How to Use a Camera Lens With an Astronomy Camera

Camera lens vs. telescope: pros & cons

With some telescopes having such short focal lengths these days, it is reasonable to ask: why should I use a camera lens over a telescope? In this section, we’ll go over the pros & cons of owning each for astrophotography.

Camera Lens Pros

• Affordability: Camera lenses, especially manual focus ones like the Samyang / Rokinon 135mm f/2, are very affordable at around $400 USD, or even less if you buy used.
• Versatility: Camera lenses offer more versatility than telescopes, as you can still attach a normal mirrorless or DSLR camera for other types of photography. 
• Wide field of view: With a variety of wider focal lengths available, camera lenses allow imagers to access wider angle astrophotography than possible with a telescope.

Camera Lens Cons

• Not designed for astronomy: Camera lenses lack some of the nice features that telescopes have, such as better optics designed for sharp stars, rack & pinion focusers, and tube rings for easy mounting. In addition, the bayonet mount means you need to use an adapter to attach an astronomy camera.
• Limited back focus: As we’ll discuss in the next section, camera lenses have a limited flange focal distance or back focus. At most, the available back focus is around 46mm, which greatly limits you from attaching accessories. Telescopes, especially Petzval lenses, usually have at least 55mm or longer, allowing you to add more accessories like a camera rotator into the image train.

How to select a lens for astrophotography

With endless options for camera lenses, choosing a lens might feel overwhelming for astrophotography. Here are some qualities to look for in a camera lens, regardless of which camera you use it on:

• Fast focal ratio / f-stop: As most of space is quite faint, it’s best to choose a lens with a fast maximum focal ratio aperture. Most ideal lenses start around f/2.8 or faster like f/2 or below. The more light you can let in, the better your images will be.
• Fixed focal length: Having a lens with great optics is highly desirable for astrophotography. Though not required, we strongly recommend a lens with a fixed focal length (also known as a prime lens) such as 50mm. If possible, avoid zoom lenses as they usually perform worse than prime lenses.
• Sweet spot for reach: Most astrophotographers using camera lenses typically find that a focal length range of 50mm to 200mm hits a sweet spot for wide field imaging (not for static tripod Milky Way shots). Wider than 50mm can cause issues with the transmission of astronomy filters, and at longer than 200mm it is typically best to use a telescope instead.
• Newer optics: Recent camera lenses (within the last two decades or so) designed for digital cameras are a better choice for astrophotography thanks to advancements in coating technology. Older lenses designed for film cameras may lack the coatings to produce sharp stars.
• Designed for DSLR, not mirrorless: At first glance, it might seem that a modern mirrorless camera lens would be the ideal choice for pairing with an astronomy camera. In practice, however, this often doesn’t work because mirrorless lenses typically don’t provide enough back focus. See more in our Flange Focal Distance section below.
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• What is flange focal distance?

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• Flange focal distance, also known as back focus in the astrophotography world, is a fixed point at which a lens or telescope is designed to reach focus. This varies by manufacturer and even lens mount, so it’s important to consider this when selecting a lens.
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• For example, let’s say your lens’ flange focal distance is 46mm. That means you have 46mm of space to add your astronomy camera, a lens adapter, and/or accessories. Most cameras consume around 17.5mm of this, which would leave just 28.5mm for accessories in your image train. More modern mirrorless camera lenses may only have a flange focal distance of around 16mm, which is too short to attach an astronomy camera as there is no room for attaching an adapter.
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• If you do not connect the correct accessories to get to your camera lens’ flange focal distance, you may not be able to reach focus at all. It’s possible to reach focus within a few millimeters, but you’ll likely have unwanted optical aberrations such as elongated stars near the corners.

Best lens mounts for astrophotography with a camera lens

Below is a list of camera lens mounts, their flange focal lengths, and our ranking for how suitable they are for attaching an astronomy camera:

Best camera lenses for astrophotography

The Samyang / Rokinon 135mm f/2 is our top pick for astrophotography. It’s so popular in the community that many accessories already exist for it, including 3D printed rings, ZWO EAF mounts, and more, which you can shop here!

1. Samyang / Rokinon 135mm f/2

   • Fast focal ratio
   • Good focal length for mosaics
   • Very affordable
   • Available in both Canon EF and Nikon F mounts
   • May have copy to copy issues with sharp stars
   • View images taken with the Samyang / Rokinon 135mm f/2

2. Canon EF 200mm f/2.8L II

   • Longer focal length with decently fast focal ratio
   • Also very affordable
   • Designed for full frame cameras
   • View images taken with the Canon EF 200mm f/2.8

3. Sigma 105mm f/1.4 Art

   • Ultra-fast focal ratio
   • Available in both Canon EF and Nikon F mounts
   • Significantly more expensive
   • View images taken with the Sigma 105mm f/1.4 Art

How to connect an astronomy camera to a camera lens

Connecting your astronomy camera to a camera lens will be different for everyone, as the flange focal distance of each lens mount varies, plus factors such as monochrome cameras vs. color cameras come into play.

Fortunately, if using commonly available gear such as a ZWO astronomy camera and a Canon EF or Nikon F lens, there are off the shelf solutions for attaching your camera.

When selecting an adapter to mount your camera, we strongly recommend choosing one that allows the ability to insert astronomy filters. These dual purpose filter drawers allow you to insert a filter as well as adapt the lens bayonet mount to your astronomy camera. Below is a connection diagram plus the correct filter drawers for Canon EF & Nikon F lenses: