Image Processing
Combining landscapes with dark skies.
How to put together a stunning night scene by blending two separately shot images
S kyfall is an image I had in mind before shooting it. Having shot a similar image on the opposite side of the Godley Valley in New Zealand, I knew that this mountain waterfall would make a striking foreground for the Galactic core. The previous image was finished in black and white and I always had a monochrome image in mind when shooting this one.
So how would you go about producing an image like this? Because it was being shot in a dark-sky reserve only starlight illuminates the landscape. This means that in the darker landscape areas our signal-to-noise (SNR) ratio (photons hitting the sensor versus various electronic noise) is low and will produce a noisy, single image. Stacking images is a way to solve this problem. This means you need to commit to blending a final image of the sky with one of the landscape to prevent the Earth’s rotation causing star trails.
Landscape first
This image was composed of two landscape-orientated frames from an 85mm lens, cropped to portrait orientation. The landscape requires a much longer exposure to gather enough signal to extract detail. So 13 two-minute exposure images at f/3.2 were stacked to let in a reasonable amount of light and retain sharpness. The ISO was set at 3200. If you’re shooting with a lens that you’re confident can be opened wider while retaining sharpness, it’s always good to let more light in and raise the SNR. Generally, a longer exposure at lower ISOs is likely to be better than the equivalent total exposure at high ISOs.
Shooting the landscape gave time for
the core to move into position. The sky was shot using a tracker for longer exposures without trailing and a smaller aperture for sharper stars. Exposure-wise, you don’t need to use as wide an aperture or as much time to get a clean image of the sky, so this was five images stacked at f/4, 60 seconds at ISO 6400.
Processing was in Lightroom and Photoshop. Separate images were stacked using the median filter. In Lightroom this is done by selecting images to stack, right-clicking and selecting ‘Edit’ in ‘Open as layers in Photoshop’. You’ll want to check that your layers are aligned.
For the landscape shots, if you’re confident they’re aligned you can begin stacking. For tracked shots it’s best to make sure. This can be done automatically by selecting all layers and ‘Edit > Auto align layers’. If you have a lot of landscape in the frame you may need to mask it out and some lenses may need manual alignment. Once aligned, select all layers
and right-click ‘> Convert to smart object’. This combines all layers before stacking. Select ‘Layer > Smart Objects > Stack Mode > Median’. This will filter out noise and leave a much cleaner image.
Combining the images
When stacking is complete the two images need to be blended via masking. This can be tedious but if you want the highest quality for a certain image, there is no better way. For this image, the tops of the mountains were easy to blend. It was done by pasting the landscape image on top of the tracked sky image and adding a layer mask. The layer mask icon is at the bottom of the ‘Layers’ panel in Photoshop or under the ‘Layers’ menu.
Selecting the mask, choose a black brush and erase the blurry sky to reveal the tracked sky. You can use broad strokes for the most part but you need to zoom in and carefully trace around the edges where the landscape meets the sky. For mountains or
straight structures this is quite easy; for trees, not so much. Once finished, you can flatten the image and save it in a format that retains all the information.
For the final look of this image, I used Lightroom. After using the saturation slider to convert to monochrome, I made contrast and clarity adjustments.
Finally, with desaturation leaving a warmer tone, adjustments were made to cool it using Split toning, adding cooler blue in both the shadows and highlights.
I think this image benefitted from some natural elements which enhanced the overall look, such as some light fog above the mountains which appears to enlarge the brighter stars and make them stand out. For me, this gives the effect of the dusty galactic core region being like snow from an approaching blizzard, or churning water from a larger waterfall above.