HDR Using Layer Masks

Some objects have both very bright and very dim regions. Two well-known examples are M31 (the Andromeda Galaxy) and M42 (The Orion Nebula). If you expose for the dim portions you're likely to saturate the bright areas, essentially clipping them off the white end of the histogram.

We'd really like to have both bright and dim areas in our image, so some extra work is required. During Acquisition you'll want to use exposure times both long and short to capture the entire object. After stretching you'll have one exposure (we'll call it "core") and another ("outer") that need to be combined. Photoshop layers makes this easy.

  1. Open the core and outer images.
  2. Select the core image (this is the one with the core exposed correctly). Select and copy it with Ctrl-A and Ctrl-C.
  3. Select the outer image and paste the core onto it as a new layer (Ctrl-V). The darker core image will be on top, the lighter outer image with the clipped core will be on the bottom.
  4. Select the core layer and create a Hide All mask for it (Layer > Layer Mask > Hide All)
  5. Set the foreground color to White (upper left square of paired squares near bottom of tool bar)
  6. Choose the Paint Brush tool, set its size appropriate to the region you'll be masking in, Mode = Normal and both Opacity and Flow to 100%.
  7. Paint on the core layer with the brush. You'll see the darker core superimposed onto the brighter underlying layer. Get the core completely painted in. (This may look rather rough at first.)
  8. Apply a strong Gaussian blur to the core layer. Vary the radius to obtain a reasonably smooth transition from the outer region to the core. Don' worry if the core still seems too dark, we'll fix that next.
  9. Adjust the opacity of the core layer to refine the transition. This will allow some of the overexposed outer layer through.
  10. When you're happy with the effect, merge the layers and save.

For increased dynamic range you can use several layers or repeat the process several times.

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