Exposing for Star Trails

There are two exposure methods for creating Star Trails photographs: single timed exposures or interval exposures. Single timed exposures are finished photographs in the sense that they can be immediately shared or post-processed aesthetically. Interval exposures require post processing in specific Star Trails stacking software to actually create the Star Trails photograph. I recommend the latter (interval exposures) but it's good to understand both processes and the benefits and challenges of each.

90 minute Star Trails photograph
90 Minute Single Exposure

Set up (applies to both methods)

Assuming you've decided on your composition, you should place your camera on your tripod, set your composition, and lock everything down. For the majority of Star Trails photographs you're going to want to set your focus at infinity and your aperture at its widest setting (somewhere between f1.4 - f4.0 on most lenses). Be careful with the infinity focus. I'm used to infinity being the very end point of the focus ring on my lenses so it's easy to do without looking. But some lenses, including the Nikon 14-24mm I originally rented, actually rotates past the infinity mark resulting in blurred photos (that flashlight came in handy once I figured out what the problem was).

Once you have your focus and aperture set, then it's time to set camera ISO and shutter speed. This is where things get a little subjective and it's best to run a couple of test photographs before committing to a final base exposure. If the sky is well past sunset (near black sky) then I'd test with ISO 1200-2000 and a 20-30 second exposure. You should get a good star field (including the milky way) and only slight movement in the stars themselves. Of course, even with the best cameras, ISO 1200-2000 is going to introduce some noise which can be mitigated with post-processing. Closer to dusk (60-90 minutes after sunset), when there's still some color in the sky, you can generally drop your ISO to more noise manageable levels. In moon light scenarios (50-70% illumination is my preference), I tend to drop my ISO down to the 800-1250 range. These choices are subjective: test and find your preferences.

Straight Exposure Method

With your camera set up and ready to go, your first option is to simply do a long exposure (basically anything over 30 seconds fits this category). As I said previously, you'll need Bulb "B" exposure capability and a remote cable release. The timing of your exposure depends on how long of a star trail you wish to capture. The pictures below demonstrate progressive examples at 15, 30, 45 and 60 minutes.

15 minute Star Trails photograph
15 Minute Exposure
30 minute Star Trails photograph
30 Minute Exposure
45 minute Star Trails photograph
45 Minute Exposure
60 minute Star Trails photograph
60 Minute Exposure

One of the advantage of longer exposures is the option to use lower ISO settings (less noise) and/or smaller apertures for greater depth of field. The 90 minute exposure at the top of this page used a very reasonable 400 ISO and f6.3 aperture to give me some depth-of-field in the foreground.

Digital Noise from an ~30 min exposure
Noise from a 30-min exposure

Long exposures with digital cameras have a major drawback: noise. Even with my D800e noise was a problem and required significant post processing (spotting) to remove. This is especially true it you wish to make a sizable print. The D800e offers in-camera high ISO noise reduction but this requires nearly equal time to process in camera as the exposure. A 90 minute exposure took an additional 90 minutes to process. That's 3 hours of total time. And while the heavy noise was reduced in the most contrasty areas of the image, I still found significant noise throughout the image.

It's hard to suggest this method unless you enjoy spending hours at the computer cleaning up spots to get a good printable image. On the flip side, the photographs have their own pleasurable aesthetic. And image taken close to sunset or sunrise can minimize the noise problem. This is one area where the grain of tradition film still has a leg up on digital sensors: essentially no noise. Film does have reciprocity problems which cause significant color shifts but this is generally much easier to deal with in processing.

Interval Exposure Method

This second option, the more popular method, is to create a sequence of exposures that are subsequently "stacked" using post processing software designed specifically for creating star trails. While, in theory, you could accomplish the exposures manually, the better option is to use a camera that supports interval timed exposures. With interval timed exposure you can create a sequence of 30 second exposures over the time period you wish to expose (i.e. a 30 minute exposure would require 60 single shot 30-second exposures). Check your camera's manual for instructions and run a couple short tests.

With 30 second exposures as your base line, I found ISO 2000 about right to capture good detail in each exposure for post processing. One additional consideration is file type. Targeting a 60 minute Star Trails photograph requires 120 30-second images. On a D800e in full RAW format at 36.3MB, that's about 4GB of total file space eaten up by just one sequence. I'd planned for this as I hoped to be able to ultimately create a 16 x 20 print from the final image. But your goals may not be so ambitious, so going smaller RAW or even JPEG may suit you. Just make sure you have enough memory space on your card. Nothing worse than getting through 100 of 120 images and having your card fill up.

The Interval Timed Exposure Problem

Interval timed exposures have one persistent problem across cameras. There is a time gap (where the camera writes the exposed image to the memory card) that must be accounted for in your settings. Not accounting for the time gap means you'll end up with significantly less total exposures than desired. In my initial testing with the Nikon D800e, a setting of 10 total 30-second exposures at 30-second intervals yielded only 5 photographs. As I increased the time interval my net photographs increased with a 33 second interval ultimately doing the trick. Different camera systems will require different adjustments to cover the time gap. Make sure to test your camera before going out.