How to Choose the Best Focal Length for Deep Sky Objects

Selecting the correct focal length for observing or photographing deep sky objects can have a profound effect on the quality of your images and the level of detail you capture. With so many options available, it may be difficult to know which one to pick. In this article, I’ll provide some tips and tricks for selecting an optimal focal length when photographing deep sky objects.

Before anything else, it is essential to comprehend what focal length is and how it impacts your images. Focal length refers to the distance between a lens and a camera sensor or film; shorter focal lengths offer a wider field of view while longer ones magnify the image and give you a narrower scope. When photographing deep sky objects, aim for a focal length that provides enough magnification to capture all relevant details without losing too much sky beyond it.

andromeda galaxy at 150mm focal length
Andromeda Galaxy at 150mm focal length

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.

Using a Variety of Focal Lengths

I have overcome the problem of the need for different focal lengths by having two alternative setups which will enable me to change the focal length according to the target I want to capture.

I have my telescope and camera (ZWO ASI533 MC PRO) which is 650mm and has a square sensor which is fairly small. Alternatively, for more widefield shots I can use a DSLR camera plus different lenses. Currently, the lenses I have are 17mm, 24mm, 50mm, 85mm, and 150mm. I could actually do with a lens of about 300mm to make the range quite comprehensive.

Having focal lengths of 17-50mm is great for Milky Way astrophotography.

Ursa Major (the Big Dipper) captured at 50mm focal length

As you can see in the photo I took above of the Big Dipper, using a DSLR camera and 50mm lens enables widefield shots of constellations including landscapes. The result can be quite stunning!

The other lenses are good for larger targets such as Pleiades, North American Nebula, or Andromeda. The telescope takes care of the rest. If I get more money to spend in the future I would also consider something in the 1500-2000mm range to capture smaller objects such as smaller nebulae, galaxies, and planetary nebulae.

One of the most popular focal lengths 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.

For instance, when observing large objects like the Andromeda Galaxy or Pleiades Cluster, a shorter focal length such as 500mm may be ideal to capture all of it. I would even say from my experience that 300-400mm 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 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 is 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 in a technique called mosaic. I found this was a good technique for the Christmas Tree Nebula and the California Nebula for example because 650mm focal length was too much to get the whole of the nebula into one frame.

Comparing Ideal Focal Length for Different Deep Sky Objects

ObjectFocal Length (mm)
Andromeda Galaxy300-500
Orion Nebula300-500
Pleiades Cluster50-150
Ring Nebula800-1200
Whirlpool Galaxy800-1200
Rosette Nebula400-600
Lagoon Nebula300-500
Eagle Nebula400-600
Triangulum Galaxy800-1200
Veil Nebula300-500
comparing the focal lengths needed to image 10 deep sky objects

The above figures are only rough estimates and in practice, you may prefer to experiment. I have imaged the Pleiades at 150mm and found it to be very small. I would like to capture it at about 400mm which I think would be perfect. In my telescope at 650mm, it just fits but some of the stars are cut out of the frame and it would have been nice to include them although they are beyond the blue nebulosity of the star cluster.

When selecting a focal length, another factor to consider is your camera sensor size. With full-frame cameras, longer lenses are possible without sacrificing image quality; on the other hand, crop sensor cameras may require you to use shorter focal lengths in order to not lose too much detail from each shot.

Finally, the ideal focal length for deep sky objects depends on several factors including the object you are observing or photographing, your camera sensor size, and personal preferences. Experiment with different focal lengths to find which works best for you and don’t be afraid to try something new; with practice and patience you’ll be able to capture stunning images of our universe.

Hopefully, now you’ll understand the importance of focal length in the composition of your astrophotos. I wish you clear skies and good fortune in your astrophotography! If you have any questions for me I’d be glad to receive them just ask me a question here.