Photo Stacking and Long Exposures – Part 2: Taking the Photos


Welcome to the second part of my series on photo stacking  — a technique for taking a series of photos and turning them into one long exposure. As a quick review, photo stacking relies on taking many short exposures of the same scene, with the exact same exposure. The photos are then combined together to appear as if they were one long exposure.

Part one covered the basic equipment and software needed for photo stacking. In this part, we’ll talk about how to plan and execute a photo shoot for future photo stacking. Our focus is especially on star trails: long nighttime exposures which show the motion of stars through the sky. Next time, we’ll discuss the post-processing required to create the stacked photo.

Preparing and setting up

Although I will come at this from the perspective of star trails, you can apply these same basic principles to almost any other kind of photography which will involve stacking.

In the first part, I discussed the importance of having three main items: A camera (with lens), a tripod, and a way to get your camera to take many photos at pre-defined intervals (usually called an intervalometer). While it’s important to have these, there are some other important steps to take before heading out for a night of long exposures. Let’s start with the most important: where will you go?

  1. Identify a location. This is the artistic part: find somewhere where you’d like to take some nighttime photos. There really are no limits, but one important thing to consider is light pollution. Even a small city 20 miles away can pump out an awful lot of light, so a remote location is very desirable. On the other hand, it is possible to take dramatic cityscape star trail photos, if the atmosphere is clear enough. Also consider the likelihood of people with cars, flashlights, etc. ruining your carefully set-up photos.
    "Hoist Stars" by David Clark

    Silhouettes can make excellent star trail subjects

  2. Explore your location during the day. I’ve learned from hard experience, so that you don’t have to: do not explore a new location for the first time at night! Once you’ve identified a potential location, scout it out during the day. Explore it, take some sample photos, and find some good subjects. Once you’ve decided on a general composition, identify possible set-up locations. An ideal set-up location is flat (to hold your tripod and equipment), reasonably accessible (so you won’t be falling off cliffs in the dark), and far away from light pollution, traffic, and other people. You’ll want to be very familiar with your site, so that when you come to take photos, you can move in, set up, and be prepared without having to make important decisions under the pressure of oncoming night. (This brings me to a pet peeve: some photographers seem to think that they have to go to a location, find a scene, take one photo, or it’s not real art. Heck no! Spend tons of time. Take lots of sample photos. Look at them, change things, and take them again. He who dies with the lowest shutter actuation count is not the winner!)
  3. Outfit yourself. Collect appropriate clothing and gear for yourself, before thinking about your camera setup. While your exact outfit will depend on your location, season, and weather, you should always be prepared with warm, layered clothing, a hat, gloves, and at least two light sources. Even in the middle of summer, nights can get quite cool, and they’re always dark. Know your area to understand what gear is most appropriate. If you’re planning to camp out, be sure to bring the ten essentials for camping. Incidentally, camping under the stars while your photos take themselves can be a wonderful experience.
  4. Collect your hardware. Much of this was discussed in part 1 of this series. At a bare minimum, you’ll need to bring a camera and tripod. If you’re not going to use a built-in intervalometer (or if your camera doesn’t have one), you’ll also need to bring whatever external controllers you need. For my setup, this means a laptop, and a USB cable to connect the laptop to my camera. I also like to bring spare batteries and a box to set the laptop on — especially if I’m going to be in snow, this will help protect those valuable electronics!

Pick a night

Lake Superior stars -- what happens when clouds interfere

The next step is to decide when you’ll take your photos. If you’re aiming for star trails, you’ll need a nice clear night, preferably low in humidity. Sometimes this requires flexibility — a night will be unexpectedly clear, even when the forecast said otherwise. If you can manage it, grab the opportunity! Otherwise, follow your local forecast and try to plan ahead.

Some artistic choices to consider: Clouds can create an interesting effect, but because you’ll be taking fairly long exposures, they will usually “smear”. This may or may not look really awesome — it’s up to you! Meteor showers are another common choice for long exposures, as you get an effect of streaks of light radiating out from a source. Airplanes and some man-made satellites (such as the space station) will also show up in photos, making very obvious, bright lines.

On the day of the photographing

So, you’ve got everything set: you’ve chosen a site, scouted it out, collected your gear, and now the day has arrived. You’re getting ready to head out — here are some things to think about.

  1. Arrive well before sunset! Plan lots of time for your photo stacking adventure, including arriving at your location  at least an hour before it gets dim. You don’t want to have to hike in to your favorite location, set up your gear, compose your shot, and troubleshoot with the pressure from a quickly darkening sky. To determine when you should arrive, you will need to find the time of twilight. Many weather sites list sunrise and sunset times,  but my favorite tool is The Photographer’s Ephemeris, or TPE. TPE is a very flexible app which will show extremely detailed sunrise, sunset, and altitude information for any location and date you want. You should be especially aware of the difference between civil, nautical, and astronomical twilight. These are three different types of twilight, each one denoting steadily later times and greater amounts of darkness. You should arrive at your site at least half an hour before civil twilight, but don’t plan to take photos until at least nautical twilight. For the darkest possible skies, wait until astronomical twilight. However, the slight glow from a setting sun can provide a nice background for your first star trail shot.
  2. Pre-compose and pre-focus. It can be extremely hard to compose an image when it’s dark out. In DSLRs, the image in the viewfinder is nearly impossible to see in the dark — this is especially true when lights from the controls and LCDs in the viewfinder destroy your night vision every time you look through it. Focusing is even harder: if you have a specific subject on which you want to focus, it’s likely to be too dim for your camera to successfully autofocus. Manual focusing in the dark is difficult for the same reasons as composing. Compose and focus when there’s still light, and leave your camera in that position for the rest of the night.
  3. Secure and protect your gear. After setting up your camera, it’s important to ensure that it stays in place. This usually involves tightening down the tripod’s head position. You may also want to place some sort of protection or stabilization around the tripod’s base, so that you don’t accidentally hit it or trip over it at night. If available, I use milk crates to block off the legs of the tripod. If not, cardboard boxes, rocks, or even small snow drifts can help. Yes, snow drifts. You equipment can handle it!

