Canon makes a lot of different lenses. The total number of zoom lenses is almost bewildering. Which are the best for astrophotography? It's often hard to tell from the normal daytime tests that most reviewers do. Using a lens for astrophotography is very different than normal daytime photography. For the most part, the lenses listed below are recommended because I have either used them myself, or I know someone I trust who has used them specifically for astrophotography.
The following lenses and astrophotographic performance descriptions apply to APS-sized 1.3x, 1.5x and 1.6x crop-factor sensors. In general, lens performance gets worse in the corners as the sensor size gets larger. These evaluations are subjective and not quantitative.
Prices and availability are subject to change without notice. Once you are up to about 400mm of focal length, at the price of a Canon L lens, you can start considering a telescope. A scope will work better on the stars, and you can use it visually for observing also. A camera lens, on the other hand, is a bit more versatile. You can use it for daytime sports or wildlife photography, and you can use autofocus and autoexposure with it. Canon EF-S Lenses Canon makes a special series of lenses, called EF-S, which are specifically made for smaller DSLR-sized sensors. These lenses are more compact and less expensive but still perform well optically on a 1.5x or 1.6x crop sensor. These EF-S lenses usually have a rear element that protrudes more into the camera body and covers a smaller image circle. When these lenses are used on a larger sensor they will vignette and have poorer performance in the corners so Canon does not recommend their use on full-sized sensors like in the Canon 5D.
Canon made some fine manual-focus lenses, such as the R, FL, and FD series, for their film cameras before they came out with the EOS system and auto-focus. Unfortunately, these lenses can't really be used on DSLR camera bodies for astrophotography. When Canon redesigned their cameras and lenses for autofocus, they changed the distance from the lens flange to the sensor. Older FD lenses have a register (flange to sensor distance) of 42 mm. Modern EOS lenses have a register of 44 mm. That means that for infinity focus, older lenses come to a focus 2mm in front of the sensor, so they won't work for astrophotography. Nikon Lenses on Canon Bodies You can also use Nikon lenses on Canon EOS EF lens-mount bodies. I have several old manual-focus Nikon lenses that I use on my Canon DSLR, and they work very well for astrophotography. You can do this because the register distance on a Nikon lens is 46.5 mm, so there is room for a simple mechanical adapter between a Nikon lens and a Canon EOS body. A good quality, inexpensive adapter is available from Fotodiox.com. They also offer many other adapters to use different lenses on different camera bodies. Nikon lenses used on Canon bodies require manual focus and must be stopped down for metering, but these two drawbacks are not important for astrophotography. Sigma Lenses for Canon Sigma also make lenses in the Canon EF lens mount. These lenses are less expensive, and some are quite good for astrophotography. Quality control seems to be a bit more of a problem though, so individual samples of lenses can vary. Test them if you can before you buy them. Sigma makes some Canon-mount lenses that are very good for astrophotography on a budget. In particular these specific lenses are recommended for astrophotography:
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