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Moon Set over Marin, CA
(33 nine-second exposures)
Figure 4
West Again
(53 1-Minute exposures)
Figure 5
A compendium of tips learned while taking images like those shown in the Image Gallery and the figures below. Note that some star trail related tips also apply to other low-light photography scenarios.
NOTE: If you want to get hands on training in a great dark sky location you might consider joining me at a Star Circle Academy Workshop.
TABLE OF CONTENTS
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.
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Moon Set over Marin, CA |
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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?
That is a relatively short time. If that same shot is zoomed in to 200mm 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. The sky is darkest when the all of the following are true:
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 discernable in the sky, and even when the moon is full or nearly so star trails are usually possible - they just require more planning.
Clouds - especially high, thin clouds can add some dramatic "milkiness" to the image - as can the Milky Way itself.
![4 Hours in Pleasant Valley [6047-6109]](http://farm3.static.flickr.com/2589/3813388062_ccc636ce37_m.jpg) |
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:
**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 which is based on its focal length times the crop factor (also
called focal length multiplier). Stars at the celestial equator travel 15
degrees in the sky 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. More,
commonly a wider angle lens is used, say 17mm. To get edge-to-edge trails at
17mm, a 4.5 hour exposure is needed! Stars near the north celestial pole
(i.e. near the North Star, Polaris) move as well, but the trails will be much
shorter in the same period of time.
Please note that a "good" star trail will require as many as 3 hours of exposure (longer if you can), 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 "wants" to expose a photo based on the assumption that it is capturing daylight and 18% grey. An auto-calculated exposure will make your night look like day time complete with a blue sky! The best way to determine your optimum exposure is to 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." So, for example, 10 seconds * 1200 ISO = 12,000 @ f/4. Next decrease the ISO and lengthen the exposure proportionately. The best ISO setting is under 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? Ok, 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.
| Here is a correct exposure in a bright moon scenario - the exposure is about
1/3 of the way from the "dark" edge to the "bright" edge.
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Later the same evening the moon light caused the image and the histogram
to look like this:
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In the second image the stars are getting washed out by the moon glow. At this point there are three possible tactics to employ: 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 by increasing the f-stop the result of that action just makes longer exposures possible - to a point.
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. Generally 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 stack. Changing the ISO is not recommended either as variations in star trails may occur as a result. It is best to just change the exposure duration. Having the initial or final exposures correspond with Nautical or Civil twilight may provide enough light for foreground illumination.
Additional exposures highly recommended include "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! This is very 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 noise. One caveat is that the dark frames should be taken in the same location and with the camera at the same temperature because noise varies greatly with heat. One dark frame is recommended before the sequence of exposures begins and two (or more) after the exposure stack ends.
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.
Figure 9
You may be wondering what exactly 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, and darkest.
There are several tradeoffs to consider. When taking a star trail there are three choices: 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:
| Exposure Length | Advantages | Disadvantages |
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| Short <= 30 seconds |
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| >= 30 Seconds, <= 8 Minutes |
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| > 8 Minutes |
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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.
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12 Normal Images Stacked |
Stack of Images at Pfeiffer Beach |
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).
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 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 or eliminate the gaps.
If the shot will run unattended, it is a good idea to double and triple check all of the following:
Picasa3, 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 StarTrails.exe. If you can not use Startrails.exe or Image Stacker because you mistakenly bought a Mac instead of a computer ;-) Photoshop CAN be used but it is painfully slow and cumbersome. Moreover to reduce the cumbersome factor you need Photoshop EXTENDED and one of the scripts for "maximum."
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.
The following list summarizes how I do my processing.
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:
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 40D and 50D cameras, putting the camera in mirror lock-up mode just does not wor 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 handle 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 comes from the mirror movement. This is especially true for telephoto arrangements and when the camera is not "heavily" secured. For 20 second or longer exposures the mirror lock up should make little or no difference.
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 40/50D)
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.
Timescapes Timelapse: Mountain Light from Tom @ Timescapes on Vimeo.
Karla Ormsby a professional writer can spot nonsense when she reads it. Any nonsense that still remains after her thoughtful review is entirely my fault, not hers. Thank you, Karla for your help!
![Moon Break [5_019889-99]](http://farm5.static.flickr.com/4035/4210363871_cbae27d39f_m.jpg)
