A compendium of tips learned while taking images like those shown in the Image Gallery and the figures below. Some star trail related tips also apply to other low-light photography scenarios. This version is revised significantly from the original version and has been moved to the StarCircleAcademy.com blog.
TABLE OF CONTENTS
- Low Light Rewards
- What Kind Of Image?
- Sky Considerations
- Preparing the Photographer for the Night
- Preparing the Camera for the Night
- Getting the Right Exposure
- To Stack or Not To Stack
- Optimum Star Trail Strategy
- Processing Images
- Additional Tips
Obtaining images at night or in very low light conditions can be surprising and rewarding. The camera reveals things that the unaided human eye cannot see. Our eyes only discern shades of gray in low light, but the camera sees color. Turning out the light does not make a thing go away. The chair still remains in the dark room as is painfully clear when “discovered” by means of the shin. Although our eyes can not see color in the dark or collect photons (light) over long periods of time the camera can. The amazing photographs from the Hubble Space Telescope are sometimes created by exposing the sensor for minutes – or hours – at a time. Because there is so little light it takes a long time to collect enough photons to reveal what is there. The camera can also “see” wavelengths (colors) that the human eye cannot – for example, infra-red and ultraviolet.
Many kinds of images are attainable at night or in low light that cannot be obtained under “normal” conditions. For example a flash, or combination of strobes or gels can be used to illuminate a scene to dramatically alter the strength and direction of the light. In the same way long exposures allow for illumination of key parts of an image with a flashlight, headlight, candle light, or glow stick. If the exposure is long, the photographer can even move through the image while it is being exposed and illuminate different parts of the scene in different light. This technique is called light painting.
It is possible also to combine multiple exposures with different illumination in each. And, starting with a sequence of shots it is possible to combine them into a time-lapse movie, or into a single image.
The first thing to decide is what kind of sky image to capture. Capturing star trails
is different from capturing a star-filled sky.
Just as motion blur occurs when the shutter speed is too slow for a moving subject the long exposures needed for night photography allow the earth to rotate enough to streak the stars: no kidding!
To obtain a natural looking starry sky the exposure must be less than the time it takes for the stars to noticeably streak. How long is that?
- Divide 500 by the focal length in 35 mm terms to get the maximum number of seconds of exposure without noticeable blurring. A 28 mm lens on a 1.6 crop factor camera yields: 500 / (1.6 * 28) = 11 seconds. (You may see this described as the 600 rule, but my experience is that 600 will result in streaks. For much more detail on the rule, see here.)
11 seconds is a relatively short time. If a 200mm lens is used streaks begin in 1.6 seconds – a very, very short time for a night exposure.
For star trails the limiting exposure time will be due to one of the following issues: battery life, the amount of background glow in the sky, or the relative noisiness of the camera sensor. More on these issues in a moment.
This one is pretty simple. Seek the darkest skies possible. Dark skies are found far away from city glow and when there is no moon in the sky. But don’t despair if you live near heavily light polluted cities or only have a chance to shoot in strong moonlight. The sky is darkest when the all the following are true:
- There is no visible glow in the direction of the photographs from any man-made sources. Sometimes such glow is noticed only in long exposures!
- The sun has set for at least 90 minutes or it is at least 90 minutes before sunrise.
- The moon has set for at least 60 minutes or it is more than 60 minutes before it rises.
NOTE: The moon is a very useful tool to get some light on your foreground objects – particularly when the foreground is large and distant from the camera. The best star trail shots usually either start as the moon is setting or end when the moon is rising. If there is no moon, shooting can start or end at twilight.
If the Milky Way is visible from horizon to horizon the environment is definitely dark enough and star trails are possible. Even when only a few stars are discernible in the sky, and even when the moon is full or nearly so star trails are usually possible – they just need more planning – usually shorter exposures.
Clouds – especially high, thin clouds can add some dramatic “milkiness” to the image – as can the Milky Way itself.
This one is pretty simple. Bring layers of clothes and dress more warmly than expected. A thermos of hot beverage, snacks and perhaps a blanket or sleeping bag and a comfortable chair are all useful. While it is very tempting to use a flashlight or a headlight to see in the night and it is wise to bring one of each better overall night vision is achieved by turning off ALL light and allowing the eyes at least 15 minutes to dark adapt. Turning off all lights should include turning off the camera’s various displays, or at least turning them to the lowest brightness level possible.
