Sky Exposure: Star-stacked blend of 10 exposures, each at 14mm, ISO 12800, ƒ/2.8, 10 seconds
Foreground Exposure: Foreground Exposure: Blend of two exposures, each at ƒ/2.8, ISO 1600, 8 minutes.
Nikon D810A, Nikkor 14-24mm ƒ/2.8
Get ready to stay up all night taking in the wonder of the stars. Astrophotography, which is technique of capturing long-exposure photos of the heavens is immensely rewarding. It combines recreational time spent in the outdoors and photography to create images of the cosmos that are largely hidden from view. A few simple steps can be the difference between successful images of the sky and a blurred jumble of lights.
Before we start, I want to note that the images in this article are the result of blending multiple exposures together in Photoshop because arriving at a a low-noise and in-focus wide-angle image at night usually requires multiple exposures, something I’ll cover in detail.
Choosing A Camera And Lens
Until recently, you needed a generous budget to capture high-quality images of pinpoint stars in the sky, but over the past few years cameras have come a long way, and you can get good results with a mirrorless camera and a fast wide-angle lens, such as a Sony a6000/a6300 or Fujifilm X-Pro2 and the Rokinon 12mm ƒ/2 lens. The Olympus O-MD series of cameras has a built-in feature to “stack” multiple exposures into one final image, and the results can be exceptional.
The highest-resolution night images are still captured using full-frame cameras, such as the Nikon D810A (a camera tuned specifically for astrophography), Canon 5D series, Sony a7 series, and the Pentax K1—whose built-in GPS allows the camera to move the sensor to capture perfect, sharp stars. The truth is though that you can produce amazing night sky images with any modern cameras.
You want a large-aperture wide-angle lens that is sharp with minimal chromatic aberration even at its largest aperture, or at least at ƒ/2.8. To capture as much light as possible in the sky over a 30-second or shorter exposure, your lens needs to be pretty sharp even at its largest aperture, or you’ll get soft stars.
Capturing The Stars
To capture pinpoint stars—as opposed to long star trails—you need to use exposure times that are generally 30 seconds or less. The actual exposure time will depend on your focal length. A general rule of thumb is the “500 Rule”, which states that your focal length (35mm equivalent) divided into 500 gives you an exposure time that would have small amounts of star trails.
Using this formula, a 50mm lens would need a 10-second exposure—500/50mm=10. However, I personally find this rule often results in star trails that are too long, so I usually back off the exposure time.
You also need to use a high ISO in order to capture enough light from the dark sky. Generally, ISO 3200 or higher is a good start. An ISO setting that’s too low won’t capture enough detail in the sky, and one that’s too high might result in more noise than stars. Experiment to find the best results with your camera.
When shooting single exposures of the sky, I’m frequently at a focal length of 14mm, with my aperture set to ƒ/2.8, an exposure time of 25 seconds, and an ISO between 6400 and 12800 on my Nikon D810A.
Since you need to capture the stars at a very wide aperture, focused at infinity, and a very high ISO, the foreground will be out of focus and noisy. By taking additional exposures of the foreground at different focus distances and lower ISOs, you will have a cleaner and in-focus foreground that you can blend with your sky exposure in Photoshop to create a single image that has pinpoint stars and the entire scene in focus.
When possible, I like to shoot my foreground exposures at ISO 1600 or lower. The exposure time can vary anywhere from one minute to 30 minutes, depending on the ambient light. You can use light painting to brighten the foreground for a shorter exposure time, but this can lead to harsh shadows and specular highlights. I usually prefer to capture the scene using ambient light.
You can create an image that has truly pinpoint stars with exceptionally low noise by using a technique known as “star stacking.” Using this technique, you take multiple very short and very high ISO exposures of the stars, and then align and average those exposures together in software. The result has pinpoint stars due to the short exposure time, with very low noise due to the averaging of the noisy images.
