How to shoot star trails: Intrigue vs. in your face

Shooting star trails is a rewarding part of night photography, be it in complete darkness in nature (because they become an important compositional element in the darkness) or in the middle of a city (because nobody expects to see those and also because it is challenging to bring them out and contrast with the city lights).

Unfortunately, there seems to be more misinformation than good advice out there and I keep seeing what are considered "professional" (usually daytime and landscape) photographers who teach people to crank up the ISO to 1600 (or higher) and keep shooting 30 second images to be combined into star trails. To generalize this is bad advise for many reasons. I've seen a presentation at last year's NECCC conference, where the presenter suggested, and actually tried - live! - to combine 340 high ISO, 15 second shots into one star trail. And all of that in resource hog Photoshop. Why he attempted or suggested that is beyond me. If he had shot 8 minute images, he could have done the same with about 20 photos. Needless to say, the demo was terminated by rebooting the computer. Not a good presentation, I'd say, I almost walked out of so much incompetence.

So, how does one go about shooting star trails? 

Because star trails look best with some foreground feature, the way I teach star trails is to figure out the correct exposure for the ambient environment that forms the foreground. Of course, you can consider purposely underexposing the foreground to be a silhouette only, which might look good, but seeing the landscape usually looks better. You can use high ISO preview to quickly figure out the correct or best exposure. 

I also believe that star trails look best if they are "the gravy on top" rather than the main subject. In some cases (see above), they do make good main elements, but even then, I feel that less is more. Subtle star trails add intrigue, over the top circles are crazy and get attention, but won't often be considered "fine art". Many people advise to open the aperture wide, which is the right approach to shoot the Milky Way and star points, as the number and brightness of stars goes up with wider apertures. Star trails formed from Milky Way shots, however, usually look way too busy and over the top, see the example at left. And, of course, I realize that some of this is personal taste. As always, my advise here is just a good starting point, there are no hard rules in photography, but following the below guarantees that you will get decent star trail photos.

So, start out by choosing your apertureThe best range and compromise between brightness, number of stars and sharpness/depth of field is usually in the f/4 to f/8 range with the sweet spot being f/5.6-7.1. My personal favorite is f/5.6.

Next off is ISO and, as for regular night photography, photos have the best quality at lowest ISO setting. Digital cameras are MADE for their native ISO setting (usually 100, sometimes 200). Any increase in ISO from its native setting quickly degrades photo quality: you get more noise and the dynamic range of your camera diminishes. The Nikon D750 has about 14.5 stops of dynamic range (translating to about 23000 steps of luminance that the camera can distinguish). At ISO 6400, that camera has only a little more than 9 stops of dynamic range left (less than 600 steps between pure black and pure white!!). Highlight and shadow control go out the window at such high ISO settings, which is why most Milky Way shots look very muddy in the foreground. Hence, the recommendation is to use lowest possible ISO for highest possible quality.

Now that we settled on an aperture and fixed the ISO, exposure time is the last step. Use high ISO preview to maximize your exposure time and "expose to the right". Outside the city, exposure times may be rather long, 30 minutes or longer in really dark areas. Modern cameras can handle that and if you take only one exposure, you can use Long Exposure Noise Reduction (LENR). However, if you want a decently looking star trail image, you need to shoot for at least 30 minutes, and at least an hour when including the North Star (Polaris). It gets really interesting at 90 minutes plus. Waiting for another hour or two for the noise reduction, though, is painful at best. 

Note that this advise does not apply to Milky Way shots, where exposure times are limited to 20-30 seconds.Time lapse photography also works better with short exposures and has its own needs for settings. 

There is a better way of doing this: compose your trail of multiple individual shots, preferably in the 4-8 minute range, which minimizes shot noise and amp glow and risk of loosing everything with one battery failure, but also minimizes the number of photos. You will need to turn LENR off because it would create a sequence of trails and gaps. However, you can take ONE (only 4 or 8 minute long!) dark frame before and/or after the sequence (same ISO and exposure time, but with the lens cap on) and subtract that from each of the individual shots in software in order to reduce what I call "ugly noise", the non-random shot noise and amp glow of the digital sensor, which gets worse with exposure time (and higher ISO). 

The above first trail in Death Valley is composed of 90 four-minute photos for a total of about six hours (without any noise reduction, as you can tell if you blow it up - back then I didn't know dark frames). Some frames were light painted for a more interesting foreground and I included a frame that still had some light in the sky to not be completely black. With my newer (less noisy) cameras, I would probably choose 8 or even 15 minute individual shots for this.

