The high resolution of digital sensors provide quite a challenge for normal photographic lenses. These lenses were made for general daytime photography. Shooting pinpoint light sources across a wide field if stars is the most difficult test possible for a camera lens. Telescopes are corrected to give their best performance at relatively slow focal ratios at infinity. Camera lenses must cover comparatively large fields at various subject distances at much faster focal ratios. So it is not really fair to compare the performance of a camera lens to an astronomical telescope, especially when the camera lens is used wide open. To cover wide fields at fast focal ratios, camera lenses employ multiple elements in multiple groups. In some cases a dozen or more elements. Telescopes, because they work at much slower focal ratios, usually only need 3 or 4 elements for a refractor, and only one mirror for a reflector (the second mirror in a Newtonian is only a flat mirror to turn the light cone at a 90 degree angle to make it come out of the side of the tube). Compound designs, such as a Schmidt Cassegrain, are compromise designs to make a much more portable design, but even they produce diffraction-limited images with only a mirror and one lens. Multiple elements in camera lenses lead to some light loss because there are so many air-to-glass surfaces, and additional contrast loss, even with multi-coating. Also, alignment, centering and collimation of all of these elements is critical in a camera lens. All lenses will also exhibit "vignetting", or more properly, "geometric light falloff." This is an uneven circular field illumination when used wide open, which continues to improve as the lens is stopped down 3 to 4 stops. Optical aberrations such as astigmatism and coma are generally much improved by the time the lens is stopped down two stops. Camera lenses, however, can be used for shooting the stars! In most cases they must be stopped down a stop or two from wide open to improve coma, astigmatism and chromatic aberrations enough to produce usable stars. Even lenses that work very well wide open, such as the Nikon 180mm f/2.8 ED and Canon 200mm f/2.8, will get sharper if they are stopped down one or two stops. The quality of lenses can vary from one to another, even in the same model because of variations in production. 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. This is especially critical for astrophotography. You may find a used lens that looks pristine on the outside but may have lens elements out of collimation on the inside because the lens was dropped or knocked around in a way that didn't leave markings. Zoom Lenses Many DSLR cameras come with a very inexpensive zoom lens. These lenses usually perform ok for daytime work, but generally do not perform as well as fixed-focal length lenses, especially for astrophotography. They contain more elements in more complicated optical designs, and are usually slower in terms of their focal ratios. Some of the latest zoom lenses, in particular the Canon L series, can perform fairly well, but are very expensive. An exception is the Canon EF-S 18 - 55mm f/3.5 - f/5.6 IS II. It is a remarkably good inexpensive lens for astrophotography. Lens Speed To shoot scenic twilight scenes, constellations, meteors, or comets on a fixed tripod it is important to get the fastest lens that you can. "Fast" is this case refers to the focal ratio and means a lens with a large aperture in relation to its focal length. The focal ratio is defined as the focal length of the lens divided by the aperture. So a lens with a 50mm focal length and 25mm aperture has a focal ratio of f/2 (50/25 = 2). Camera lenses do not have their apertures printed on them, nor are they advertised. Only the focal length and f/ratio are given, but it's easy to calculate the aperture with this formula: Aperture = focal length/focal ratio. Counter-intuitively, smaller focal ratio numbers mean larger apertures for a given focal length. For example, a 50mm f/1.4 lens has a larger aperture than a 50mm f/2 lens. Likewise, a 300mm f/2.8 lens has a larger aperture than a 300mm f/5.6 lens. The wider the aperture of the lens, the "faster" it is in terms of speed. Wider apertures collect more photons and shorter exposures can be used. Try to get a lens that is at least f/2.8 for fixed tripod shots so you can use relatively short exposures. This will lessen the amount of star trailing you get when shooting on a fixed tripod. If you are shooting on a polar-aligned equatorial tracking mount, the f/stop of the lens is not as critical because you can always shoot longer exposures without trailing. Canon EF-S and Nikon DX Lenses Sensors in DSLR cameras come in different sizes. Some, like the Canon 5D and Nikon D3 are "full-sized" at 36 mm x 24 mm (the same size as traditional 35mm film). Most are smaller, about 22 to 24 mm on the long side by 15 to 16 mm on the short side. Covering a larger sized sensor at the focal plane is more demanding for a lens' design. Camera manufacturers realized that they could design a set of special lenses for smaller sensors that would be more compact and less expensive and