Night photography: Stacking star trails and clouds in the same image
This 15 minute full moon photo was taken at a yard that restores and repairs vintage buses. The image was lit by a combination of moonlight, and light from the nearby highway. The green glow on the underside of the power lines is from a light outside of the building. Three 5-minute long exposures were stacked together for the final image. The image stacking technique allows you to shoot without using in-camera noise reduction, which helps with shooting productivity and battery life. The star trails look the same as one 15 minute exposure, but sometimes this can create a strange effect when there are clouds in the sky.
When you're shooting long exposures, the amount of cloud definition depends on two factors: how fast the clouds are moving, and the length of the exposure. For full moon night photography, fast-moving clouds usually show a good blend of movement and definition when exposing for 2-4 minutes. When the clouds are moving more slowly, longer exposures are possible. If you expose too long, the sky can simply turn white without definition.
In the photo above, each 5 minute exposure captured a distinct amount of cloud movement. When the image was stacked for star trails, the additive cloud definition created a slightly ribbed pattern in the sky. If I hadn't pointed out this effect would you have noticed? Because the sky is about a 50/50 split between stars and clouds, I think it works in this image. The clouds can start to look unnatural if you try to stack more images, or use shorter exposures.
Each image could stand on its own if I decide that I don't like the effect. Stacking also allowed me to have different options for the amount of light from cars on the highway and road on the right. The highway shows the cumulative 15-minute exposure, and the road ended up looking better with just the red tail lights of one exposure.
If you've tried stacking images for star trails that also have a lot of cloud movement, how did they turn out?