When photographing with a digital single lens reflex (DSLR) camera each of the following guidelines should be heeded (in fact, we have a checklist with even more detail):
- Use the fastest, widest lens for capturing the sky such as f/2 or f/2.8 at 17mm (or wider) if possible. f/4 or f/4.5 will work, too.
- Keep extra batteries accessible.
- Know the camera controls and where the buttons are without resorting to flashlights or artificial light.
- Use a sturdy tripod, well supported and weighted down. Insure that you can change the memory card or the battery without moving or removing the camera on the tripod.
- Turn off all the accessories that drain battery life – e.g. turn off the “image preview” feature and the on-screen display of shooting data.
- If using a timer with an initial delay, the camera may auto-power off before the first image is taken. This means the camera must be able to return to the settings needed, not back to different power on settings. For many Canon DSLR models, it may be necessary to use the C1, C2 or C3 (customer user settings) function.
- Pre-focus on infinity and remember to TURN OFF AUTOFOCUS! The moon or a bright light in the distance are great tools for focusing the camera in the dark if the daytime focusing is forgotten or has been changed accidentally. On some camera models if Auto Focus (AF) is not turned off, the camera may refuse to take a picture since it can not focus in the dark.
- Set the camera to manual mode. Normally “BULB” mode is used. Bulb mode keeps the shutter open as long as the shutter button is pressed.
- Attach a programmable release cable (preferred) or locking remote release cable (second best), or be ready to have your finger, or a jury-rigged apparatus for holding your shutter button down (e.g. pebble and tape or rubber band).
- Set white balance to daylight.
- Turn OFF long exposure noise reduction.**
- Turn OFF high ISO noise reduction.
**Q: Why turn off long exposure noise reduction?
A: Long exposure noise reduction will introduce gaps in time between exposures. When taking a SINGLE shot “long exposure noise reduction” can be left on. But remember that when left on the exposure will take up to twice as long to complete. High ISO noise reduction may not be as effective in-camera as out of camera. Try it both ways.
Q: How long do I have to expose to get a trail?
A: As discussed earlier, the length of the trail depends on the field of view of your lens. The field of view can be calculated from the focal length and the crop-factor (also called focal length multiplier). Stars at the celestial equator travel 15 degrees each hour. If the field of view is 15 degrees – as it would be at 100 mm – one hour of exposure produces edge-to-edge star trails. A wider angle lens such as 17mm requires 4.5 hours to get edge-to-edge trails! Stars near the north celestial pole (i.e. near the North Star, Polaris) move as well, but the trails will be much shorter and more curved in the same period of time.
Please note that a pleasing star trail requires as many as 3 hours of exposure (longer if possible), so be prepared with a spare battery… or use an excessively massive battery rig.
A histogram of a night image will be skewed to the left. That is as it should be… it is a dark image after all. The camera – if allowed to – will expose a photo based on the assumption that it is capturing daylight and 12% grey. An auto-calculated exposure will make your night look like day time complete with a blue sky! To determine the optimum exposure start with the highest ISO possible and take an exposure for say 10 seconds at f/4. If the histogram is between 1/3 and 1/2 of the way across, then this is your “exposure factor.” 10 seconds * 1200 ISO = 12,000 @ f/4. Next decrease the ISO and lengthen the exposure proportionately. The best ISO setting for star trails is near to or less than 400. In our example, decreasing to 400 (dividing the ISO by 3) means we have to triple the exposure from 10 seconds to 30 seconds. Our exposure factor will remain constant: 400 * 30 = 12000. Next test a 30 second exposure and check the results. Still good? Then reduce the ISO by a factor of four (100 ISO) and expose for four times longer: 120 seconds (two minutes). Changing the f/stop will also necessitate a change in one of the other elements of the exposure factor.
Decreasing the f/stop from f/4.0 to f/3.5 means a 30% shorter exposure is needed. Increasing from f/4.0 to f/5.6 means either the exposure time or the ISO must be doubled.
This is a good exposure in a bright moon scenario. The exposure histogram is about 1/3 of the way from the dark edge at left to the bright edge at right.
Later the same evening the moonlight caused the image and the histogram
to look like the above.
