In my experience, some of the best zoom lenses are just acceptable for astropho-tography. Most zoom lenses aren't. A zoom lens, even a high-technology zoom lens from a major manufacturer, is designed to be tolerable at many focal lengths rather than perfect at one.
Dealers have learned that a zoom lens gives a camera "showroom appeal" -it's more fun to pick up the camera, look through it, and play with the zoom. People have gotten used to extreme zoom lenses on camcorders, which are low-resolution devices not bothered by optical defects. And some photographers just don't like to change lenses (especially if there's a risk of getting dust on the sensor). For these reasons, zoom lenses, including those with extreme ratios, have become ubiquitous.
One look at Nikon's or Canon's published MTF curves should make it clear that a mediocre fixed-length lens is usually better than a first-rate zoom. If you have nothing but zoom lenses, try a non-zoom ("prime") lens; you're in for a treat.
If you do attempt astrophotography with a zoom lens - as I have done -beware of "zoom creep." During the exposure, the zoom mechanism can shift. You may have to tape it in place to prevent this.
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