I like to photograph the night sky on clear, moonless nights when the only light comes from the canopy of stars. I also like to create composites for my final image. To do this I make exposures of about 3 minutes for the foreground and 25 seconds for the sky and combine them in Photoshop.
Unfortunately, even on the darkest nights, the camera will capture the orange glow of a distant city.
The photo below, made from Yavapai Point at the Grand Canyon was exposed for three minutes. Notice that the canyon is properly exposed but the stars are forming trails because of the long exposure. Notice the glow in the sky just above the horizon on the left. It comes from the City of Las Vegas which is 175 straight line miles away.
Here, the sky is exposed for the stars but the canyon is underexposed. The glow from Las Vegas is still clearly visible.
I combined the above two images to make this composite.
Here’s how I did it. First, I opened both photos in Adobe Camera Raw and made various adjustments. What they were is really not important because this article is about creating a composite from two images. What is important is that I made the same adjustments to both images. I’ve found that making separate adjustments for each layer tends to make the composite look fake and fixing it is more difficult than just using the same adjustments.
I opened both images in Photoshop. I made a copy of the image with the properly exposed foreground (CTRL J) and using the move tool, placed it as a layer on top of the image with the properly exposed sky. Using the quick selection tool, I made a selection of the sky. Because the intersection of the horizon and the plateau is virtually flat, the quick selection tool did a great job. With the sky selected, I held down the ALT key and clicked on the create a layer mask icon on the Layers pallet. That created a black mask which hid the selected the area on the top layer and revealed the selected area on the bottom layer creating the perfect composite.
But what about the glow from Las Vegas? Here’s how I fixed that. I simply selected the sky layer and then chose EDIT, TRANSFORM, SCALE and dragged the handle at the bottom corner to make the sky layer larger. I then used the move tool to move that layer down until the glow was below the foreground layer. With the original foreground selected and making sure I had selected white as the foreground layer, I clicked on the add layer mask icon. This action revealed the sky of the transformed layer. It was a simple matter. Because the sky layer is larger, you can also move it from side to side to get that layer in the best position.
Is there a downside do using Transform? Yes, it has the effect of using a longer lens so the slight trailing in the stars from the 25 second exposure will be a bit larger but that will only matter if you decide to print an enlargement of your photo. But even then, depending on your camera, the effect will be minimal.