still provide good optical performance. Canon uses a EF-S designation for their particular models. Nikon uses a DX designation for these type lenses. These EF-S and DX 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 and Canon and Nikon do not recommend their use on full-sized sensors. High-End Lenses Canon lenses and Nikon lenses both include high-end, high-performance lenses. In addition to being very expensive, they utilize special glass, such as ED (extra-low dispersion) or fluorite, as well as aspheric lens surface curves. Canon's L series, and Nikon's ED series, in particular, provide very fast maximum apertures 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. Canon has a 24mm f/1.4 L, 35mm f/1.4 L, 50mm f/1.2 L and 85mm f/1.2 L. By comparison, Nikon's fastest shorter lenses are a 35mm f/2, 50mm f/1.4 and 85mm f/1.4. None of these lenses by Nikon incorporate any special ED glass or aspheric curves however. Even with their special glass and exotic designs, these high-end, short-focal-length lenses will not be perfect when used wide open shooting star fields. Their performance will be good, but there will be some coma present especially at maximum apertures. Like every other lens, their performance will improve when stopped down. High-End Lenses vs Telescopes Both Canon and Nikon make fast high-end lenses in the 300mm to 600mm range with apertures from f/2.8 to f/4. These lenses are very expensive, costing thousands of dollars. At this price point you are into the same price range a expensive high-end apochromatic refractors. This naturally raises the question of which work better for astrophotography. If you need an optical system for both daytime photography and astrophotography, these high-end lenses can work. If you need a long focal length with a very fast aperture, say for nature or sports photography, you won't really be able to use a telescope at all. You can, however, use a 400mm f/2.8 lens to shoot the stars, if you have a sturdy mount capable of handling the weight and if you don't mind compromising a bit on optical performance when the lens is used wide open. The optical performance of these lenses on starfields are usually pretty good when used wide open. Their performance, like most other lenses, will improve when they are stopped down. But if you are going to spend $6,800 on a giant piece of glass like this only to stop it down, then you are definitely better off going with an apochromatic refractor with a telecompressor/field flattener. These scopes are made to do one thing - image stars at infinity. They also make excellent instruments for visual observing. If you put a high-power eyepiece on an expensive super telephoto, you will be very disappointed in it's visual performance. So, if you need dual use for daytime and astrophotography, get a long telephoto. If you need dual use for astrophotography and visual astronomy, get an apochromatic refractor. Canon Lenses 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 in the corners degrades as the sensor size gets larger. 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 in the corners degrades as the sensor size gets larger.
Older Canon Lenses 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 the new auto-focus EF 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. This is not a good thing for astrophotography! The older FD lenses can not be used at infinity on an EOS body unless an adapter with an optical transfer lens is used. This usually degrades the performance of the lens enough to be undesirable for astrophotography. So basically, you don't want to use older Canon FD lenses on EOS EF bodies for astrophotography. But if you have one of these rare Canon optical adapters, you can certainly try it! There are also some third party optical adapters to use older FD lenses on new auto-focus bodies, but they are of poor optical quality. Nikon Lenses for astrophotography Nikon is proud that they have not changed their lens mount, and have maintained backwards compatibility with older Nikon lenses with the newest Nikon camera bodies. You can also use Nikon lenses on Canon EOS EF lens-mount bodies. This is because the register distance on a Nikon is 46.5 mm, so there is room for a simple mechanical adapter between a Nikon lens and a Canon EOS body. An excellent quality, inexpensive adapter is available from Fotodiox.com for $28 on the internet. 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.
Other Manufacturers Third-party manufacturers, such as Sigma and Tokina, also make lenses in the Nikon F and Canon EF lens mounts. These lenses are less expensive, but usually not quite as good. Quality control seems to be the major problem with third party manufacturers so individual samples of lenses can vary. Test them if you can before you buy them. Sigma, in particular, has a decent reputation for making some lenses that are good for astrophotography on a budget. In particular these specific lenses are recommended for astrophotography:
*Prices and availability of all items are subject to change without notice by the vendors and manufacturers. |
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