Technical details: Three exposures of 7 minutes were stacked for 21 minute long star trails (7 minutes at f/9.5, ISO 200). There is a 1 second interval between exposures -- any longer would show gaps in the star trails. The Canon 5D Mark II is clean at 7 minutes without the need for noise reduction, as long as the ambient temperature is not too warm.
On the first image I did not light paint. The 2nd and 3rd exposures had different light painting. After the first three images were complete, I reviewed the light painting on the back of the camera. The interior and side of the car looked great in image #3, but there was a hot spot above the front grill. I did a fourth exposure of 3 minutes at f/8 to re-do the light painting on the front of the car. Below are all 4 images in Lightroom.
Here are the steps involved in post-processing the final image:
- The first 3 images for stacking star trails were processed in Lightroom with identical settings
- The 4th image was processed in Lightroom just for the light painting
- On the top menu in Lightroom, I opened all four images into one file using: Photo -- Edit In -- Open as Layers in Photoshop
- The 2nd and 3rd image were set to Lighten blending mode to stack the star trails
- I selected the sky and made a layer mask on the second and third image to hide the light painting but keep the star trails
- I added a layer mask to the 4th image, and filled it with black
- Using a soft brush at 20% opacity, the light painting from the 3rd and 4th image was added using layer masks
- A Selective Color adjustment layer was used to make subtle changes to the color of the car and also the sky
I hope this behind the scenes look at creating a light painted image with long star trails is helpful.