How to select Scope I.

How to select Scope I.

1. Optical type:

There are many monocular types on the market, but not many suitable for birding. First, you must rule out reflective telescopes, such as Newton's. One has to view the scope from the side of the main body, and the result is like an inverted image. Telescopic telescopes are also not very good for birding because of their extremely long focal lengths and high magnification. Also, both the primary and secondary mirrors are silver-plated, which causes light loss and a pale yellow color. In addition, the secondary mirror also causes the incident light to diffract, resulting in reduced sharpness. So in general, the binoculars used for birding are basically refracting. Monoculars with large apertures and long focal lengths are not suitable for bird watching. As a result, the mainstream binoculars we can see today are smaller than 100 millimeters in diameter. Short focus refractor.

2. Objective lens caliber:

It is generally recommended that the diameter of the binoculars for birding be 50-90 mm, and the diameter is closely related to the size, weight and multiple of the telescope. Too small a aperture results in a telescope with low effective magnification and cannot be used for fine observations, while too large a aperture means more volume and weight. Easy work. At present, the mainstream scope caliber is around 80mm. For less avid birders (like me), I like to carry a small 55mm scope when I'm out and about. For female birders, it is between 50-65mm, the caliber is not taxing on the body, and matching tripods and cloud heads can also be lighter.

3. Material of objective lens:

Common point light source mirror objective lens is made of ordinary optical glass, high-end point light source mirror is made of ED glass, and top point light source mirror is made of fluorite. ED glass and fluorite are expensive blanks with low yield and are therefore very expensive to sell. In terms of optical quality, the dispersion coefficient of ED glass is lower and that of fluorite is lower. A lower dispersion coefficient means that the different wavelengths of light emitted from the object point are more concentrated to the image point. Visually, the image has almost no color edges and is extremely sharp and contrast.

4. Objective lens structure:

The most common objective lens uses a double-gel structure, which is two pieces of glass with different refractive indices bonded together to obtain achromatic effect. High quality objective lenses will have a double separation structure, that is, the appropriate separation of two lenses with different refractive indexes. Two variables (the curvature of the middle lens and the distance between the lenses) can be added for better aberration correction compared to the double-glued construction. Influence. Better objective lenses employ a tripartite structure known as a tripartite lens, but this structure seems to be used more in refracting astronomical telescopes and is very rare to be used in bird-watching monoculars.

5. Three parameters of the eyepiece:

Exit pupil distance - the maximum distance between the eye and the lens of the last eyepiece to see the entire field of vision. Generally, it is more suitable between 15-20 mm. If it is too small, the eye must be attached to the eyepiece to see the full field of vision. It's too small. It's prone to dark shadows. Field of View - Field of view here does not refer to the size of the telescope field of view, but to the eyepiece. A 50-60 degree view will yield satisfactory results. Field of view of the telescope = field of view of the eyepiece/magnification of the telescope. Focal length - This is a very important parameter because telescope magnification = objective focal length/eyepiece focal length, and choosing eyepieces with different focal lengths can combine different magnifications. (About magnification: Birding often takes place during the day and is heavily influenced by light. If it is cloudy, the magnification should not be too large, otherwise the diameter of the exit pupil is too small, and the image will become dark. If the sun is full and the atmosphere is transparent, the magnification can be very good. Properly larger, but imaging at too large a magnification is due to light refraction caused by fluctuations in atmospheric density, which in turn causes the image to blur. Commonly used magnification is between 20-60 times)

6. Coating:

High-quality coatings can not only increase the brightness of the imaging, but also improve the color reproduction of the image, reduce flares and glare, and help to see bright and clear images. The best coatings will have low reflectivity multilayer coatings on all air contact surfaces of the lens. Conventional models will be coated with a relatively simple blue anti-reflection film. Some junk models will be covered in a shiny red film, so don't buy one (a product that attracts customers with a red film won't be a rigorous job, and is questionable anywhere else besides the coating).