1. Optical type:
There are many types of monocular on the market, but not many are suitable for bird watching. First of all, reflective telescopes, such as Newtonian ones, must be ruled out. People must observe from the side of the scope body, and the result is like an inverted image. Catadioptric telescopes are also not very suitable for bird watching, because their focal length is extremely long and the magnification factor is too high. Moreover, both the main mirror and the secondary mirror are silver-plated, which will cause light loss and yellowish color. In addition, the secondary mirror can also cause diffraction of incident light, resulting in a decrease in clarity. So generally speaking, the monoculars for bird watching are basically refraction type. Monoculars with too large aperture and too long focal length are not suitable for bird watching. Therefore, the mainstream bird watching monoculars we can see at present are all with a diameter of less than 100 mm. Short focus refractor.
2. Objective len caliber:
It is generally recommended that the diameter of the monocular for bird watching is 50-90 mm, and the diameter is closely related to the volume, weight and multiple of the telescope. Too small a caliber will lead to the low effective magnificiation of the telescope, which cannot be used for fine observation, while a too large caliber means a larger volume and weight. easy job. At present, the caliber of mainstream spotting scopes is around 80mm. For less fanatical birders (such as me), I like to bring a 55mm small-caliber spotting scope when I go out. For female bird watching enthusiasts, it is between 50-65mm, The caliber will not cause physical burden, and the matching tripod and head can also be lighter.
3. Objective len material:
The common spotting scope objective len is made of ordinary optical glass, the high-end spotting scope objective len is made of ED glass, and the top spotting scope objective len is made of fluorite. The blanks of ED glass and fluorite are expensive, and the processing yield is low, so the selling price is also very expensive. In terms of optical quality, the dispersion coefficient of ED glass is low, and the dispersion coefficient of fluorite is lower. The lower the dispersion coefficient means that the light of different wavelengths emitted from the object point is more concentrated to the image point. From the visual sense, it will be found that the image has almost no color edges, and the clarity and contrast are extremely high.
4. Objective len structure:
The most common objective len adopts a double-glue structure, which is to glue two pieces of glass with different refractive indices together to obtain an achromatic effect. The high-quality objective len will adopt a double separation structure, which is to properly separate two lenses with different refractive indices. Compared with the double cemented structure, two variables (the curvature of the middle lens and the distance between the lenses) can be added to obtain better aberration correction. Effect. A better objective lens adopts a three-separated structure, that is, three separated lenses, but this structure seems to be used more in top-level refracting astronomical telescopes, and it is very rare to be used in bird-watching monoculars.
5.Three parameters of eyepiece:
Exit pupil distance - the maximum distance between the eye and the last eyepiece lens to see the entire field of view clearly. Generally speaking, it is more suitable between 15-20 mm. If it is too small, the eyes must be attached to the eyepiece to see the entire field of view. Too small Far and prone to dark shadows. Field of view - the field of view here does not refer to the size of the telescope's field of view, but to the eyepiece. A field of view of 50-60 degrees will obtain satisfactory results. The field of view of the telescope = the field of view of the eyepiece / the magnification of the telescope . Focal length - this is a very important parameter, because the magnification of the telescope = objective len focal length / eyepiece focal length, choosing eyepieces with different focal lengths can combine different magnification. (About magnification: Bird watching activities are often carried out during the day, which is greatly affected by light. If it is cloudy, the magnification should not be too large, otherwise the diameter of the exit pupil will be too small and the image will be dim. If the sun is sufficient and the atmosphere is transparent, the magnification can be fine. Appropriately larger, but imaging at too large a magnification is due to the refraction of light caused by atmospheric density fluctuations, which instead leads to blurred images. Generally, the commonly used magnification is between 20-60 times)
High-quality coating can not only increase the brightness of imaging, but also improve the color reproduction of the image, reduce flare and glare, and help to see bright and clear images. The best coatings will have a low reflectivity multilayer coating on all air contact surfaces of the lens. Traditional models will be coated with a relatively simple blue anti-reflection coating. Some garbage models will be coated with sparkling red film, so don’t buy such telescopes (products that rely on red film to attract customers will not be a rigorous work, and other places except coating are also questionable)