Binoculars are essential for bird-watching. The choice of binoculars depends on your budget, your choice of terrain and the level of your professional involvement in nature watching.
If you are a beginner or buying for a child you should go for a reasonably priced or even cheap pair with sufficient light gathering qualities. Binoculars come with two numbers such as 8x40. The first is the magnification. For bird-watching 7-10 inclusive is fine. The second is the diameter of the objective lens (mm), that is the lens furthest from the eye through which the light enters. If you divide the diameter by the magnification - you should get a value of 5 or better for good light gathering including at dawn or dusk or if you operate in a forest environment. So 8x40 or 10x50 will both yield a value of 5. If the value is greater than 5 (7x50) you will have even more light gathering power for working on subjects in shadowy conditions at closer range. Thus If you operate in open spaces with large distances, a larger magnification such as 10 will be preferable to a smaller, but the larger the magnification, the heavier the binoculars may be and this can lead to hand shake. For practical birdwatching binoculars of magnification 7-10 inclusive is more than adequate with an objective diameter that yields a value of 5 if divided by the eyepiece. The smaller the magnification, the closer you can focus and this can make certain binoculars attractive for closer up work like butterfly watching as well as birds. You should ideally have close focusing of 10 feet or less.
When the light enters the binoculars, it has to go through several lenses and at each lens some of the light will be reflected leading to a dimmer light reaching your eye. It is better to have fully coated lenses which minimize the reflected light and the highest standard of prism is made from a type of glass called BAK4 - and it is useful to look to this level at least of glass quality. Most binoculars have lens coatings and this is what gives them their colors (red, blue or green). These coatings are very similar to those used on anti reflective spectacles. You can check the quality of the light getting through by holding the binoculars at arms length and looking through the eyepiece lenses (the ones that join the eye). You should see a point of light where the light is entering through the objective lenses (make sure the caps are off and the unit is facing a source of light). The larger andmore circular this is, the better the coating. If the spot of light is square, there is a loss of light and clarity. The diameter of the spot of light is called the exit pupil measurement and this is what you calculate (in mm) by dividing the objective diameter (in mm as above) by the magnification.
Traditional binoculars are called poro-prism binoculars. The eyepiece and objective lenses are staggered. Roof prism binoculars are more sophisticated but not necessarily better. In these it looks like the light goes straight through though actually the light is refracted even more (bent, reflected and twisted). Poro-prism binoculars gather light more readily and offer better depth perception in judging distances. If you get a reasonably priced, light pair of binoculars (around or less than 2 pounds (1kg)) - at about 8x magnification this should be fine to start with. Companies like Bushnel and Audbon or organizations like the RSPB all sell good pairs like this.
Roof prism binoculars can be more compact and fully sealed unlike poro-prism ones. This can make them more waterproof and fog proof and there will be no condensation issues if they are sealed with an inert gas like nitrogen inside. These are more expensive to buy as they have more lenses inside which need to be fixed in place and minimize light and color distortion.
The eyepiece lenses can have rubber eye-caps to help spectacle wearers and binoculars often give "eye relief" figures, which is the distance that the eyepiece lenses can comfortably be held away from the eyes with a clear image. If you wear spectacles try and get eye relief of at least 15mm.
OK, now I'm going to make your life easy and recommend three websites (two in the same place) which effectively answer the above question better than I can. You can find more information than I can provide and one of the pages provides a detailed list of brands and prices - it is a lot better for you to check them yourself before you buy. I wish you well in your purchase as bird-watching is definitely an environmentally friendly activity - good for you and in the long run the birds that you see. Some manufacturers actually engage in helping to research and conserve birds, and if you get your binoculars from them, part of your money will also assist avian conservation.
http://www.birdwatching.com/optics/binoculars1.html
http://www.birds.cornell.edu/Publications/LivingBird/Winter2005/Age_Binos.html
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