Equipment review:
Birdwatchers’ Binoculars for Starwatching? - the Swift Audubon 8.5x44
The island of Öland, off south-east Sweden, is a bird-watcher’s paradise - a large part of the migratory birds pass this island, going from Northern Europe and Siberia to Southern Europe and Africa, and back. It also offers amateur astronomers fewer cloud than the mainland, and next to no light pollution. On a summer visit mainly for a star-party, my close friend wanted binoculars useful for both stars and birds, so we went to the ornithology association’s shop. There were many models, from exclusive to popular - but the cheap department store binoculars were not there. It was love at first sight with this Swift model - the wide field, bright, clear and sharp, was outstanding. Apparently this model is their best-seller for bird-watching - customers interested in astronomy must be a very small minority. But would love be as strong after the first night? - yes indeed! Watching the summer Milky Way overhead, with its myriad of stars, was an experience of joy and beauty to be remembered.
Why would you use binoculars for amateur astronomy? They offer rich-field views of larger sky areas than telescopes do, and using both eyes helps tremendously in perception (if you don’t believe this, put a cap over one of the objectives, look for a while and note the difference when you remove the cap). Binoculars are robust, easy to carry along, and ready for use in a moment. Even for the telescope owner trying to star-hop in unfamiliar areas, binoculars are invaluable for surveying the area and looking for good guidepost stars.
So what is so special about this Swift model? On the exterior it is an unremarkable gray, conventional Porro prism model - not extremely light and compact, not waterproof or armored. But looking into it, what strikes you is the unusually wide apparent field of some 70 degrees. This is wide by any standard, met by few telescope eyepieces and exceeded by fewer - such as the Nagler types. This is 40% more in angular diameter than ordinary 7x50 or 10x50 models with 50 degree apparent field, and twice the apparent angular area! What’s more, the eye relief would be good enough for a more conventional field but is outstanding for a wide-field model - with glasses you can still see all or most of the field. Also, the true field is an unusual 8.25 degrees (144m/1000m or 430 ft/1000yds), to be compared to the common 7 degrees for a 7x50 or 5 degrees for a 10x50 model.
The mechanical design is conventional with cast housings and center focusing, with separate adjustment of the right eyepiece. There is no rubber armor. It is certainly as well built and sturdy as any instrument in its price range - it will not take the constant hardships of hard-core bird-watching as well as the models costing 3 times more, but the amateur astronomer’s instruments shouldn’t really need that. I am sure most of us will find it light enough for comfortable viewing, and easy to hold (a standard adapter can be used for mounting on a tripod or other binocular mount).
Optically, it is well made with BaK-4 type Porro prisms (the BK7 prisms of cheap binoculars lose some of the light from the edges of the objectives), and multi-coated optics. The field is sharp - but as with any wide-angle model you will find some small unsharpness near the field edge if you look for it. This is no cause for concern - as with all wide angle instruments, you cannot and do not move your direct vision far from the center of the field. Since your eyeballs rotate through their centers, looking towards the field edge makes your eye pupil move sideways enough to make it miss the exit pupil (where the light passes, after coming from the eyepiece). If something interesting appears near the edge, you move the instrument instead to center it.
The objective size, 44 mm, is slightly smaller than the 50 mm often recommended, and so is the exit pupil of 5 mm. Does it matter? Conventional ”wisdom” says that a 7 mm exit pupil is best since it gives the brightest images, but if you compare a 7x50 (with 7 mm exit pupil) to a 10x50, you will clearly find you can see fainter objects with the latter, despite (or rather because of) its smaller exit pupil. With a wide angle instrument, eye positioning is rather critical, and I believe a larger exit pupil than 5 mm would restrict eye movement too much. The Porro prisms make the binoculars somewhat larger and heavier than roof prisms do - but instead, they offer wider field and lower prices. You may find binoculars with 50 mm lenses that are no bigger and heavier than this model, but only if you are prepared to sacrifice the wide field for the 0.28 magnitude gain in light - and I wouldn’t.
So, even if the design specifications were made with bird-watching in mind, I believe they are excellent for astronomy.
So how about me? After a year of borrowing my friend’s, I bought a pair for myself, and pensioned off my old 10x50s!
Nils Olof Carlin