A 400mm f/5.6 ED camera lens (bottom) is compared to a 420mm f/6 ED telescope (top). The ED telescope is optimized for stars and, even with a field flattener, costs about 1/2 the price of a used ED camera lens of similar focal length and aperture.

Ordinary camera lenses can be used to take some wonderful astrophotos. Their shorter focal lengths compared to telescopes make them much more forgiving of errors in tracking for long exposures.

Camera lenses, however, were not really made to shoot star fields. A star field, with point light sources of varying brightness across the entire field of view, is one of the most difficult tests for a camera lens.

Unlike a telescope, which is optimized to work at infinity, camera lenses are much more complicated optical systems that are designed to work at many distances from up close to infinity. They cover a much wider field of view than telescopes. They rarely work best when used wide open.

You will find that with most camera lenses, the overall sharpness and aberrations improve as they are stopped down. There is only one problem with this for astrophotography though. As they are stopped down, the aperture gets smaller, and they gather less light. This is not a problem in the daytime where there is plenty of light, but it is really a problem for faint deep-sky objects.

So using a camera lens for astrophotography will always be a compromise between the desirability of using the lens wide open to gather light, and stopping the lens down to improve performance.

Telescopes are corrected to give their best performance at infinity at relatively slow focal ratios. As the focal ratio gets faster, they become more complicated to design and construct, same as a camera lens. But camera lenses usually work at much faster focal ratios and cover larger fields of view. Constructing a camera lens that accomplishes this is a more difficult task. That is why camera lenses have multiple elements in multiple groups.

Some zoom camera lenses may have 16 individual lens elements in four groups inside the lens. Some of these groups must move internally relative to each other for focusing and zooming. Each of these elements must also stay centered and collimated. This is a much more difficult task than constructing a telescope with a single objective made out of two or three elements, such as in today's modern apochromatic refractors. The objective in a refractor stays fixed, and all you have to do to focus is move the camera in and out with the focuser.

At long focal lengths, an astronomical telescope will usually out-perform a camera lens at infinity. If you don't believe this, just put an eyepiece on a camera lens. You will be surprised at how bad the image is. This doesn't mean that scopes can't have their own problems, or that they can't be poorly made. But telescopes are designed to work at their maximum aperture and focus stars. Indeed, telescopes are not designed to be stopped down.

Normal photographic lenses, as well as telescopes, are really tested by the high resolution of digital sensors. Every aberration of a telescope or lens will be recorded in minute detail by a DSLR sensor. So the optical quality of a lens or telescope has become more critical with the widespread use of digital sensors.

In general, you will find there is not much of a conflict between using a camera lens or a telescope. For the most part, you will be using camera lenses for wide angle shots of constellations and the Milky Way, and a telescope for high magnification for individual celestial objects such as nebulas, galaxies and star clusters.

Lenses vs. Telescopes - The Bottom Line For wide-angle shots you will need to use camera lenses. For longer focal lengths of more than about 300mm, a telescope is a better choice for astrophotography.