You can do the alignment and averaging in Photoshop by masking out the foreground in each image before doing the alignment, then putting the aligned layers into a single Smart Object and choosing the Median blend mode for the Smart Object.
If you’re on a Mac, you can use the program Starry Landscape Stacker (available on the iTunes Mac App store) to automate much of the star-stacking process for you. You can then take the resulting star-stacked image and blend it with your foreground exposures just as you would with a single exposure for the sky.
Chasing The Aurora
Photographing the Northern Lights is a little different than astrophotography, in that your exposure time depends on the brightness and movement of the aurora. Generally, you’re forgoing pinpoint stars and doing whatever is necessary to get a good aurora shot. If the aurora is bright enough, you can use a very short exposure to capture the pillars and arcs of light and freeze them more, but if the aurora is very dim, you may need to do a longer exposure to capture enough light—at the cost of blurring the aurora movement. I’ve used shutter speeds of anywhere from one second to 30 seconds with the aurora, depending on my location and the conditions. Other than that, the foreground exposure technique and blending foreground with the sky in Photoshop is the same.
For star stacking at 14mm and ƒ/2.8 with my camera, I normally use ISO 12800 for 10 seconds, and take 10 exposures. You should experiment to find what works with your setup. Some cameras will have a lot of magenta sensor noise on the edge of the frame when shooting a really high ISO for a short time, so keep this in mind and see how your camera performs. You may need to bring the ISO down anywhere from 1/3 of a stop to a full stop to get rid of the magenta noise with these short ten-second exposures. When I was using a Nikon D800E, I needed to shoot at ISO 5000 or 4000 for star stacking to avoid the magenta noise problem.
Dealing With Hot Pixels
Hot pixels are the bane of night photographers. They happen excessively with long-exposure, high-ISO photos. The easiest but often most time-consuming way to get rid of them is to use Long Exposure Noise Reduction (dark frame subtraction) in your camera. This setting causes the camera to take another exposure at the exact same settings as the actual exposure but with the shutter closed, and then use that “dark frame” to map out the hot pixels in the actual exposure before writing out the file. This effectively doubles your exposure time. If you’re doing star stacking, you don’t need to use dark frames since the averaging process will get rid of hot pixels, but single-sky shots and long-exposure foreground shots often have lots of hot pixels. If you don’t want to wait for the dark frame exposures, there are ways to do it in software, but this can be time-consuming, as well.
Find A Dark Sky And Shoot!
Now that you know the basics, get out under a clear and moonless night sky, away from light pollution, and practice.
Essential Gear For Astrophotography
|Full-Frame Camera. While you can get amazing results with even crop cameras these days, you’ll have the least frustration and best quality with a modern full-frame camera.
Bright Wide-Angle Lens. You’ll want ƒ/2.8 or better, and on full-frame 14mm or so is great for ultrawide shots.
Sturdy Tripod. For long exposures, sometimes in the wind, you’ll need a reliable and sturdy tripod and head.
Remote Timer. You’ll likely need a remote release or intervalometer to do exposures longer than 30 seconds and star stacking.
HeadLamp with Red LED. Save your night vision by using a headlamp that has a red LED mode, available in many headlamps these days.
Hand Warmers Or Other Lens Heater. For keeping moisture off your lens at night.
Cotton Cloth & Air Blower. Cotton soaks up water, microfiber doesn’t. You may need to wipe the moisture from your lens and/or dry it with the blower.
Eyepiece Shade. If you’re using a DSLR and the eyepiece doesn’t have a built-in flip-down shade, then you’ll need something to cover the eyepiece to prevent light leakage during long exposures.
Adam Woodworth is a landscape photographer, fine-art printer, award-winning filmmaker, and software engineer. He has had a love of photography for most of his life, and one of his main focuses is landscape astrophotography. To learn more about his techniques, check out the video tutorial available on his website: www.adamwoodworth.com.
Updated August 10, 2016
Published November 30, 2015