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| [size=+2]Camera Lenses for Astrophotography | |
[size=-1] | | [size=-2]Canon 50mm and 16-35mm lenses. |
The high resolution of digital sensors provide quite a challenge fornormal photographic lenses. These lenses were made for general daytimephotography. Shooting pinpoint light sources across a wide field ifstars is the most difficult test possible for a camera lens.
[size=-1]Telescopesare corrected to give their best performance at relatively slow focalratios at infinity. Camera lenses must cover comparatively large fieldsat various subject distances at much faster focal ratios. So it is notreally fair to compare the performance of a camera lens to anastronomical telescope, especially when the camera lens is used wideopen.
[size=-1]To coverwide fields at fast focal ratios, camera lenses employ multipleelements in multiple groups. In some cases a dozen or more elements.Telescopes, because they work at much slower focal ratios, usually onlyneed 3 or 4 elements for a refractor, and only one mirror for areflector (the second mirror in a Newtonian is only a flat mirror toturn the light cone at a 90 degree angle to make it come out of theside of the tube). Compound designs, such as a Schmidt Cassegrain, arecompromise designs to make a much more portable design, but even theyproduce diffraction-limited images with only a mirror and one lens.
[size=-1]Multipleelements in camera lenses lead to some light loss because there are somany air-to-glass surfaces, and additional contrast loss, even withmulti-coating. Also, alignment, centering and collimation of all ofthese elements is critical in a camera lens.
[size=-1]All lenseswill also exhibit "vignetting", or more properly, "geometric lightfalloff." This is an uneven circular field illumination when used wideopen, which continues to improve as the lens is stopped down down 3 to4 stops. Optical aberrations such as astigmatism and coma are generallymuch improved by the time the lens is stopped down two stops.
[size=-1]Cameralenses, however, can be used for shooting the stars! In most cases theymust 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 180mmf/2.8 ED and Canon 200mm f/2.8, will get sharper if they are stoppeddown one or two stops.
[size=-1]Thequality of lenses can vary from one to another, even in the same modelbecause of variations in production. If you are going to buy anylens specifically for astrophotography, you should try to test it firstwith your equipment to see if it satisfies your requirements. This isespecially critical for astrophotography. You may find a used lens thatlooks pristine on the outside but may have lens elements out ofcollimation on the inside because the lens was dropped or knockedaround in a way that didn't leave markings.
[size=-1]Zoom Lenses
[size=-1]Many DSLRcameras come with a very inexpensive ($100) kit zoom lens, such as an18 - 55mm f/3.5 - f/5.6. These lenses usually perform ok for daytimework, but generally do not perform as well as fixed-focal lengthlenses, especially for astrophotography. They contain more elements inmore complicated optical designs, and are usually slower in terms oftheir focal ratios.
[size=-1]Some of the latest zoom lenses, in particular the Canon L series, can perform fairly well, but are very expensive.
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Lens Speed
[size=-1]To shootscenic twilight scenes, constellations, meteors, or comets on a fixedtripod it is important to get the fastest lens that you can. "Fast" isthis case refers to the focal ratio and means a lens with a largeaperture in relation to its focal length.
[size=-1]The focalratio is defined as the focal length of the lens divided by theaperture. So a lens with a 50mm focal length and 25mm aperture has afocal ratio of f/2 (50/25 = 2).
[size=-1]Cameralenses do not have their apertures printed on them, nor are theyadvertised. Only the focal length and f/ratio are given, but it's easyto calculate the aperture with this formula: Aperture = focallength/focal ratio.
[size=-1]Counter-intuitively,smaller focal ratio numbers mean larger apertures for a given focallength. For example, a 50mm f/1.4 lens has a larger aperture than a50mm f/2 lens. Likewise, a 300mm f/2.8 lens has a larger aperture thana 300mm f/5.6 lens.
[size=-1]The widerthe aperture of the lens, the "faster" it is in terms of speed. Widerapertures collect more photons and shorter exposures can be used.
[size=-1]Try to geta lens that is at least f/2.8 for fixed tripod shots so you can userelatively short exposures. This will lessen the amount of startrailing you get when shooting on a fixed tripod. If you are shootingon a polar-aligned equatorial tracking mount, the f/stop of the lens isnot as critical because you can always shoot longer exposures withouttrailing.
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Canon EF-S and Nikon DX Lenses
[size=-1]Sensors inDSLR cameras come in different sizes. Some, like the Canon 5D and NikonD3 are "full-sized" at 36 mm x 24 mm (the same size as traditional 35mmfilm). Most are smaller, about 22 to 24 mm on the long side by 15 to 16mm on the short side.
[size=-1]Covering alarger sized sensor at the focal plane is more demanding for a lens'design. Camera manufacturers realized that they could design a set ofspecial lenses for smaller sensors that would be more compact and lessexpensive 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.
