Many of us believe that astrophotography requires a ton of know-how plus the investment of highly specialized (and expensive) gear to create successful images. The truth is that you can capture astrophotography with any camera and any lens. But, as the saying goes, your mileage may vary.
If you already have a camera and lens, you might want to head over to my three-part series Basic Backyard Astrophotography. But, if you want to talk lenses, stay here because we are about to take a deep dive into what glass, or types of optics, you might want to slap on the front of your mirrorless or DSLR camera to get the astrophotographs of your galactic dreams.
Wide-Field vs. Deep Sky
You will often see the terms “wide-field” and “deep sky” floating around the Internet cosmos in discussions regarding astro images. In general, wide-field refers to astrophotography that covers large portions of the sky. Deep sky usually refers to astrophotography that focuses on a single object like a nebula. (Note: These two terms do not have official definitions.)
Your camera lens’s focal length determines what kind of astrophotographs you can capture. Wide-field images—i.e. images of the Milky Way galaxy stretching across a great expanse of sky and maybe the foreground and landscape, as well—are accessible to almost any photographer. Deep sky images—stunning photos of distant nebula and galaxies—often require specialized equipment beyond a camera and lens.
Focal Length / Field of View
The focal length of the lens and the sensor size of the camera determine the field of view of that camera-lens combination. The resulting field of view determines how much of the night sky you are going to capture with your setup—and whether you are capturing wide-field or deep sky images.
To avoid going down the rabbit hole that is the discussion of crop factor, this article will only refer to the lens field of view associated with full-frame equivalent focal lengths. If the mention of “crop factor” already has your brain spinning, click the above link for an in-depth discussion.
The shorter your lens’s focal length is, the wider the field of view and the more sky you can capture. The longer the focal length is, the narrower the field of view, and the more you can capture distant objects with detail. But there is a catch. A huge catch…
Rotation of the Earth vs. Shutter Speed
The round Earth is constantly spinning on its axis while the stars above remain in (relatively) constant positions. That fact is the core challenge of astrophotography. How do you make low-light exposures that require long shutter speeds and prevent the motion of the earth from making the stars look like they are in motion? Or do you give up the dream of having the stars appear as points in your photograph and let them streak through the frame?
Because we are discussing lenses here, let’s focus on the goal of creating crisp, sharp stars in our images and then tie that discussion back into lens focal length.
When we attempt to get star points, we need a shutter speed fast enough to freeze the motion of the earth (or appear to freeze it). To calculate our shutter speed, we can use the “Rule of 500,”* where we divide 500 by the full-frame equivalent focal length of a lens to figure out our maximum shutter speed before we start to see the stars trail.
Let’s use this formula with a wide-angle lens and a telephoto lens:
500/14mm lens = 36 seconds
500/200mm lens = 2.5 seconds
A 36-second exposure of the night sky with a wide aperture (more on this later) and good ISO performance will be a recipe for a nice wide-field photograph. On the other hand, a 2.5 second exposure at any aperture and almost any ISO will likely leave you with a very dark image.
The focal length/shutter speed takeaway? A wide-angle lens allows you to leave your shutter open longer, letting in more star light, and still have star points. A telephoto lens demands a very short shutter opening, and thus a dark image, necessitating the use of star tracking mounts to allow longer exposures with telephoto lenses. Tracking mounts spin the camera and lens at a precise rate to counter the rotation of the earth.
*There are variations of this “Rule” that include using 600 or even 200 depending on your camera’s resolution.
Light Gathering vs. Aperture
Exposure is all about light—and at night, when the stars are out, light is at a premium. With the knowledge that our focal-length-based shutter speed is limited by the rotation of the earth, we need our cameras to maximize light gathering in other ways. The Exposure Triangle teaches us that aperture and ISO are our only other options for increasing the exposure. While it is tempting to crank up the ISO, we all know that high ISO generates noise that can cripple astro images. Aperture is important in astrophotography, and you will want to get a lens with a large maximum aperture—generally wider than f/2.8.