In the second image the stars are almost washed out by the moon glow. When the sky becomes too bright there are three possible remedies: make shorter exposures, reduce the ISO, or stop altogether and wait for the moon to be less prominent. While it may be tempting to decrease the aperture the result of that action will enable longer exposures, but reduce the contrast in the stars.
While looking at the exposure histogram, it is a good idea to check sharpness. It is hard to focus at night but if necessary a series of tweaks on the focus followed by exposures can at least improve the sharpness. The best night focus trick I can offer is to focus on the moon if it is available.
The best focus is at the hyperfocal distance for your lens and f-stop. For a 17mm lens at f/4 that distance is about 30 feet. Focusing at 30 feet can be accomplished with a bright flashlight.
The best images are often composed from sky exposures for the stars and one or more foreground exposures. The foreground exposures may need to be much longer than the sky exposures. Changes to the aperture or the focus for any of the exposures may cause a misalignment of the images and degrade the quality of the result. Changing the ISO is not recommended either as variations in star trails may occur as a result. Therefore it is best to only change the exposure duration. Having the initial or final exposures correspond with nautical twilight may provide enough light for foreground illumination.
I also recommend capturing “dark frames.” A dark frame is an image taken with all of the settings used for the “sky exposures” images but with the lens cap on! A Dark Frame is important if you have either a noisy camera or a lot of sky glow. Dark frames can be used to enhance the contrast of the image and to help control hot pixel noise. One caveat is that the dark frames should be taken with the camera at the same temperature and settings because noise varies greatly with heat. For more tips about dark frames and how they can be used please see this article.
Varying the Aperture
Figure 9 is a composite of 8 four-minute exposures taken with a 50mm f/1.4 lens using a Canon 50D digital camera. All noise reduction was turned off. The pictures were taken directly overhead in the author’s backyard and included a nearly full moon and light pollution from the city of San Jose, California. From the lower left to the upper right are increasing f-stop (smaller apertures). Two star paths are labeled. One at the bottom shows a very bright star that is clearly visible in all of the images. Another star trail
just above the upper labels is almost invisible until f/5.6 or less. By inspection the best exposures – the ones that show the best contrast and still manage to keep the sky dark – are the f/5.6 and f/8 exposures. The f/4 exposure reveals some faint stars that are barely visible in the other images, while at the extreme the f/22 image at 800 ISO is very noisy. The f/16 image at 400 ISO is noisy as well.
You may be wondering what is stacking. There are several different kinds of stacking, but what this treatise addresses is brightness compositing (called lighten mode in Photoshop). In brightness mode, the brightest pixel at each position from all of the images in the input appears in the resulting output image. Other forms of stacking include addition, average, darkest, and a hodgepodge of bizarre statistical methods.
When taking a star trail there are three options: one long exposure (often not practical due to battery, sky glow or camera noise limitations), several intermediate length exposures (e.g. one to ten minutes each) or a hundred or more short exposures (20 to 30 seconds each). If a programmable timer is not available then many short exposures may be the only viable option.
Here are the pros and cons of each:
|Less than 31 seconds||
|More than 31 seconds but less than 8 minutes||
|8 minutes or longer||
Stacking can accomplish more than creating star trails. Stacking can be used, for example to smooth out rough water just as is accomplished with a long exposure. Figure 10 shows what happens when 12 separate “normal” daylight images are stacked. Any single image showed rough water while the combined images smooth out the water.
| DayLight Images Stacked
Beach at Night
Same as Figure 11 with a Twilight Exposure
Figure 11, a star trail stack when combined with a single dark frame from twilight produces Figure 12 showing how you can use multiple exposures to combine things together. Stacking can allow a shot with the moon and a shot without the moon to be combined cleanly. Figure 12 could be made even more appealing if combined with an image taken with illumination on the face of the rock (e.g. at dawn).
Minding the Gap
When stacking photos of stars images it is not unusual to find gaps in the trails. There are a number of reasons for these gaps. To understand the causes first imagine the camera sensor as a large two dimensional array of light “buckets.” Each bucket collects light (photons) for the duration of the exposure. When the exposure completes the shutter closes preventing any more light from entering any of the buckets. The camera then measures the number of photons collected in each bucket and creates an image from the millions of measurements. It takes time to close the shutter, complete the measurements and write out the resulting image to a memory card. After the processing time completes the camera is able to take the next exposure.
