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).
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?
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 aperture. The
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
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.
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).
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.
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.
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
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.
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.).
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
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.
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.
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
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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How to shoot star trails: Intrigue vs. in your face
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