Star Trails: Single Long Exposure or Stacking?
I often get asked, is it better to shoot a single long exposure for star trails, or stack several shorter exposures from a timelapse. Shooting a single long exposure is easier if you have a simple remote that supports locking the shutter open in bulb mode. You can use any stop watch as a timer. Shooting a timelapse to stack images is a little more sophisticated and requires an intervalometer that is either built into most Nikons or can be run on many Canons via Magic Lantern. Many remotes also have an intervalometer function such as the Promote Control. There is no right or wrong method to shooting star trails, but I prefer stacking over a single long exposure and here are some of my reasons why:
1. If you ever want to render a timelapse video, obviously you will need multiple frames vs. one long exposure. At 24fps or 30fps, you’re going to need to take a lot of 30 second exposures to get a 5 to 10 second video clip, but it leaves the option open if you shoot enough frames. I like this flexibility.
2. Noise reduction is a lot easier with a bunch of shorter exposures vs. one long exposure. Long exposure noise reduction is different from high ISO noise reduction, and the longer the shutter is open the more hot pixels you will get. Shooting a half hour exposure and then waiting for a half hour long exposure noise reduction is a colossal waste of time and battery power. Instead you can shoot a "dark frame" of just the hot pixels by putting the lens cap on and shooting at f/22 but keeping the same ISO and shutter speed, then apply that dark frame to all the other timelapse frames via PixelFixer (free) on the RAW files before importing into Lightroom or editing. It's important to do this right before or after your timelapse so you have the same ambient temperature on your sensor. Using this dark frame method lets you disable long exposure noise reduction on all the rest of the shots cutting your shooting time in half.
3. It’s much easier to clone out airplane trails and other objects that get in your image from a bunch of short exposures vs. one long one.
4. The same is true for blending in things you might want to include like trees or other foreground objects lit up by light painting or vehicles driving by. Lit up objects will not show up much at all in a single long exposure (figure 10 to 30 seconds of light out of a total of 30 minutes—barely visible), but many stacking methods will grab the brightest pixels of each frame and blend it properly. Of course, you can manually blend layers in Photoshop with either method of shooting so this isn’t a big deal.
5. A dead battery, tripod vibration, something walking in front of the lens, clouds in the sky, an unexpected light source like car headlights, and a host of other unexpected things could ruin an entire long exposure, but only screw up a few frames with the stacking method, particularly if it's at the end or beginning of a timelapse. This alone is the best reason for stacking multiple shorter exposures in my opinion. But neither approach is right nor wrong. In film days we had to do a single exposure, but digital opens up new opportunities.
6. Comet trails are much easier to create by stacking images. With a single long exposure you’d have to gradually adjust your aperture over time to fade the stars in or out for comet trails. With stacking you can change the opacity or brightness of each layer gradually with StarStaX or Advanced Stacker Plus (my favorite stacking plugin for Photoshop).
Hope that helps! Happy shooting!