If the time between the end of one exposure and the beginning of the next is long enough any moving light source (stars, airplanes, meteors) will move enough during the non-exposure time to skip over some buckets leaving partially filled and unfilled buckets – dark pixels.
When a light source is moving very slowly across the sensor any interruption of light results in at least one bucket in the path collecting fewer photons. The under-filled bucket will produce a darker pixel. The bucket analogy is, of course, oversimplified because in reality even point light sources like stars cast light onto multiple light buckets at once. In most modern digital cameras the color of a pixel is created from three or more adjacent buckets each measuring a different color of light. The fact that starlight is collected across several pixels at a time gives rise to a second cause of gaps – the algorithms used to combine the images.
People like sharp images. Sharpness comes from processing the adjacent pixels in such a way that increases the contrast. So a “dim” pixel near several “bright” pixels will be brightened while a bright pixel near several dim pixels will be darkened. This processing which happens on an exposure by exposure basis is also performed when combining exposures. Using a different method to combine exposures can reduce the gaps.
- Pick your spot. Weigh down a sturdy tripod and make sure everything is solid. Wind and walking nearby should not rock the rig. If you are set up on a beach, make sure you are far enough from the surf that water will not wash near or under your tripod.
TIP: Be mindful of how you mount your camera on the tripod! You should be able to change both your battery and your memory card without having to unmount, move or dislodge the camera. If you have a shoulder strap on the camera, you may want to remove it to prevent the wind from wobbling the camera.
- Frame your shot. Do a high ISO test exposure to confirm focus and operation. You can also make a guesstimate at what length exposure is needed to properly expose the foreground. I have found that a well shot high – ISO exposure can be combined with the later shots to get a strong feeling for the sky.
- Set the camera back to the target ISO, put on the lens cap, format your memory card and take an exposure equal to the majority of your images with Long Exposure noise reduction OFF. This will be one of your dark frames.
- Start the automatic exposure sequence being sure to look at your first shot and allow enough time to be sure your second shot also starts immediately after the first one ends (three seconds is suggested). If the first shot is OK, delete it and
the one you interrupted.
- Install a freshly charged battery and start the automated sequence over again. You may wish to first delete your prior shots so you do not later wonder why you have gaps in your star trail.
- Take a nap, walk away, get some exercise… Come back in an hour or so to confirm everything is still working. Check the battery condition and check the lens for dew – still OK? Let the shooting continue.
- As the current automated exposure completes, quickly reconfigure your camera to turn ON long exposure noise reduction and setup to take a LONG exposure – your goal in this shot is to get the foreground exposed pretty well and still have a long single star trail shot that is not washed out. Alternatively, you can make your exposures coincide with twilight at sunset or sunrise and use one or more shots from that (see Figure 12).
- When your foreground exposure completes (after noise reduction), put the lens cap back on, reset to your automated exposure settings and take two more dark frames (again for noise reduction). Make sure to keep the camera in the same climate so the dark frames are taken at the same temperature as the prior frames.
- Now you should check your last long exposure – the one exposed for the foreground. If it’s not right, you may need to increase the ISO and expose again. You may also need to swap batteries. Note that any shot you take now will have an unusable star trail due to the large gap in time between this step and the end of step 7. Therefore this extra fix up is ONLY for getting a foreground. You may need to resort to light painting of various forms. Heck, it’s worth a try even if your previous foreground exposure was great.
- Only now can you pack up or repoint your camera.
If the shot will run unattended, it is a good idea to double and triple check all of the following:
- The ISO is set correctly (e.g. 200 to 400).
- The f/stop is correct (2.8 to 5.6 recommended)
- The camera is set to “bulb” exposure mode.
- Auto exposure bracketing is OFF.
- Exposure mode is set to single exposure (not high speed or continuous)
- Your camera will return to the above settings if it enters auto-power down mode.
- The intervalometer and the camera have sufficient battery.
- The delay on the interval timer is set appropriately. It is best to
start your first exposure near nautical twilight.
- The exposure length is set correctly (e.g. 3 minutes, 57 seconds)
- The delay between exposures is 3 seconds for older cameras but no less than one second for all cameras.
