Lenses for Piggyback Astrophotography
By Clayton Kessler
October 24, 2001
So! You have been listening to me spout off about how easy it is to take piggyback astrophotos and maybe you have even tried it. The pictures came out well and now you want to branch out but you only have a 50mm lens for your astro camera. The question is do you have to "break the bank" to pick up a few lenses and just what lenses should you look for? Lets start out by looking at the requirements.
If you went to your closet or out to a used camera shop or a flea market to get an older manual SLR camera for astrophotography it probably came with a 50mm lens. The newer cameras all seem to come with a zoom lens of some kind. These are nice for general photos but I try to avoid the use of zooms for astro shots. Zoom lenses have many optical elements and a complex mechanical setup that can make astrophotography harder than it really is. The many optical elements make flare a common occurrence and the complex mechanics mean the lens can shift during a long time exposure. While you can use a zoom lens if you have one I would avoid zooms for any lens that you purchase for astrophotography. What you want is a non-zoom lenses in several focal lengths. Of course, that begs the question Which focal lengths?
Well, let me start by telling you what to avoid. I already stated my opinion about zoom lenses use them if you have them but dont buy one with astrophotography in mind. The only other ones to avoid, IMHO, are the "mirror" lenses. At first glance these seem an incredible bargain. You can pick up a nice 500mm f8 lens for less than $100.00 sometimes lots less. The big problem in that they are very sensitive to correct focus and they are VERY hard to focus on astronomical objects. My suggestion is to leave these alone and stick to the "normal" lenses for now.
(Star Trails taken with a 17mm Tameron Wide Angle Lens)
A wide angle lens is a great choice for extending your lens collection. Focal lengths from 24mm to 35mm are all good and available at a reasonable cost. Generally speaking you will have to "stop" the lens down a stop or two to avoid stretched out stars in the corners. I have a wonderful Canon 24mm f2.8 lens but if I shoot it wide open I get stars that look like seagulls in the corners. Mostly I shoot this lens at f4 or slower and it performs like a champ. Because of the short focal length these require even less guiding than a 50mm lens. They are my first choice for Aurora shots and star trails. Speaking of wide angles a nice "fish eye" lens seems like it would be a lot of fun but I find that once you take a few shots with super wide lenses like this the novelty wears off. In the end I have used my 17mm lens much less than I thought. Oh well I am going to try it on meteors next month and see how that works.
(Zodicial Light taken with 24mm f2.8 Canon lens)
(Constellation Orion taken with 50mm f1.4 Canon Lens)
The "short" telephotos are also very nice. 135mm, 180mm and 200mm lenses from various manufacturers are available at some very nice prices. Keep in mind that you can pay hundreds of dollars for that Canon 200mm f2.8 or that Nikon 180mm f2 but you can get results that are as good with a used Soligor 200mm f3.5 that cost $35.00 it just takes longer. As the focal lengths get longer the requirement for guiding gets greater. A 30-minute exposure with the 200mm lens will probably require that you put an illuminated reticule eyepiece in your scope and make periodic tracking corrections. The 135mm size will require less guiding and will also frame many constellations quite nicely.
(Antares and M4 in the center of Scorpius - taken with a 200mm f3.5 Soligor lens)
(Orion's belt taken with 200mm f3.5 Soligor lens)
(Orion's belt and sword taken with a 300mm Sigma lens)
Longer lenses 300mm, 400mm, and up require a sturdy mount, lots of support and accurate guiding but you can find some great deals on manual focus lenses in the f5.6 to f6.3 range in these focal lengths. Also keep in mind that as the focal length of the lens increases the chances on the proper focus being the marked infinity setting decreases. These lenses must be focused each time you use them for best results.
(Rosette Nebula taken with 300mm f4 Sigma Telephoto Lens)
What kinds of lenses to buy? Well, it is always nice to get the lenses from the same manufacturer as your camera but you can save substantial money by looking at used and after-market lenses. Another way to save money is to look for lenses that are "cosmetically challenged". After all you will be using these in the dark and they will be covered in dew at some point or another. You want the optics to be OK but scratches on the barrel who cares? I like to keep an eye on eBay, several used camera stores and the local camera shows. I have lenses from Canon, Sigma, Soligor, Vivitar and Tameron. They all work well. One of my favorites is a 200mm f3.5 Soligor that I picked up for $35.00 at a camera show. It is sharp to the corners and the infinity setting is at good star focus. These features make the lens a joy to shoot with. I picked up a nice 400mm f6.3 "T" mount lens in the spring and I hope to put it to good use this fall. I also found a nice older Canon FL series 35mm lens at an Ann Arbor camera store. This should frame the larger constellations quite well. Like any other "smart shopper", go slow and take your time. You will find the lens you want at your price eventually just be patient! In the mean time use the lens or lenses that you have and take some pictures!