Night Photography Basics Part 2 – Full Moon, Star Trails, and Auroras

May 2, 2012
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Moonlight illuminates Glen Canyon and the Colorado River near Page, Arizona. (Mike Cavaroc, 2008)
Moonlight illuminates Glen Canyon and the Colorado River near Page, Arizona.

In Part 1, I discussed the ideal settings for shooting a dark night sky under a new moon, as well as what all those settings mean. If you’re not comfortable working in Manual Mode (M) on your camera, you should go back and read it to make sure you’re up to speed. This section will assume that you’ve got the basic understanding of M Mode and how it works.

This time around, I’ll be discussing how to alter those settings to account for a full moon, how to capture star trails, and also how to photograph the northern or southern lights, aka the Aurora Borealis or Australis, respectively.

Understanding The Histogram

Before moving further, it’s important to understand the histogram as displayed within the camera. Put simply, the histogram shows you the light that was captured in a given scene. To get it to show up, review an image on your camera and hit the Info button (on Canon cameras) once or twice to get it to show up with the image. If you use Adobe Lightroom for post-processing, you’ll see it near the top-right. Likewise, it can also be activated in Photoshop under the Window menu. All you see is a graph, but this graph tells you exactly what was captured and whether or not you need to adjust some settings to try again. At the very left edge of the graph is what the camera reads as black, while at the very right of the graph is what it reads as white. Your ideal image should fall within those two lines, peaking on neither edge. With new moon shots you’ll typically see it on the left side of the graph since the light is much more faint. As long as it’s not completely against the left edge, you’ll have enough information to pull out in post-processing, though it’s much easier to work with an image if it’s properly captured to begin with. You want there to be enough light to make it look like it’s pushed off of the left edge. With full moon shots especially, you might see what are called "blinkies." These indicate that the exposure captured too much light, which you’ll see on the histogram as information all the way to the right. This is information that cannot be recovered. There was too much light, so the amount of light pouring into that one spot caused it to record nothing but solid white. If you see this, it’s a good time to make adjustments and try again. Learning to read the histogram will help you better understand your images and create better overall images.

How to Photograph Under a Full Moon

While it might seem like a night sky is a night sky, there’s actually quite a bit of difference in light depending on whether there’s a full or new moon. In the first part, I discussed how a wide-open aperture along with a generous ISO is best for capturing the stars and Milky Way Galaxy. Under a full moon, however, there’s plenty of light available to do full landscape shots, particularly if you live in a winter environment, which is also because the Milky Way and all but the brightest stars are drowned out in the moonlight.

Since a full moon will drown out many of the dimmer stars, you don’t need to worry about opening up your aperture wide open since the brightest stars will come through regardless. A full moon shot shouldn’t be about the abundance of stars available as with a new moon, but rather, since there’s plenty of light to work with, you should try and make a good landscape shot that includes stars above. This will give an almost other-worldly appearance as your landscape will appear like a daytime landscape, but will have stars up above it.

With normal daytime landscape exposures, you’d typically want to shoot for an aperture of around f/16 or so. Since that wouldn’t allow in enough light to account for the stars, you’ll want to set it to something between f/8 and f/11. This will allow for a good crispness throughout the scene (provided there isn’t anything in the immediate foreground), and will leave the aperture opened up enough to let the stars still shine brightly. Dropping down your ISO to something around 800 or 1000 should put your shutter speed in the range of 20-30 seconds, just as we wanted before, depending on the amount of moonlight available. If you have a good telephoto lens and want to get in close on the moon, you’ll want as low of a shutter speed as possible since the moon is moving much faster than you might think.

The key point to remember about a full moon is that more landscapes and less sky (than with a new moon) serve the photo much better.

UPDATE – 2012/05/03: If you want to photograph a full moon rising, the first thing you’ll need is a telephoto lens. Naturally, the longer the better. On a cropped sensor, anything 300mm or higher should work great. Focus will be difficult, given the amount of light that will gone by the time a full moon rises. As a result, you can either focus right on the area the moon will be rising while there’s still daylight, or wait until the moon begins to peak above the eastern horizon. The latter will force you to react and adjust quickly, but keep in mind the best shots are usually a few minutes later when most of it is behind a certain object, so you have some time to work with. Getting it against an object as simple as a tree will really bring out the scale of it. If you have the capability to set your lens to infinity, it should be suitable for the moon.

To see where the moon will specifically rise, there’s an incredibly handy program called The Photographer’s Ephemeris. This will show you exactly where both the moon and sun will rise and set each day, allowing you to plan exactly where you want to be to get a certain landmark in the scene.

Once set up, you’ll want your aperture at f/11, or somewhere nearby. If you’re zoomed in well on the moon and have it taking up a good bit of the frame, you can drop your ISO down to something between 100-400, leaving your shutter speed to something in the range of 1/250th of a second. If it doesn’t match that exactly, don’t worry about it. Just make some necessary adjustments and make sure your moon isn’t overexposed causing "blinkies." If you see any highlights getting blown out, lower your shutter speed and try again. Likewise, if it looks too dark, you can bring up the ISO a bit along with the shutter speed, but keep in mind that the moon moves quicker than you think, so keeping your shutter speed to something relatively quick is ideal.

If the moon isn’t filling up very much of the frame, possibly because you’re using something less than 300mm, you might want to focus more on capturing a scene with the moon rising, rather than trying to get as much of the moon as possible. In this instance, you’ll want to raise your ISO to get some more details in the land. In either case, exposing for the moon will cause your landscape to be silhouetted. You can try techniques to blend two or multiple exposures to balance them out, but usually the contrast is so great that it looks a bit odd trying to get anything but one or the other.