The takeaway? The wider your maximum aperture for any lens pointed at the stars, the more light you can gather for a given shutter speed and ISO, and the shorter your exposures can be.
Wide-Angle Lenses for Wide-Field Astrophotography
Because you can photograph at longer shutter speeds while rendering stars as points, wide-angle lenses are the easiest gateway to enter astrophotography.
Wide-angle lenses allow you to capture expansive shots of the Milky Way or big-sky star-scapes. They also let you capture interesting foreground features and dramatic landscapes inside the same frame.
How wide angle is wide angle when it comes to star photos? The most popular wide-angle full-frame focal lengths for astrophotography seem to be between 14mm and 35mm. Cropped sensor APS-C astro shooters enjoy focal lengths around 10mm to 24mm.
Telephoto Lenses for Deep Sky Astrophotography
If you want to get up close and personal with deep sky objects like the Andromeda galaxy or Orion nebula, you need to go telephoto with your optic.
What focal length is best? Well, that really depends on the capabilities (tracking and payload capacity) of that aforementioned tracking mount and what objects you wish to capture. Pushing past the 300mm mark will allow you to capture many deep sky objects with fantastic detail.
To reiterate, the rub here is that large aperture telephoto lenses are big, heavy, and expensive and will require a hardcore tracking mount to get exposures long enough to gather the dim starlight.
Astrophotography Lens Miscellany
While the big focal length decision boils down to wide-angle/wide-field photos or star-tracked telephoto images, there are some other lens-specific conversations that can be had to guide your purchasing decision:
Autofocus vs. Manual Focus—Modern cameras and autofocus systems are usually capable of locking focus on bright stars or planets, and that is a very nice bonus for astro-photographers. But having a manual focus lens is OK, as well—after all, telescopes are manual focus, too. So, regardless of what focal length you are shooting, autofocus is not required for compelling astrophotography. Don’t hesitate to enjoy the magical mechanical manual focus pleasures of vintage lenses when out shooting the stars!
Prime vs. Zoom—This debate persists in almost every genre of photography and the arguments for each side are similar. Zoom lenses give you the flexibility to adjust your field of view and stellar composition—great for those nights when you want to isolate a part of the sky or include a specific foreground landscape in your composition. Prime lenses (usually) have wider maximum apertures, are (usually) lighter and smaller than their zoom brethren, and (usually) offer (marginally?) better optical quality. The bottom line here is that there are amazing lenses for astrophotography that zoom and don’t zoom.
Catadioptric / Mirror / Reflex Lenses—With exceptionally long focal lengths for the price and size, catadioptric lenses, also known as mirror or reflex lenses, offer a tantalizing gateway into deep sky astrophotography. Before diving in, know that these lenses have limited light-gathering capabilities due to their small maximum fixed apertures—demanding longer shutter speeds that will, unless photographing the moon, usually require a star-tracking mount.
Digiscoping—Skipping photographic lenses altogether, compelling astrophotos may be captured through spotting scopes or telescopes via your digital camera and a digiscoping adapter. These scopes are decidedly not going to provide wide-angle views of the heavens, so know that you’ll be wanting to get outfitted with a star tracker here, as well.
Astrograph—An astrograph is a telescope specifically designed for astrophotography and not usually compatible with telescope eyepieces. Since they are telescopes with simpler optical designs than comparable telephoto camera lenses, they are often lighter (fewer lenses and no mechanical aperture diaphragms) and feature relatively large apertures. A star tracker will be required for these.
Binoculars—Binoculars won’t lend themselves to digiscoping, but, while your camera is making star photos, we encourage you to keep your eyes on the skies and soak in the amazing views that are only afforded by a pair of stargazing binoculars!
Do you have any questions about the right lens for your astrophotography needs? What lenses have you pointed at the stars and loved? Let us know in the Comments section, below!