Just focus on infinity and aim it at something interesting, such as the Big Dipper, Orion, or Sagittarius. We will discuss in detail how to focus and how to determine the correct exposure in a later sections.
While camera lenses are not really made for shooting star fields, they definitely can be used for this purpose and can take some wonderful astrophotos. Lens Aberrations Shooting a starfield is one of the toughest tests that you can come up with for a camera lens. Pinpoint light sources across the field will reveal a lot of aberrations in a camera lens such as astigmatism, spherical aberration and coma. See section 907, Optical Aberrations, for some examples. Some camera lenses do perform well on starfields, but many need to be stopped down a stop or two to correct for aberrations and get the best performance. The problem here is that when you stop a lens down, you make the aperture smaller and it collects less light. This means you will need longer exposures. The sensors in digital cameras these days are capable of recording very high resolution. This also means that they better record every aberration that a lens has. Since there is such a range of quality of lenses out there, you will just have to test the lenses you have to see if they perform up to your expectations for astrophotography. A simple series of test exposures will reveal a lot about your lenses for astrophotographic use. Shoot a test frame of a starfield with the lens carefully focused on infinity. Start with the lens wide open, and then stop down 1 stop for each subsequent test frame. Double the exposure each time you stop down 1 stop. Then examine the stars at 100 percent magnification in the center and corners of the field. This test will reveal the f/stop that your lens works best at. It will usually be several stops down from wide open. However, deciding which aperture to actually shoot at will be a compromise. Your f/2.8 lens may be sharpest at f/8, but stopping down that much will make your exposures very long. So you may have to settle for less-than-perfect performance in the corners of the lens to get a reasonable aperture, and exposure time, to shoot at. Usually stopping down the lens one or two stops from wide open is a good compromise. Almost all camera lenses will show some vignetting. This appears as a darkening in the corners of the frame. It's usually not too much of a problem unless the image's contrast is increased a lot. Stopping down will improve vignetting also. The quality of lenses can vary from one to another, even in the same model because of production variations. If you are going to buy any lens specifically for astrophotography, you should try to test it first with your equipment to see if it satisfies your requirements. In general though, you should just start out using what you already have. Zoom Lenses Many DSLR cameras come with a very inexpensive kit zoom lens. Older lenses like this did not work very well for astrophotography. Newer models however, such as the Canon EF-S 18 - 55mm f/3.5 - f/5.6 IS autofocus zoom work very well for their price for astrophotography. This lens came with my Canon 1000D (Digital Rebel XS), and I was very pleasantly surprised by how good it was for astrophotography, even used wide open. It does have some blue color fringing and a bit of astigmatism in the corners, but I can live with that if I need to shoot wide open. Or if I want to use a longer exposure, I can stop the lens down one stop and make it perform better. Some other zoom lenses, particularly inexpensive longer focal length zoom lenses with high zoom ratios, often do not perform well for astrophotography. You will just have to test the lens you have to see how well it works. In general, a fixed-focal-length lens will work better at a larger aperture compared to a zoom lens. For example, a 50mm f/1.8 lens might work well for astrophotography stopped down to f/4. But a 18mm to 55mm f/3.5 to f/5.6 zoom lens will not be able to work at all at f/4 because its widest aperture at 50mm of focal length is only f/5.6. If we stop the zoom lens down two stops to f/11 improve it's optical performance, it is really going to hurt its light gathering ability because of the smaller aperture. If you require a 1-minute exposure with the 50mm lens at f/4, to get the same equivalent exposure with the zoom lens at f/11, you will need an 8 minute exposure to collect the same amount of photons. If you took 8 one-minute exposures at f/4 with the 50mm lens and stacked them, another method we will discuss for improving your images in the advanced techniques section, you would have 8 times more photons than a single 8-minute exposure at f/11 with the zoom lens. If you have a choice, for astrophotography purposes, you will almost always be better off with a faster fixed-focal-length lens. But this is not to say that you can not take pictures at all with a zoom lens, because you can! High-End Lenses Both Canon and Nikon make very expensive high-end, high-performance lenses. They have lens elements that are made out of special, expensive glass. The primary purpose of these designs is to give good performance sports photography or low-light conditions with a wide maximum aperture. These lenses can abe used for astrophotography if you can afford them. Canon's L series, and Nikon's ED series, provide very fast maximum-aperture telephoto lenses. Canon's L series extends down to their wide-angle lenses, normal and short telephotos, but Nikon doesn't quite have anything to match them in the shorter focal lengths. Nikon telephotos, on the other hand, seem to be a little better than Canons. Even with their special glass and exotic designs, these high-end lenses will not be perfect when used wide open shooting star fields. Their performance will be pretty good, but there will be some aberrations present, especially at maximum apertures. Like every other lens, their performance will improve when stopped down. Aiming and Framing Don't be surprised if you can't see much at all through a camera lens when you point it at the stars. This is particularly true of wide-angle lenses with slow apertures. If you use something like a right-angle finder, don't be surprised if you can't see anything at all through it, even if the lens is focused correctly. These devices make the view very dim. To frame the field that you are interested in, you can just try aiming the lens by dead reckoning, or trail and error. Take a test exposure and adjust the framing. Another way to aim the lens is by adapting a small 1x-power red-dot finder to fit into the external flash slot on the top of the camera. Then aim the red dot at the center of the field you want to shoot.
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