. First, you should practice becoming more familiar with the various options.

1 Open your starting image in Photoshop CS5. Go to “Image,” then “Adjustments,” and finally “HDR Toning.”

After High Dynamic Range (HDR) Toning, however, new details emerge from the previously “finished” photo.

2 The HDR control box will appear. Choose the following input parameters for your image: “Preset” = Default (assuming this is the first time you use HDR); “Method” = Local Adaptation.

3 “Edge Glow” is the trickiest part of HDR. It’s a way to microadjust the contrast of your image’s features. Start with “Radius” = approximately 100, and “Strength” = approximately 0.50. You will fine-tune these values later. Don’t worry if your highlights get too bright at this point.

4 “Tone and Detail” allows you to adjust the image even further after “Edge Glow” using five different sliders: “Gamma,” “Exposure,” “Detail,” “Shadow,” and “Highlight.” Try the following:

The HDR Toning dialogue box in Photoshop CS5 is found under “Image,” then “Adjustments.”

For “Gamma,” start by slowly moving the slider to the right while watching your highlights and brightest stars. At some point, you will recover all of the detail that washed out in the highlights. Going past this point will render them gray.

“Exposure” defines how the image is exposed overall. If you try to make the photo brighter, the highlights will wash out again, especially around the brightest stars. You can use this variable to darken the image slightly if adding contrast makes it too light. (“Curves” also adjusts the density of any part of the image, and it is more density specific.)

“Detail” is a powerful tool, akin to high-pass filtering on steroids. Moving the slider to the right will emphasize detail, but don’t go nuts. Use this control with discretion.

Finally, “Shadow” and “Highlight” should be fairly obvious: They control the densities of the shadows and highlights tonal zones.

In the next article, you’ll see how to use all of these controls to bring out the fine detail of your image.