How to Photograph Star Trails

Perseid meteors streak across star trails above Jackson Hole, Wyoming. (Mike Cavaroc)
Perseid Meteor Shower Star Trails

In the previous post, I mentioned that you don’t want to create a star trail photo by leaving the shutter open because it would introduce too much noise to create a practical image with. It worked great with film, but in the days of digital, there’s another technique that works much better that applies to both a full and new moon. Using an intervalometer for your camera, you’ll want to record a series of shots taken one after another over a certain amount of time. For star trails, an exposure of 20-30 seconds is ideal, with a total of roughly 100-200 shots will create some nice looking trails. Just make sure you allow enough time for the image to save onto the memory card before starting on the next one, otherwise it could cause you to lose images along the way in the sequence, creating noticeable gaps. Anything more than a few seconds and you might not be capturing images fast enough to create a seamless trail. For this reason, a fast memory card in your camera is not just recommended, but also required.

Once those shots are back on your computer, you have a few different options of how to process them. The basic principle is that using Photoshop (ideally, or some program that works in layers), you’re going to set one image on top of the other as layers. If you’re new to layers, in basic terms specific to this example, the program has the capability to allow you to place one photo on top of another, allowing one image to "sit" on top of another. This can be repeated as much as your computer resources will allow. With that said, if you’re on an older computer, you may have some waiting around to do with this part. Otherwise, this can be repeated until all the captures are in one photo, layered on top of each other. Once in, each layer should be set to the Lighten mode, which allows for the brightest elements of the previous layer to show through the one above it.

Since this can be incredibly tedious, scripts have been made to make this easier, allowing you to click a few buttons and let the computer do the work. Both Dr. Brown’s Stack-a-Matic and StarStax will automate this process. Understanding what these scripts do, though, will give you a better understanding of each phase in the process, hence the previous paragraph. While StarStax is a stand-alone application, Stack-a-Matic is made to be installed into Photoshop. Thus, the latter will give you a bit more flexibility in tweaking the end result in terms of modifying each individual layer, should minor adjustments need to be made. If you’re new, or unfamiliar with Photoshop, StarStax might be the program to stick with initially. However if you’re comfortable with Photoshop and working with layers and masks, you’ll want to give Stack-a-Matic a try.

California-based photographer, Jim Goldstein, did an excellent video tutorial on this entire concept, so if you’d like a video course from beginning to end, including setting up your camera, all the way to the finished image, check out his Star Trail Video Course here. You could learn something even if you feel you have a good understanding of it all.

Shadow Mountain in Bridger-Teton National Forest is silhouetted in front of northern lights above Jackson Hole, Wyoming. (Mike Cavaroc)
Shadow Mountain in Bridger-Teton National Forest is silhouetted in front of northern lights above Jackson Hole, Wyoming.

Photographing the Aurora Borealis

Since I live in the northern hemisphere, I’m simply going to refer to them as the northern lights or aurora borealis, however the exact same principles apply to the southern hemisphere as well. Unless otherwise stated, we’re going to assume there’s a new moon, or close to it, out.

Depending on where you are, the northern lights can add an element of trickery into shooting. Ideally, you should have a shutter speed no more than five seconds or so with an ISO of about 800-1000. This will preserve the "waves" of the northern lights and not blur them into one solid color, such as what a longer exposure will do. However this also assumes ideal conditions such as up in Alaska or Canada where they’ll appear regularly overhead. For most of us, this means we’re going to have to count on the coming peak of the solar maximum to push them down into the lower 48 (for those in the U.S.). Most of the time, and pretty much every time I’ve seen them thus far since moving to Jackson Hole, Wyoming, the northern lights have been on the northern horizon, whether faintly or strongly. As a result, the light isn’t quite as strong, which means I have to compensate with shutter speed and ISO. As with any new moon shot, you still want your aperture wide open, unless they’re overhead and producing noticeable light, in which case you can begin to apply settings more along the lines of a full moon. The last thing you should change is your shutter speed so that you can have every chance you have to capture the "waves" and "ribbons" of the northern lights.

Let’s start with the ideal settings: an aperture of about f/8.0, a shutter speed of about five seconds, and an ISO of around 800-1000. For those of us with them on the northern horizon, the first thing to change would be the aperture. Open it up as wide as it will go, ideally f/2.8 or lower. Look at your histogram and see if that solved it. If it still appears very dim, the next thing to adjust would be the ISO. Which setting to go to was discussed thoroughly in Part 1. On my Canon 7D, I wouldn’t go higher than 5000. Test another shot and if it still appears too dark, only then should you begin increasing the shutter speed, again, no higher than 20-30 seconds. Sometimes you’ll have no choice but to make everything that high, but capturing the glow of the northern lights successfully is better than not getting them at all.

The great thing about digital is that there’s no harm in experimenting with multiple different settings to see what comes out best. With most night shots where the moon isn’t producing significant light, you want your exposure on the histogram to be on the left side, just not all the way to the left. This will preserve many of the more subtle colors while also not absorbing too much light from the stars. Just like with anything, it takes a bit of practice in learning the best settings for each instance, but in time, you’ll know immediately what to set everything to just by looking at the night sky. So, get out there and enjoy one of the most peaceful and sublime times to shoot!

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