The time saving through this way of assembling the trail is that one short dark frame is sufficient to reduce noise on all of the images, as long as they all have the same exposure time and ISO and ambient temperature doesn't change dramatically. Moreover, if you shoot 4-minute photos at low ISO, you may not need any dark frame subtraction on modern cameras, as they are clean enough. And finally, minimizing the number of photos makes it much easier and quicker to composing the final star trail or to mask out unwanted parts in Photoshop.

Inside the city, exposure times may be very short because of all the ambient lights. The photo on the right is taken at Boylston Street in Boston. The maximum exposure time I could squeeze out at f/5.6 inside this heavily lit courtyard was 10 seconds and this is a composite of 170 images for a total of almost 30 minutes. It took more than usual post processing by increasing clarity, highlights and sharpness (applied with the paint brush only to the sky portion) to bring out the star trails, but it works quite well. In less brightly lit areas in the city, it is easy to achieve 20 and 30 second exposures, however, making for less processing (such as the raised platform below). 

How to assemble star trails?

If not taking a single, very long exposure star trail image (I encourage you to try it out and learn from it!), most people choose to stack multiple images together. This can be done in Photoshop or with dedicated software. 

1) Photoshop
The very brief instruction is to load all images as individual layers into Photoshop, then highlight them all and choose the LIGHTEN blending mode to blend them together. Save the file and you're done. The main advantage of this is that you will be able to mask out unwanted portions in some of the frames (add a layer mask to that frame), such as bad light painting, a car driving by or a person with flashlight walking through your shot, an airplane trail etc. The main disadvantage is that Photoshop is a resource hog and it may take hours to do this with many photos on normally configured computers. 

Photoshop also does not close the gap that you will encounter between the individual trails. Contrary to popular belief, those gaps are NOT from the one second interval that intervalometers need between shots. Just like you can shoot stars for 2-5 seconds without seeing much star elongation, you can actually interrupt the exposures (e.g., to check correct exposure) for a few seconds before the gap becomes noticeable (because the star will have moved to the next pixel). The gaps that appear in normal blending come from a "rounding" of the ends of each trail, which is caused by the algorithm that cameras or the raw file converters use to process the raw-raw data from the Bayer sensor (it would be interesting to know if the Sigma cameras with their Foveon sensor have the same issue). Nothing you can do to avoid it, I hope that somebody will fix this eventually. Russell Brown has a star trail automation script for Photoshop.  (Russell Brown's Stack-A-matic; I found installation and use of this script very unintuitive, I was not able to make it work on my PC.).

2) StarStax
In the meantime, what you CAN do about the gaps is to use free software such as StarStax,
which works for all major operating systems (StarTrails is also free, but only for PC and there is paid software of the same name for Macs). StarStax is very easy to use, you drag and drop images into its interface, click a button and you're done. Biggest plus is that it has a lighten blending mode that also closes the gaps and you have some control over the sensitivity of that gap closure. It also lets you subtract dark frames for noise control and it has "meteor" modes, which slowly blend in or out the trails to make them look like falling stars. Aside from those features, the other main advantage of this program is that it is lightning fast compared to Photoshop. The disadvantage is that it saves the output only as JPG or 8-bit TIF, not as PSD or 16-bit TIF (a licensing issue) and, of course, you cannot mask individual foregrounds.  To save as TIF, you need to type in the extension .tif for your output file, otherwise it saves as JPG, by default.

3) A combination of both
The Death Valley star trail at the very top is made up of 90 four minute photos. About 20 of them were light painted, the rest are ambient shots. Assembling 90 photos in Photoshop is painful even on the latest computer hardware unless you have dedicated work stations with lots of RAM and high performance drives. What I did there was to stack the 70 ambient photos together in StarStax in a matter of minutes. That created a very long star trail with 20 gaps because I left out the light painted images. I saved that as a TIF file.

I then loaded those 20 light painting photos into Photoshop, stacked them using the lighten mode, which created a chaotic foreground as well as a spotty star trail. I then added a layer mask to each photo and masked out everything I didn't want in the foreground, without touching the sky portions. Usually, I kept only the light painting from 2-4 photos. I saved that output as my first version .psd. 

Then I took that first light paint version from PS and stacked it together with the 70 image stack from StarStax (I am stacking only two images at this point!) to get the complete 6-hour star trail with some good looking foreground. 

Then I went back into PS, masked out the light painting I just saved and unmasked some other light painting, saved that as a version 2 .psd. Stacking this version 2 with the original 70 photo ambient stack (again, just two images!) created a second version of the 6 hour stack. And so on. I ended up creating about 10 different versions of the same photo, all with the same 6 hour star trail, but all with different foregrounds (see left). Very substantial time saving and very manageable in PS.

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