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The principle of binoculars
Apr 11, 2018

Prism structure:

From the appearance of the binoculars, we can directly distinguish between the Roof prism system and the Porro prism system. The roof prism system and the Porro prism system are obviously different in the appearance of the binoculars. The binoculars of the Roof Prism system are of the straight-tube type; the binoculars of the Porro Prism system look like an N-shape or a U-shape, and the distance between the two objectives is farther than the distance between the two eyepieces. .


The commonly seen combination prism is the Porro series. Since this series is easier to manufacture, it is also cheaper, but it is heavier. The Roof series, because the two prisms must be quite close, so that the light path through the objective lens and eyepiece is a straight line, so the precision requirements are relatively high, more difficult to manufacture. The relative price is also relatively high. However, the well-designed Roof series optical imaging is only as good as the Porro series, but the roof prism structure is denser and lighter in weight.


Whether it is the Porro series or the Roof series, the final purpose of this combination prism of the telescope is to make a series of refraction of light, so that the final light from the eyepiece is formed as a standing image.


As for the material of the prism, if it is not borosilicate glass (BK-7 glass), barium crown glass (or BaK-4 glass) is used. The texture of BaK-4 is relatively small, the resulting image is brighter, and the surrounding image is clearer. In contrast, the price is more expensive. How do we determine the prism material of the binoculars we use?


We point the binoculars at a bright spot, let the eyepiece be about 30 centimeters from the eye, and look at the shape of the exit pupil. Because BaK-4 will produce a round shape, if the image is cut around, it is made of BK-7 glass.


Coating applications:

When light enters the glass from the air or enters the air from the glass, about 5% is reflected. Binoculars The objective lens, prism, and eyepiece of each lens are added together, typically with 10 to 16 optical surfaces in contact with the air. If these optical surfaces are not treated at all, then the total incident light is lost by about 50% due to reflection.


To reduce this harmful reflection, modern refractors are coated with single or multiple antireflection coatings on all optical surfaces. The material of the multilayer antireflection film is magnesium fluoride. The single-layer anti-reflection coating has the best anti-reflection effect for a specific wavelength and has a poor anti-reflection effect for other wavelengths. It can reduce the light reflection on each surface to 1.5%, for example, on all surfaces of a binocular. Monolayer film, the light penetration rate can exceed 80%. The reflection of light from each surface of a good multi-layer film is only about 0.25%. For example, all surfaces of a binocular are coated with a multilayer film, and the light transmission rate can reach 90 to 95%.


In general, the single-layer anti-reflection film design of the visual telescope is best for the yellow-green light transmittance of 5,500 angstroms, because the human eye is most sensitive to this one-band light. Therefore, the reflection of blue-red light is more, so we see that the lenses coated with a single-layer film are often blue-purple or red (that is, the so-called red-eye telescope on the market). The lenses coated with multilayer films are pale green or dark purple, such as the coating of camera lenses.


A single-layer film that is too thickly plated will look green, and some unscrupulous manufacturers use it as a multi-layer film. Some people think that the large-diameter binoculars do not matter even if the coating is poor, and the large diameter can compensate for the loss of light; in fact, the reflections on the optical surfaces inside the lens barrel form stray light, which reduces the contrast of the scene and makes the image blurred. Clearly, when observing the objects in the shadow under the sunlight, the target will disappear in dazzling brilliance, especially in poorly coated telescopes.


There are several kinds of coating?

The binocular mirror is usually marked with a lens coating to indicate the optical quality of the binocular. The type of coating is as follows:


Coated Optics: is the lowest grade AR coating. It only means that at least one optical surface is plated with a single-layer anti-reflection film, which is usually coated on the outer surfaces of both the objective lens and the eyepiece, and the inner lens and prism are not coated.


Fully Coated: All lenses and prisms are coated with a single film, but if an optical plastic lens is used in the eyepiece, the plastic lens may not be coated.


Multi-Coated: At least one optical surface is coated with a multilayer antireflection coating, other optical surfaces may be plated with a single layer or may not be coated at all; usually only on the outer surface of both objective and eyepieces Plated multilayer film.


Fully Multi-Coated: All lenses and prisms are coated with anti-reflection coatings. Some manufacturers have applied multi-layer coatings on all optical surfaces, "while others are only partially coated on optical surfaces. Multilayer film, still coated with a single layer on other surfaces."


Binoculars are all marked with a set of numbers such as 7x50 or 8x42. What does this mean?

The set of numbers on the binoculars, such as 7x50, is the first number "7" refers to the telescope's magnification, which is the distance that will allow each object to be viewed closer to 7 times. For example, using this binoculars to see a target as far away as 3,500 meters, it is as if we were looking at the target before 500 meters with the naked eye. The second number "50" refers to the objective (diameter) of this telescope, in millimeters (mm). The larger the caliber, the more the starlight is collected, and the better the analytical ability. The astronomical telescope should have a calibre of at least 40mm.


You may think that the high magnification of astronomical binoculars is necessary, but it is not. Telescopes that are suitable for handheld use are usually between 7 and 10 times. For a telescope of the same caliber, images with lower magnification are less shaken when held in hand, and usually have a wider field of view. Although the models with higher magnifications have larger scenes, the background of the starry sky looks slightly darker due to the smaller field of view. However, the relative brightness will be higher when looking at the moon and star clusters, and the observation of dark stars will also help.


In addition, high magnification, hand-held is not easy, resulting in severe image shake, may weaken the definition of the search target imaging. It is best to use a tripod to fix it. The magnification is too high In addition to the above situation, the pupil diameter (pupil diameter) is also considered. Moreover, high-magnification models have a short viewing distance, which can cause inconvenience in wearing glasses for observation.


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