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Whether you’re an amateur astronomer with a telescope or a budding photographer eager to capture the night sky, understanding the importance of focal length is essential.
In this article, we’ll look at the relationship between focal length, field of view, and magnification and how these factors affect your images.
By the end of this post, you should have a good idea how to choose the focal length that is right for the objects you are photographing.
Focal length will limit what kind of objects you are able to photograph, as some astronomical objects are large and some small. Therefore, you might wonder what focal length is best for you. In this article, I will give you some guidelines to help you choose a focal length suitable for most deep-sky objects.
I’d really like to hear what focal length you use, so tell me in the comments below.
What is focal length?
Focal length is something you will commonly come across when discussing astrophotography. The focal length of your setup depends on the optics of your equipment, such as your camera lens or telescope.
Focal length is the distance from the point where the light converges in a lens (A) to the camera sensor (B); a short focal length will create a wider field of view, and a long focal length will increase magnification and create a narrow field of view.
Field of View and Image Scale
Field of view and image scale are important in astrophotography so let’s see how focal length affects these. A short focal length lens will result in a wider field of view, perfect for capturing those large nebulae and star fields whereas, a longer focal length will give you a narrower field of view—magnifying your target for an increased level of detail.
Your image scale, or how many arcseconds per pixel are displayed in your light frame is directly related to the focal length. Image scale tells you what level of detail you can expect when using a telescope with a given focal length and a camera with a certain sensor size.
This image should make things a bit clearer:
For a camera and telescope setup, the focal length is the distance from the focal point of the light in the telescope optical tube to the camera sensor. We normally use either a DSLR or a dedicated astronomy camera. Neither should be used with a lens. This is called prime focus. Because there is no camera lens only the telescope optics will affect the focal length.
In a camera and telescope configuration the focal length is the distance between the point where light merges at a point in the interior of the telescope optical tube and the camera sensor. This camera could be either the DSLR type or a dedicated astronomy camera. Neither should be used with a lens. This is called prime focus. Only the telescope optics will determine the focal length.
Check the following image:
In a refractor telescope, light transmits through the objective lens and moves through the telescope tube and converges at the focal point. Light in a reflector telescope on the other hand passes through a tube and bounces off a primary mirror at the far end of the tube. A small secondary mirror moves the reflected light 90 degrees so it can go up the focusing tube to the eyepiece.
If you examine the diagram of the reflector telescope above, I have indicated the position of focal point if the mirror were not there. This should make it easier to identify how long the focal length is as mentioned above. The focal point is now close to the focusing tube.
The focuser moves the eyepiece to the required focal length required for observation. Every eyepiece must be placed in a specific position to focus because the eyepiece has a certain focal length.
Are there any questions you have about this? All you have to do is ask me something in the comment section and I will answer you as soon as I can.
How does Telescope Focal Length Affect Your Astrophotography Images?
When doing astrophotography, we use a camera without a lens, in other words, in prime focus. This means the camera sensor must be placed in the right position at the focal point of the telescope for it to be correctly focused. The focal length determines exactly where this point is.
The telescope focal length can affect your image in a number of ways including:
- a longer focal length magnifies objects so you can see or photograph smaller objects such as many galaxies.
- a shorter focal length allows you to see or photograph larger objects because it increases your field of view.
- a shorter focal length has less magnification which is no problem when you are looking at larger objects.
I have tried to capture objects like the Ring Nebula or some of the smaller galaxies and planetary nebula but cannot get enough detail with my telescope at 650mm focal length. In fact when I zoom in and crop my image I am not happy at all with the results because all I get is blurry undetailed images. I either need to use a Barlow lens which can double the focal length and magnification or use a different telescope.
For deep sky astrophotography, aim for a focal length that provides enough magnification to capture all relevant details of your target without losing too much sky beyond it.
The problem with this is that for deep-sky astrophotography, there are targets of varying sizes. From small galaxies that are tiny because they are so far away to the enormous Andromeda galaxy and from large nebulae such as the North American Nebula to medium-sized objects, we cannot limit ourselves to one focal length. One thing I have done, though, is work out what I can do with the focal length I have (650mm) and how to photograph smaller or larger objects. More about this below.
