Sometimes, the lens that comes with your camera may not be suitable for astrophotography. It may be too slow, or have too many aberrations. Or, it may be fine, but you want to expand your range of focal lengths so you can shoot different types of objects. Lenses are broadly classified into three categories. Wide-angle, normal, and telephoto. Normal is a holdover from the days of film when a 50mm lens came with most cameras, and it's field of view was considered "normal" for approximating the magnification of normal human vision. Wide-angle lenses are usually anything under about 28mm of focal length. With the crop factor of a DSLR sensor, 35mm to 50mm is now considered "normal". 85mm to 135mm would be considered a short to medium telephoto, and anything over 135mm is a long telephoto. If your camera didn't already come with a wide-angle zoom lens, the first lens to think about getting will be a wide angle. Wide angles are great for scenic shots on a fixed tripod and are more forgiving of mount tracking errors when used piggy-back for constellation shots. The Milky Way can be magnificent when shot with a wide angle lens. A really good, inexpensive lens for a beginner astrophotographer to consider is a simple 50mm f/1.8 fixed focal length lens. It is small, lightweight, and fast, even when stopped down to f/2.8. It will frame a field of view like a short telephoto lens with the 1.5 to 1.6x crop factor of consumer DSLR cameras. It can take some nice constellation shots, such as Sagittarius, the Northern Cross, or the Milky Way in Cygnus. Telephotos are great for large nebulas like the North America nebula and the Milky Way star clouds in Sagittarius. Which type lens to get first will be up to your personal preference, but wide-angle lenses with their low magnification are much more forgiving of tracking inaccuracies. As the magnification increases with telephoto lenses, problems with tracking, polar alignment and focus are also magnified. As the focal length of the lens increases, things become more critical and the degree of difficulty increases. Below are a selection of lenses in each category that I recommend for beginners in astrophotography for their combination of performance and price.
You will notice that all of these recommended lenses except the Canon 18-55mm are fixed focal-length lenses. While zoom lenses are great for daytime work, you will just get better performance for astrophotography, in a faster lens, with a fixed focal-length lens. The Sigma lenses are very fast for wide-angle lenses, and both perform very well when stopped down a little bit. This is a great lens for constellations, scenics, or aurora photos on a fixed tripod. Put it on a barn-door tracker, or equatorial mount, and you will be able to take some killer long-exposure wide-field shots of the star clouds and dark nebulas in the Milky Way. Provided, of course, that you are at a dark-sky site where these delights of the sky are visible. The 50mm f/1.8 lenses by both Canon and Nikon are true bargains. They are often overlooked these days since most DSLR cameras come with a stock wide-angle zoom lens. These lenses work great when stopped down to about f/2.8 and rival the performance of their much-more-expensive 50mm f/1.4 brothers. The Sigma 150mm f/2.8, and the Canon 200mm f/2.8 and Nikon 180mm f/2.8 telephotos all get rave reviews for their performance wide open. These long telephotos will also require some type of mount or ring to hold the lens securely. Mounting them by the camera body tripod socket alone is a bad idea because mounting a large heavy lens by a single pivot point invites rotation during an exposure leading to trailed stars. Many astrophotographers choose to mount these heavy lenses with a separate ring at the front of the lens in addition to the camera's tripod mount. Using two rings to mount this type of lens allows flexibility in aiming and framing the camera. Please note that availability and prices of these lenses are subject to change without notice.
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