How can I blend a short high-ISO star exposure with a longer low-ISO foreground exposure naturally?
Asked 10/3/2012
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I want to combine two night shots into one image: a short, high-ISO exposure to keep stars as points, and a longer, low-ISO exposure to capture cleaner foreground detail. My main problem is the strong contrast at the boundary between sky and foreground, especially around trees, where manual masking often looks obvious and unnatural. HDR tools have not worked well for this. What workflow works best for aligning and blending these two exposures cleanly?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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You could try the "Blend If" sliders in your layer options. This ought to blend in the sky within the trees, but it may block out the stars.
Better control would be to create a mask using Image > Apply Image. Once you have that mask (which will have grey tones, use levels or curves on the mask to increase the contrast of the dark sky areas until the mask is black/white. You should be able to clearly define the sky against the trees. If you are left with "holes" where the stars are, just paint over those with a brush. You might also need to feather the mask a bit.
Originally by user4191. Source · Licensed CC BY-SA 4.0
user4191
13y ago
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A good workflow is to stack the two aligned exposures as layers, then blend with a mask rather than HDR.
If your exposure/ISO choices are balanced well, the two frames should be fairly close in overall brightness; the main difference should be noise and star sharpness. In that case, put the short star exposure on top and paint in the cleaner long-exposure foreground with a layer mask.
For tricky edges like trees, make the mask from the image itself: use Apply Image on the mask, then adjust the mask with Levels or Curves until the sky and foreground separate clearly. Feather slightly if needed, and manually paint small problem areas.
You can also try Blend If sliders to help the sky show through fine branches, though this may suppress some stars.
Another simple option is to place the star exposure on top and use a Lighten/Lighter Color blend mode so only the brighter stars come through. If noise also comes through, reduce the brightness of the star layer so mainly the stars remain visible.
The key is matching brightness between the two source frames as closely as possible before blending.
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