There is a direct proportional relationship between focal length and magnification, such that if you double the focal length, the magnification also doubles. For more information on the mathematics behind this, see this explanation.
Focal length and magnification are directly connected. If you double the focal length, magnification also doubles. For more information on this, see this explanation.
Focal length is important because it changes the field of view. This means that your camera will capture more or less of the sky and the target you are trying to photograph.
Are you wondering how much a telescope costs?
Using a Variety of Focal Lengths
I use two different setups that allow me to change the focal length according to the target I want to capture.
What is the ideal lens for deep-space photography? The answer depends on the kind of target you want to photograph so that your focal length gives you the widest possible choice. Choose wide focal lengths for larger nebulae or galaxies and long ones for smaller objects like planetary nebulae.
Consider the composition and framing of the image you are trying to capture. The size of your camera sensor and crop factor affect your FOV (field of view) and focal length. This will affect your choice of focal length.
I have my telescope and camera (ZWO ASI533 MC PRO), which are 650mm, and the camera has a square sensor, which is fairly small. In fact, for wider shots, I can opt for a DSLR type camera and totally different astronomy lenses. Now the lenses I own are the 17mm, the 24mm, the 50mm, the 85mm and the 135mm. To have a wider range, I could do with an astrophotography lens of about 300mm.
Deep Space Astrophotography Lens Guide
Let’s look at some good lenses for deep space astrophotography. These will have different focal lengths and enable you to capture the maximum amount of light in the shortest amount of time.
Lenses with focal lengths of 17–50 mm are great for Milky Way astrophotography.
As you can see in the photo I took above of the Big Dipper, using a DSLR camera and 50mm lens enables wide-field shots of constellations, including landscapes. The result can be quite stunning!
When I take photos of deep-sky objects with my DSLR camera, my best lens is the Samyang 135mm. It’s a popular camera lens with astrophotographers because it takes high-quality deep-sky images.
So what makes a good astrophotography lens? It should have a low f-number so it lets more light into the aperture. The ideal would be an F3 or less. The second thing a good astrophotography lens would have is an accurate focus adjustment, which will maintain its focus for long periods of time. Check out my deeper dive into the best lens to use for astrophotography.
I have tried some lenses with the focal length I wanted for deep sky astrophotography, but the focus could not be set accurately, and I even had to tape the focus because it moved too easily and lost focus over time. It needs to lock into place or not move.
Thirdly, a good astrophotography lens should have good-quality optics that do not distort the stars. Some lenses I’ve tried put a pink halo around the stars or give strange shapes to the stars. We don’t want this.
The kit lenses you get with many models of DSLR cameras may have the focal length you want, but they may not be good quality for one or more of the reasons we have given above. I tried using a 75-300mm Canon lens and despite the great range of focal lengths, the stars were terrible quality with haloes. The focus was so poor that I needed to use tape to fix the lens in position for a night’s imaging. Better to spend the money on the best lens you can afford!
Find out much more about choosing a lens for astrophotography here.
This is a perfect astrophotography lens for those wide-field shots.
- Very fast (F1.8)
- Also available for Nikon, Sigma, and Pentax
- Amazon choice
- over 3300 reviews
The other lenses are good for larger deep sky astrophotography targets such as the Pleiades, the North American Nebula, or Andromeda. The telescope takes care of the rest. If I get more money to spend in the future, I will also consider something in the 1500–2000 mm range to capture smaller objects such as smaller nebulae, galaxies, and planetary nebulae.
The best telescope focal length for deep space astrophotography, especially for deep sky objects is between 800mm and 1200mm. This range allows you to capture detail within the object while still including enough background sky to provide context. Ultimately, however, the ideal focal length depends on which object you are observing or photographing.
When choosing your equipment or upgrading it, it’s a good idea to stick with the major telescope brands who have a range of models and focal lengths.
For instance, when observing large objects like the Andromeda Galaxy or Pleiades Cluster, a shorter focal length, such as 500 mm, may be ideal to capture all of them. I would even say, from my experience, that 300–400 mm would frame it even better. When I tried to image the galaxy with my telescope, most of the galaxy was out of the frame, and this doesn’t do the object justice!
