How to Choose a Lens for Night Sky Photography
There is a big distinction to be made when it comes to night sky photography, which is whether or not you plan on photographing the stars in the night sky. If you do not plan on including stars in your shots, things are a lot simpler for you. That is true in a lot of ways, in that you don’t need to worry as much about the clouds and weather, the impact of the moon, or light pollution.
When it comes to lens selection, it means that you can pretty much use whatever lens you want. Therefore, if that is the type of night photography you plan to do, continue using whatever is your favorite lens at present.
Issues doing Night Sky Photography
On the other hand, if you plan on shooting the night sky and capturing the stars, things get trickier. This stems from two facts. The first is that starlight is extremely dim. It is only a tiny, tiny fraction of what you have at sunset (let alone the middle of the day). Even moonlight is many times more powerful.
The second issue is that the stars are moving across the sky (or, rather, that the earth is spinning, but it appears as though the stars are moving to your camera). This is happening more quickly than you might realize.
As a result, you need to do everything possible to maximize exposure, and you need for that to happen quickly. In other words, the dim light means that you need a lot of exposure. Exposure stems from a combination of three things: shutter speed, aperture, and ISO.
In other types of photography, you might simply open up the shutter for a long time. In night sky photography, however, you don’t have that luxury since the stars are moving. That means your shutter speed is going to be capped somewhere between 15 and 30 seconds. If you expose any longer, the camera will pick up that movement and it will show up as tails or blur in your pictures. That won’t work.
That means you’ll have to look at the other two exposure options – ISO and aperture. To deal with the dim light, you will have to crank up your ISO to at least 3200 and in many cases 6400. At present, that’s about as high as you should go. Even if your camera goes up to something like ISO 25,600, as many cameras do these days if you use an ISO that high you risk noise completely taking over your picture.
Use a Fast Lens
Because of these caps on shutter speed and ISO, the only remaining exposure control is aperture. To maximize exposure and still successfully capture the night sky, you will need a fast lens. How fast?
A lens that opens up to f/2.8 or wider is ideal. A lens with a maximum aperture of f/4.0 is acceptable. Anything less than that (meaning a higher f-number) probably won’t work. This is one situation where your kit lens might serve you well. Most kit lenses open up to f/3.5 at their widest focal length, which is actually 1/3 stop brighter than your typical f/4.0 lens.
As a side-note, you might be worried about depth of field when using these large apertures. You need not worry about that though. You will always set your focus at infinity and everything in your scene will be on that plane of focus. Even if there are objects in the foreground, at wide angles things quickly go to infinity on your lens. Unless something is very close to you (say, within 10 feet or so), it will be on the same plane of focus and depth of field will not be an issue. If you want to include anything closer than that, you’ll likely need to focus stack.
