Exposure and noise reduction settings for star trail stacking
Five exposures of 6 minutes at f/8, ISO 200 were stacked for star trails. When you make multiple exposures for star trails, the interval between each shot must be 1 second or less, so there are no breaks in the trails. You can light paint during the exposures, but you won't be able to review the results on the back of the camera until the stacking is done.
The 6 minute exposure time was selected because f/8 is the optimum aperture for both sharpness and depth of field. The 5D Mark II can make clean 6 minute exposures with in-camera noise reduction (LENR) turned off as long as the temperature is under about 60°. Shooting with LENR turned off is much more efficient because you don't need to wait for noise reduction to run after every shot. This also helps conserve your battery.
Start with the light painting, and leave one dark
When you're doing a series of light painted exposures with stacked star trails, it's a good idea to not light paint on one of the images. This moonlight only image will give you a base for adjusting the lighting in post-processing if necessary. The recommended shooting strategy for a shot like the one above is to do a series of shorter exposures for light painting first. The process of light painting and then reviewing the image on the LCD allows you time to really assess your lighting and composition. Once you've nailed the light painting, you can decide if you want to commit the time necessary to fill the sky with star trails, or move on to another shot.
Shorter exposures for light painting
In this case, I actually started the series of star trail stacking images, and went to help someone with another shot. I was confident that I liked the composition, and knew I could come back to add the light painting. Half an hour later, I reviewed the moonlit images on the LCD. I made three additional exposures of 3 minutes at f/8, ISO 200 for light painting. I didn't worry about the interval between exposures, because these additional shots were only for the foreground subject, not the sky. The shorter exposure time of 3 minutes allowed me to work faster, and made the light painting more contrasty because there is 1 stop less moonlight on the foreground.
The star trail images were stacked in Photoshop using the Lighten blending mode, and the best of the 3 light painting exposures was added to the foreground using a layer mask. Bringing the light painting in on a mask allowed me to make subtle adjustments to the lighting.
Explain the light painting and win a print
Can you tell where I stood to do the light painting? The first person to correctly answer the question in the comments wins a small print of the image.
Hint: Almost all of the lighting was done from one position, and a little bit of fill was done from a second position. Look at the shadows, and explain what was lit from where.
The print will be a 6" x 9" image on 8" x 10" paper. Lower 48 only.
Update: The image was primarily lit from camera right at a 45° angle from the right of the machine using a Streamlight Stinger flashlight and a piece of Cinefoil to control spill. The tires would have been completely black without light painting, and required quite a bit of light. The shadow on the circular piece between the tires, and the shadow on the left front tires are the key to lighting direction. The interior was lit from the same position, and I pivoted slightly to add a little bit of fill to the tires on the right. A small amount of fill was added from camera left to the muffler area and the 2 metal pieces that stick out above the windshield.