Composition and Exposure

Before pulling the trigger — the camera’s trigger, that is! — let’s discuss some artistic and technical choices. In this section, I’ll briefly discuss some guidelines, tips, and pitfalls concerning the composition and exposure settings in your photos.

  • Subject. It’s very exciting to see star trails for the first time, but a star trail photo needs to contain more than just moving stars. Compositionally, star trail photos need to be good photos in their own right, stars or not. In particular, having a strong foreground is extremely important. This could be a lonely tree, a building, a ruin, a mountain, or almost anything interesting. An interesting shape in silhouette (or barely visible) will produce a striking photo — the moving stars will emphasize the shape. Negative space is also very effective for the same reason.
    Star trails over the Quincy Mine

    Stars over the Quincy Mine -- a good subject enhanced by stars

  • Stars. However, you shouldn’t ignore the stars entirely, since they will be a striking part of your photo. Here’s a brief astronomy lesson: in the northern hemisphere the north star (aka Polaris) is the star around which all other stars appear to rotate throughout the night. The earth’s axis of rotation points (approximately) towards the north star, so that as the earth turns, the sky appears to rotate around Polaris. If you compose with this in mind, you can get a number of effects. If you point towards Polaris (that is, point north), you will have an effect of stars rotating around your subject. If you point to the left or right of Polaris, you’ll get more pronounced movement in the stars, but with less curvature. Pointing towards the south will result in nearly straight star trails, appearing to come out of the horizon. Pointing overhead will give an interesting “saddle” effect. Factor this into your composition. For southern hemisphere readers, the Southern Cross is the best approximation to a constellation which points south. The same advice applies, however, with “north” and “south” flipped.
  • Exposure. Choosing a proper exposure will depend on your individual setup. However, there are certain general rules. First, you must set your camera into full manual exposure mode. The purpose of photo stacking is to combine multiple photos with exactly the same exposure — letting the camera choose a meter is a recipe for inconsistent, blotchy exposures. If your camera supports automatic ISO selection or dynamic ISO adaptation, be sure to turn that off as well. Finally, set your camera to manual focus, as mentioned above. As far as specific settings, choose a long exposure: typically at least 30 seconds, unless there are artificial light sources in your photos. Take several test photos on the night of your photo outing to see what works best for your aperture and ISO.
  • White balance and photo quality. These are very technical details, but important ones. For some reason, most human eyes are trained to believe that the night sky is blue. It isn’t, but star trail photos sometimes appear black or grey, and this can be very displeasing. I recommend setting your camera to Incandescent white balance (or a fairly low color temperature — around 3000-4000K), to achieve the pleasant blue effect. Also, shoot jpegs. This is no place for the RAW vs. jpeg debate, but the vast majority of stacking software can’t handle RAW files. So, you’ll have to use jpegs. This will also help your camera avoid delays due to saving gigantic RAW files.
  • Moon. Finally, beware of the moon! First, for exposure: The moon, even if it’s not in your frame, will still light up the sky. A typical exposure on a full moon’s night will be very different than an exposure on a moonless night. Secondly, for composition: The moon moves very fast — much faster than stars. It will “smear” across your photos if it makes its way into them. You will need to know where the moon will be during your photos, to take this into account. The Photographer’s Ephemeris (mentioned above) can help you predict this.
  • Length. The longer you’re out taking photos, the longer your star trails will appear. Of course, battery limitations can keep you from getting as many shots as you want — almost every photo featured in this article represents 1.5 hours of shots, due to my camera’s battery limitations. The apparent length of star trails is also affected by your point of view: The closer a star is to Polaris (or the Southern Cross), the smaller its apparent movement. So, star trails will appear longer if you point slightly away from Polaris, or if you zoom in and photograph a relatively small segment of sky.

Take the photos!

This part will vary depending on your setup, but the idea is simple: take as many photos as you can! Whether using an intervalometer, a built-in interval function on your camera, or an external computer controller, simply take a bunch of photos. Sit back and enjoy the night, hang a hammock, pitch a tent (but don’t let the light bleed out into your photo!), or be brave and drive away for an hour.

On second thought, do put a tent in your photo. It can make a fascinating subject, lit from within. But whatever you do, make sure it’s intentional!

The amount of time which your camera batteries will last depends very much on your camera, batteries, and the temperature. If this is your first time, I recommend staying nearby to see how long it takes — you’ll learn over time.

Try and try again

Having said a lot about planning and executing star trail photos, I want to leave you with the most important bit of advice in this whole article: Keep trying! Your first star trail shoot will almost certainly go poorly, whether due to equipment troubles, focusing problems, bad weather, or a composition which turned out to be less interesting than you thought. Don’t bet the farm on just one try. Just like all other photography, your star trail photos will improve over time. You’ll learn to deal with your equipment better, to see possible compositions with star trails in mind, and to learn the little details which make nighttime photo shoots work better. In many ways, learning to take star trail photos is much like re-learning photography with a new set of rules. Keep it up!

Next time, I’ll discuss what you can do with the hundred or so photos which you’ve obtained by the end of this article. For now, start checking the weather and dreaming of good compositions!

Previous:  Part I – Introduction

Next:  Part 3 – Stack


About Author

David Clark is a mathematician and teacher from the Upper Peninsula of Michigan. Outside of math, David is an improviser, explorer, and photographer. His photo blog documents the industrial heritage and natural beauty of Michigan's Copper Country.

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