[size=-1]These EF-Sand DX lenses usually have a rear element that protrudes more into thecamera body and covers a smaller image circle. When these lenses areused on a larger sensor they will vignette and have poorer performancein the corners and Canon and Nikon do not recommend their use onfull-sized sensors.
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High-End Lenses
[size=-1]Canon lenses and Nikon lensesboth include high-end, high-performance lenses. In addition to beingvery expensive, they utilize special glass, such as ED (extra-lowdispersion) or fluorite, as well as aspheric lens surface curves.
[size=-1]Canon's Lseries, and Nikon's ED series, in particular, provide very fast maximumapertures telephoto lenses. Canon's L series extends down to theirwide-angle lenses, normal and short telephotos, but Nikon doesn't quitehave anything to match them in the shorter focal lengths.
[size=-1]Canon hasa 24mm f/1.4 L, 35mm f/1.4 L, 50mm f/1.2 L and 85mm f/1.2 L. Bycomparison, Nikon's fastest shorter lenses are a 35mm f/2, 50mm f/1.4and 85mm f/1.4. None of these lenses by Nikon incorporate any specialED glass or aspheric curves however.
[size=-1]Even withtheir special glass and exotic designs, these high-end,short-focal-length lenses will not be perfect when used wide openshooting star fields. Their performance will be good, but there will besome coma present especially at maximum apertures. Like every otherlens, their performance will improve when stopped down.
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High-End Lenses vs Telescopes
[size=-1]Both Canonand Nikon make fast high-end lenses in the 300mm to 600mm range withapertures from f/2.8 to f/4. These lenses are very expensive, costingthousands of dollars. At this price point you are into the same pricerange a expensive high-end apochromatic refractors. This naturallyraises the question of which work better for astrophotography.
[size=-1]If youneed an optical system for both daytime photography andastrophotography, these high-end lenses can work. If you need a longfocal length with a very fast aperture, say for nature or sportsphotography, you won't really be able to use a telescope at all. Youcan, however, use a 400mm f/2.8 lens to shoot the stars, if you have asturdy mount capable of handling the weight and if you don't mindcompromising a bit on optical performance when the lens is used wideopen.
[size=-1]Theoptical performance of these lenses on starfields are usually prettygood 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, thenyou are definitely better off going with an apochromatic refractor witha telecompressor/field flattner. These scopes are made to do one thing- image stars at infinity. They also make excellent instruments forvisual observing. If you put a high-power eyepiece on an expensivesuper telephoto, you will be very disappointed in it's visualperformance.
[size=-1]So, if youneed dual use for daytime and astrophotography, get a long telephoto.If you need dual use for astrophotography and visual astronomy, get anapochromatic refractor.
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Canon Lenses for Astrophotography
[size=-1]Thefollowing lenses and astrophotographic performance descriptions applyto APS-sized 1.3x, 1.5x and 1.6x crop-factor sensors. In general, lensperformance in the corners degrades as the sensor size gets larger.
[size=-1]- Canon EF-S 10-22mm f/3.5 - 4.5 USM ($750) - good wide open, better stopped down 1 stop
- Canon EF 15mm f/2.8 ($660) - very good at f/4
- Canon EF 16mm-35mm f/2.8 L II USM Zoom ($1,500) - ok at f/2.8, better at f/4
- Canon EF-S 18mm-55mm f/3.5 - 5.6 IS Zoom ($170) - ok wide open at all focal lengths, better when stopped down one stop
- Canon EF 17mm-40mm f/4 L USM Zoom ($750) - ok at f/4, very good at f/5.6
- Canon EF 24-105mm f/4 L USM IS Zoom ($1,100) - ok at f/4, very good at f/5.6
- Canon EF 24mm f/1.4 II L USM ($1,700) - good at f/2.8
- Canon EF 28mm f/1.8 USM ($460) - good at f/2.8
- Canon EF 35mm f/2 ($300) - ok at f/2, good at f/2.8
- Canon EF 50mm f/1.4 USM ($400) - good at f/2.8
- Canon EF 50mm f/1.8 II ($115) - good at f/2.8
- Canon EF 85mm f/1.8 USM ($380) - ok at f/2.8, good at f/4
- Canon EF 85mm f/1.2 L II USM ($1,900) - good at f/2.5, very good at f/2.8
- Canon EF 100mm f/2 USM ($420) - good at f/2.8, very good at f/4
- Canon EF 100mm f/2.8 USM Macro ($525) - good at f/2.8, very good at f/4
- Canon EF 135mm f/2 L USM ($1,000) - good at f/2.8, very good at f/4
- Canon EF 200mm f/2.8 L II USM ($750) - good at f/2.8, very good at f/4
- Canon EF 300mm f/4 L USM non-IS - good at f/4, better at f/4.5
- Canon EF 400mm f/5.6 L USM non-IS $1,200 - good at f/5.6, better at f/6
- Canon EF 500mm f/4 L USM IS $5,800 - good at f/4, better at f/4.5
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Older Canon Lenses
[size=-1]Canon madesome fine manual-focus lenses, such as the R, FL, and FD series, fortheir film cameras before they came out with the EOS system andauto-focus. Unfortunately, these lenses can't really be used on the newauto-focus EF camera bodies for astrophotography.
