I thought it might be helpful to do a brief post as a starting point for astrophotography (i.e. pictures of the night sky).
It’s a huge topic and you can spend thousands on equipment such as telescopes, specialist cameras and more… but what if you want to start out with the camera you’ve already got, and nothing more?
Why it’s hard to do…
During the day, you can take a picture easily. You press the button, it’s done. Taking pictures of the night sky is a lot harder though. This is because to get a good picture, your camera needs lots of light. The night sky, by its very nature, is pretty dark though. Most cameras will offer a “bulb” setting where the shutter can be left open for long periods of time but that trick is no use either, because the night sky has another annoying habit- it moves. And surprisingly quickly! That’s fine if you want to shoot star-trails, but not if you want sharp pinpoint stars. Because of this, we need the camera set up in a particular way to stand a chance of success, which means using the manual settings.
Your location & time of year:
If your primary aim is to capture the stars, then you ideally want a dark location out in the countryside. In a town or city, the stars are drowned out by light pollution so that only the brightest are visible. The darker the location, the more stars you’ll pick up.
The time of year also matters- in the northern hemisphere, the Milky Way is most visible in the summer months, but this combines with it not getting dark enough until very late (or ever, depending on how far north you are). The winter is much darker, but you’ll not be able to image the milky way as it’ll be below the horizon – instead, concentrate on popular constellations, such as Orion.
The lens aperture:
We want to get as much light into the camera as possible. This generally means opening the lens up to it’s widest setting (the smallest “F” number). On a kit lens such as an 18-55, this might be something like F/3.5. On a “nifty fifty” you might have F/1.8.
The shutter speed:
This is a little more complicated. Because the stars move, we want our shutter speed fast enough so that the stars don’t appear to move in the final image, but long enough so that we gather a useful amount of light. To complicate matters further, this ideal speed varies with the length of lens used. This is because a wider angle lens covers a bigger portion of the sky, therefore the apparent movement of the stars over any given time is much smaller. Luckily, there’s a useful formula known as the 500 rule to work this out- simply divide 500 by the focal length of your lens, and use it as a starting point.
For example: an 18mm lens would give 500/18 = 27.7 seconds. On most cameras you’d pick 25 seconds as the nearest setting available. A 50mm lens on the other hand would give 500/50 = 10 seconds. You will want to review your images still, as the size of sensor in your camera, and the part of the sky you’re pointing at will both affect the apparent movement of the stars too, but the 500 rule is a good starting point.
ISO is the third setting that controls the overall exposure of the image- and as with the other settings, there’s a trade off. As ISO increases, not only do you get a brighter image, but your images become more noisy (showing coloured graining). As a general rule, newer cameras and those with larger sensors will exhibit less noise at a given ISO than smaller sensors or older cameras (which is why night time pictures on your 5 year old mobile phone all look terrible!)
This setting will also be affected by where you’re shooting- if you’re out in a desert or on top of a mountain, with nice dark skies and no towns nearby, then you’ll want to try an ISO of 1600 or 3200 to begin with. If you’re shooting near a light polluted area, or when there’s a full moon out, you might need to lower it to 640 or so, and accept that you’re not going to pick up many stars.
Use a tripod:
It goes without saying that you probably can’t hold your camera steady by hand for 25 seconds, so use a decent tripod.
Use the self timer or a remote release:
To avoid the camera wobbling at the start of the exposure from you touching it, use either the built in self-timer or some sort of remote release to take the shot.
Turn off image stabilization:
Image stabilization in modern cameras/lenses is a useful feature, but when the camera is on a tripod it can actually introduce blurring by trying to correct for movement that isn’t there. Turn it off to avoid this problem.
Shoot in RAW:
Yes, I know- RAW is hard. But for this kind of work, to get the best results, you should really consider it. I’ll cover raw editing in another post, but there’s a couple of reasons that specifically relate to astro work. One is that you may want to make adjustments to the white balance, especially if artificial light is causing an odd colour cast to your image. The other reason is that you get a huge amount of latitude in a RAW for pulling details out of shadows, and controlling highlights. If you want to get quick results and maybe edit later, you can always set the camera to JPG+ RAW 🙂
Focusing is hard when it’s dark! The best way, if your camera offers it, is to use live-view along with magnified view, and to focus manually- turn the focus ring back and forth while viewing a bright star, aiming to get the star as small as possible.
If you don’t have live-view then start with the infinity marking on your lens, then review a test shot and zoom in (most infinity markings are wrong) – trial and error will get you there in the end, and once you know the right point, feel free to make a mark on your lens for future use.
The sky itself is fine, but try to make your picture well composed still. The night sky with a single tree, windmill or other interesting subject silhouetted in front of it will make for a much nicer image.
The above is very much a getting started guide- you’ll want to experiment with settings specific for your location and camera. I’ll cover processing the shots in a later post.