On the other hand, when observing smaller nebulae and planets, using a longer astrophotography lens like 1500mm will allow you to get more detail. The Ring Nebula is one example where a longer focal length would be appropriate. With my setup of 650mm focal length, it is just so very small. Zooming in is not a good solution because that reveals a lack of clarity and resolution when the image is magnified.
In my astrophotography setup, my telescope and camera combination have an approximately 650mm focal length, which is pretty good for most of the targets I have tried to image. It was not possible to capture some targets in one go, so I did several panels using a technique called mosaic. I found this was a good technique for the Christmas Tree Nebula and the California Nebula, for example, because the 650mm focal length was too much to get the whole of the nebula into one frame.
Use of Lenses in Astrophotography
- Wide-angle lenses (14mm to 24mm) are great for large areas of the night sky, like the Milky Way. They provide a panoramic perspective.
- Standard lenses (35mm-50mm) provide flexibility as they can capture objects such as the Milky Way, or large nebulae.
- A large aperture telephoto lens (70-200mm and up) can be terrific if you want to zoom in on individual celestial objects like planets or distant galaxies, producing detailed images for serious deep-sky photography.
Comparing the Best Focal Length for Astrophotography Targets
| Object | Focal Length (mm) |
|---|---|
| Andromeda Galaxy | 300-500 |
| Orion Nebula | 300-500 |
| Pleiades Cluster | 50-150 |
| Ring Nebula | 800-1200 |
| Whirlpool Galaxy | 800-1200 |
| Rosette Nebula | 400-600 |
| Lagoon Nebula | 300-500 |
| Eagle Nebula | 400-600 |
| Triangulum Galaxy | 800-1200 |
| Veil Nebula | 300-500 |
The above figures are only rough estimates, and in practice, you may prefer to experiment. I have imaged the Pleiades at 150mm and found them to be very small. I would like to capture it at about 400 mm, which I think would be perfect. In my telescope at 650mm, it just fits but some of the stars are cropped off, I wish they could be included as well but the blue nebulosity of the stars is beyond that region of the star cluster.
Here’s a telescope great for astrophotography:
- 90 mm aperture
- 500mm focal length
- Double-speed focuser
- Achromatic glass for good edge-to-edge viewing
- Excellent value – Less than $300
Another essential aspect to bear in mind when choosing the most suitable focal length for astrophotography is your camera’s sensor size. That is, with full-frame cameras, longer lenses are conceivable without encountering the loss of resolution while, with cropped-sensor cameras, in order not to lose the amount of detail in the frames used it is necessary to use shorter focal lengths.
Last but not least, the focal length for deep space astrophotography is determined by factors like the subject you are shooting, size of the camera sensor, and the effect you want. Do not be afraid to try new things, with time and consistent practice you will be able to take great photos of the universe.
I think, now you understand what focal length means and the role it plays in creating the shot you intended to take. I wish you smooth sailing and lots of luck in your astrophotography endeavours! So, if you have any questions for me, I would be pleased to answer them.
Conclusion
Selecting the right focal length is not only about technical specifications. I encourage you to experiment to discover what works best for you. Check your astrophotography goals and preferences, and do not limit your choice too quickly. This real-world experience is priceless and will teach you how the different focal lengths work in reality and their impact on your final images.
As you gain experience, you’ll see how focal length can provide different views of the universe. I recommend trying a variety of focal lengths and posting your thoughts/images for the wider astrophotography community to see. Your astrophotos can motivate and teach others, share this with us by posting a comment below.
More Information
Here is a nice video that explains which focal lengths you need to use for different kinds of astrophotos.
References
[1] Focal Lengths, Apertures and F/ Numbers. Retrieved from https://spacemath.gsfc.nasa.gov/weekly/10Page30.pdf
[2] Simple formulas for the telescope owner. Retrieved from https://skyandtelescope.org/observing/stargazers-corner/simple-formulas-for-the-telescope-owner/
[3] Astro Escape. What Focal Length Can Get Which Deep Sky Object? – YouTube. Retrieved from https://www.youtube.com/watch?v=3yfRRccvGSo
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