[size=-1]When Canon redesigned their cameras and lenses for autofocus, theychanged the distance from the lens flange to the sensor. Older FDlenses have a register (flange to sensor distance) of 42 mm. Modern EOSlenses 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 agood thing for astrophotography!
[size=-1]The olderFD lenses can not be used at infinity on an EOS body unless an adapterwith an optical transfer lens is used. This usually degrades theperformance of the lens enough to be undesirable for astrophotography.So basically, you don't want to use older Canon FD lenses on EOS EFbodies for astrophotography. But if you have one of these rare Canonoptical adapters, you can certainly try it! There are also some thirdparty optical adapters to use older FD lenses on new auto-focus bodies,but they are of poor optical quality.
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Nikon Lenses for astrophotography
[size=-1]Nikon isproud that they have not changed their lens mount, and have maintainedbackwards compatibility with older Nikon lenses with the newest Nikoncamera bodies.
[size=-1]You canalso use Nikon lenses on Canon EOS EF lens-mount bodies. This isbecause the register distance on a Nikon is 46.5 mm, so there is roomfor a simple mechanical adapter between a Nikon lens and a Canon EOSbody. An excellent quality, inexpensive adapter is available from Fotodiox.comfor $28 on the internet. They also offer many other adapters to usedifferent lenses on different camera bodies. Nikon lenses used on Canonbodies require manual focus and must be stopped down for metering, butthese two drawbacks are not important for astrophotography.
[size=-1]- Nikon AF-S 14-24mm f/2.8G ED AF Lens ($1,800) good at f/2.8, very good at f/4
- Nikon AF Nikkor 16mm f/2.8 D ($910) - soft corners and coma at f/2.8, OK at f/4
- Nikon AF Nikkor 35mm f/2.0 D ($360) - coma at f/2.8, good at f/4
- Nikon AF Nikkor 50mm f/1.8 D ($135) - ok at f/2.8, good at f/4
- Nikon AF Nikkor 50mm f/1.4 D ($340) - ok at f/2.8, good at f/4
- Nikon AF Nikkor 85mm f/1.4 D IF ($1,300) - coma at f/1.4, good at f/2.8
- Nikon AF Nikkor 85mm f/1.8 D ($450) - ok at f/1.8, good at f/2.8
- Nikon AF DC Nikkor 135mm f/2.0 D ($1,270) - ok at f/2, good at f/2.8
- Nikkor AF 180mm f/2.8 D ED-IF ($915) - good at f/2.8, very good at f/4
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Other Manufacturers
[size=-1]Third-partymanufacturers, such as Sigma and Tokina, also make lenses in the NikonF and Canon EF lens mounts. These lenses are less expensive, butusually not quite as good. Quality control seems to be the majorproblem with third party manufacturers so individual samples of lensescan vary. Test them if you can before you buy them.
[size=-1]Sigma, inparticular, has a decent reputation for making some lenses that aregood for astrophotography on a budget. In particular these specificlenses are recommended for astrophotography:
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[size=-1]Suggested Starter Lens Sets for Astrophotography[size=-1]When juststarting out, the best lens to get is a simple 50mm f/1.8. It is small,lightweight, reasonably fast even when stopped down to f/2.8, and cheap.
[size=-1]If your camera came with a zoom lens, then you can certainly try it out for astrophotography.
[size=-1]The nextlens to think about getting will either be a wide angle or a telephoto.Wide angles are great for scenic shots on a fixed tripod and are moreforgiving of mount tracking errors when used piggy-back forconstellation shots.
[size=-1]Telephotosare great for large nebulae like the North America nebula and the MilkyWay star clouds in Sagittarius. Which type lens to get first will be upto your personal preference, but telephoto lenses will require a goodtelescope mount, and accurate focus, polar alignment and tracking. Asthe focal length of the lens increases, things become more critical andthe degree of difficulty increases.
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[size=-1]- Normal
- Wide Angle
- Telephoto
[size=-1]The Canon200mm and Nikon 180mm telephotos can be used wide open, althoughperformance will improve if they are stopped down. These longtelephotos will also require some type of mount or ring to hold thelens securely. Mounting them by the camera body tripod socket alone isa bad idea because mounting a large heavy lens by a single pivot pointinvites rotation during an exposure leading to trailed stars. Manyastrophotographers choose to mount these heavy lenses with a separatering at the front of the lens in addition to the camera's tripod mount.Using two rings to mount this type of lens allows flexibility in aimingand framing the camera.
[size=-1]Expect tohave to stop the other lenses down a minimum of one to two stops to getacceptable performance, and perhaps three stops to get very goodperformance on a full-frame DSLR camera.
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