- The number of exposures is correct (e.g. for 4 hours of exposures at 4
minutes each you will need 60 exposures or more)
- The timer has been started!
- The timer, camera and batteries are secure.
For a complete list see “The Stacker’s Checklist“.
StarStax, and StarTrails.exe or Image Stacker are about all you need for pretty impressive results. In some cases Photoshop is a more effective tool for repairing images, but it is not nearly as fast or as easy to use for stacking as programs dedicated to this task. You may find the StarCircleAcademy Stacking Action useful, however. If you would like a thorough walk through of the process, you can attend a StarCircleAcademy Webinar on Creating Star Trails - or attend one of our single or multi-day workshops.
What does “stacking” do? To put it simply, it creates the final image by picking the brightest pixel from each image. Stacking can also employ averaging which has the great benefit of reducing noise. Professionally shot astrophotography uses many, many images combined together to produce the spectacular results seen in places like the Astronomy Picture of the Day.
Planning For Star Trails
As described in the preceding paragraphs there are many considerations for planning a great star trail. Finding an interesting foreground, determining when the atmospheric and celestial conditions will best suit that location, scouting to find the best spots to photograph the image and so on. Here are some of the tools and tips that apply:
- If shooting an object to the West a half-full moon (or less) rising in the East can provide light on the foreground.
- If the object of interest is to the East, a setting sun or setting 1/2 full moon can be a good foreground illuminator.
Mirror Lock Up
The first press of the shutter button moves the mirror out of the way, the second press starts the exposure. When the exposure completes, the mirror moves back down. On my Canon cameras, putting the camera in mirror lock-up mode just does not work work as you would like. In lock-up mode the requirement of two presses of the shutter to take an image makes this mode worthless for star trails because an intervalometer can not be configured to generate two presses. Why use mirror lock-up at all? The theory around mirror lock-up is that certain exposures in the 1/20 to 2 second range are affected by the mechanical vibration that come from the mirror movement. This is especially true for telephoto arrangements and when the camera is not well secured. For 20 second or longer exposures mirror slap will make little or no difference. One way to minimize mirror slap is to use Live View mode – but that eats more batteries.
What Focal Length Lens Should I Use?
The one element of the focal length that matters is that the shorter focal lengths give smaller width trails. Theoretically stars are points of light, but as you can see from pretty much any astronomy or star trail photo there is some bending of the atmosphere that moves around the point light source and widens the image.
Another element of the focal length is that the shorter the focal length, the longer you have to expose to fill the frame with a star trail (as was briefly described under “What Kind of Image”). Shorter focal length lenses have a wider angle (field of view), and that affects how long an exposure is needed to get a noticeable trail.
If you use a telephoto lens you can fill the field of view with a star trail in less time than if you use a wide-angle lens. For example: 488 mm focal length can be achieved on a 1.6 crop factor camera (Canon 50D) using a 1.4 Tele-extender and a 200 mm lens. The field of view of this lens is 4.5 degrees by 3 degrees. It takes a star at the celestial equator 12 minutes to move 3 degrees so you can have an edge to edge star trail in about 18 minutes using the telephoto lens. With a 30 mm lens the field of view is 40.5 degrees in one direction and 27.6 in the other. Since 27.6 is more than nine times bigger then 3, you have to expose nine times longer 108 minutes (one hour, 48 minutes) to allow that same star to sweep across the entire (narrow) field in the frame. Finally, from a compositional standpoint, a wider angle lens allows you to include more terrestrial or celestial real-estate in the image. There is one additional advantage, too. Wide angle lenses have closer hyperfocal distances. The hyperfocal distance is the distance from the camera at which everything from that point to infinity is in focus. In a nutshell this means you can include a nearby tree, building, cactus or other interesting foreground object and both it and the stars will be in focus.
- Star/Night Photography Technique
- Creative Night: Digital Photography Tips and Techniques by Harold Davis.
- Photoshop Darkroom by Harold Davis (also see Photoshop Darkroom II)
- Imaging Software
- Shot Planning Tools (EXCELLENT)
For a video that will make you appreciate how marvelous the
night sky can be, Check out Tom’s work on Vimeo. Also see timescapes.org
Timescapes Timelapse: Mountain Light from Tom @ Timescapes.