How to Choose a Lens for Night Sky Photography

A Post By: Jim Hamel

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.

Shoot with a Wide-Angle Lens

You may be familiar with something called the Reciprocal Rule. It will help you make sense of why you need a wide angle lens for night sky photography. That rule states that the slowest shutter speed (exposure time) you can use when shooting handheld and still avoid blur in your pictures is the reciprocal of your focal length. So, for example, if you are using a 100mm lens, your minimum shutter speed should be 1/100th of a second.

This rule shows that there is always a direct correlation between focal length and the longest shutter speed you can use to avoid blur. That also holds true when it comes to photographing the stars. When you photograph the night sky, the wider the lens you use, the longer you can open up the shutter.

How wide of a lens do you need? There is no clear answer. If you are familiar with photographing the stars or the Milky Way, you may have heard of certain rules of thumb where you divide a number by the focal length of your lens to get a maximum exposure time (such as the Rule of 600, 500, or even 400) before the stars start to arch and blur.

For example, using the Rule of 500, a 24mm lens would allow you to use a shutter speed up to 20 seconds (500 divided by 24 mm is 20.8 seconds). But if you use a 16mm lens instead, you can expose up to 30 seconds (500 divided by 16 is 31.25). As you can see, wider is better. Here is how it works out for the widest focal lengths.

Before moving on, let me point out two things about this standard. First of all, it is not absolute, and there are those who advocate for a stricter standard (meaning that you should use even shorter shutter speeds for these focal lengths). Be sure to test out different speeds and see what works best for you.

Second, if you use a crop factor camera, be sure to adjust these numbers to the Effective Focal Length, which is based on a full-frame or 35mm format (i.e. 20mm on a 1.5x crop factor camera is effectively a 30mm, so the maximum exposure then is only 16.7 seconds before the stars arch).

Measuring Sharpness

It goes without saying that you want a sharp lens. But how do you determine what is a sharp lens and what isn’t? The best way, of course, is to try them out for yourself. It may not be practical to test out a whole series of lenses though.

We all rely on lens reviews. I’m certainly no expert on optics, so I definitely do. These are usually extremely helpful, but sometimes it is difficult to get an apples to apples comparison of different lenses. To do that, there is an extremely helpful resource called DXO Mark.

The reason DXO Mark is so helpful is that they score all lenses in the exact same manner. For example, they give scores for sharpness, distortion, and vignetting (as well as other criteria, and an overall score) and grade each lens the same way. That lets you look at numbers very quickly, rather than comparing images from different lenses and attempting to quantify the differences. If nothing else, it gives you a good place to start.

Of course, sharpness isn’t the only thing you need to consider. Lenses all have different amounts of distortion as well. One type of distortion that is important in night photography is called coma distortion. It adds little wings to points of light, which in the case of photographing stars is not ideal. Unfortunately, this is not something that is often included in lens tests, so you’ll have to check for this with whatever lens you are considering.

Narrowing Your Choices Down

We have now established the three most important criteria for picking a lens for your night sky photography. Your lens needs to be wide, fast, and sharp. In terms of how wide, how fast, and how sharp, that is up to you. But you can use your own criteria to create a list of available lenses.

For example, you might decide to only consider lenses that are 20mm or wider, open up to f/2.8, and are at least moderately sharp.

Once you narrow the list down to your particular brand of camera, you will probably find that the list is not very long. There may actually only be a few models from which to choose.

The Top Picks

Once you start looking at the widest, fastest lenses made, the list quickly gets pretty short. Once you add in the reviews and ratings, some pretty clear choices emerge. Of the Big Three manufacturers, I think you would have to strongly consider the following lenses (if you can afford them, as they are quite pricey):

That’s not to say you necessarily need to rush out and purchase one of these lenses. The point of this article is really to help you establish the criteria for picking your own lenses. At the same time, I don’t want to be coy about these obviously great lenses, as they fit the criteria for night sky photography very well.

A Sleeper

When you start putting price limitations on lenses, the list gets short real quick! Most lenses in this range cost well over $1000, and many approach $2000. You may be looking for something more affordable, as I was. Once I looked at the list further, the Tokina 16-28 mm f/2.8 jumped out at me.

It is under $700 in most places and meets all three criteria of being wide (16 mm), fast (f/2.8), and sharp (scoring a 22 on my camera per DXO Mark). I picked one up and like the results. It works great, although there is definitely some coma distortion going on. Still, for the price, it is a great option. This lens is available for both Canon and Nikon cameras.

A Wildcard

There is another affordable option if you don’t mind manual controls – Rokinon lenses. They have several wide angle options to choose from, ranging from 12-24mm. They are prime lenses and are very fast. All have maximum apertures of at least f/2.8. These are all manual lenses though, which means that you will need to focus manually. It also means that you will set the aperture with a ring on the lens, rather than with your camera.

Still, that should not be a problem with night sky photography. You just set the lens at its widest aperture and set the focus at infinity. You might never change it. Because of these manual controls, you can often pick up these lenses for well under $400 (check out the Rokinon 14mm). Again, this is just another good option if you don’t mind manual control lenses.

Note: Rokinon also has 12mm and 8mm fish-eye lenses available with mounts for Canon, Nikon, Sony, Fuji, and Micro 4/3 cameras if you want to go really wide.

The Lens for You

Just remember that if you are considering a lens for night sky photography, you’ll want to limit the lens selection to wide-angle lenses (generally 24mm or wider). You can get away with longer focal lengths, but you’ll need them to be extremely fast (probably f/2.0 or greater).

Once you have narrowed down your selection in this fashion, look at fast lenses. Try to get one that opens up to f/2.8 or better. That usually means you are looking at fairly expensive lenses, but as mentioned above there are some affordable options that will get the job done. After that, be sure to check the DXO Mark ratings. But don’t stop there – a simple Google search for the lens(es) you are considering will likely yield a lot of reviews. I am partial to The Digital Picture and DP Review as well.

Hopefully, this article has gotten you familiar with the criteria you need from a lens for night sky photography and has spurred a few ideas. It isn’t meant to limit your alternatives, so if there are other lens options I’ve missed, please let me know in the comment area below.