When you buy glasses, how to choose the lens ? (1)

When refer to how to choose the lens. we will talk about the refractive index, coefficient of wear resistance, spherical selection, Abbe number, anti-fouling ability, brand selection, and functional choice.





   1.Refractive index
The refractive index of the lens is determined by the material of lens.
In the case of the same thickness of the center, the lens of the same degree and the same material, a thinner edge of the lens has a higher refractive index; when the brand is the same, the refractive index means the different usage of the material, and the price of the lens can also be judged. The higher the refractive index, the higher the price.
Refractive index selection reference: (For reference only, do not completely follow it)
1.56: 0 — 400° (-4.00D)
1.60: 300° (-3.00D) — 600° (-6.00D)
1.67: 400° (-4.00D) — 800° (-8.00D)
1.74: 800° (-8.00D) or above2.

2. Wear resistance coefficient

The reason why the wear resistance coefficient is mentioned is that the resin lens is not wear-resistant when it enters the market, and it is basically worn after wearing the lens for half a year. However, with the development of the technology, the surface hardness of the ordinary resin lens is 2-3H, and after enhancing its hardness, its hardness reaches 4-5H.Now the hardness of the resin lens has exceeded 6-7H. Adding a hard film solves the disadvantage that the lens is not wear resistant.
It hardness can not be compared with the glass sheet. But the glass sheet are mainly used to provide high refractive index, such as: 1.8, 1.9, the refractive index.
By the way, talk about PC tablets.
Light, tough and not easy to break are significant advantages of PC tablets.
Teach you a skill to identify the hardness of the lens, you can use the same material to circle  on the sample back and forth. According to the depth and thickness of the scratch, you can roughly detect the surface hardness of the lens.

3. Spherical and aspherical surface

Cutting a part of a lens with a convex or concave shape from a sphere is called a spherical lens.
Since the spherical lens is difficult to avoid aberrations (especially astigmatic aberrations, such as bending aberrations), there are lenses that correct the disadvantages of spherical lenses, that is, aspherical surfaces (usually referred to as single-sided aspherical lenses).
Aspherical lens: refers to a rotating surface (such as a parabola) with the same non-circular cross section on all meridians.
The biggest advantage is that there is no object distortion in the edge field of view, and the edge of the lens is about 1/3 thinner than the ordinary spherical lens, so the price will be relatively higher. There are double-sided aspherical lenses on the market. Its view is clearer and the imaging effect is better. Of course, the price is also very expensive.

4. Abbe number

Abbe number is the dispersion coefficient, which is an important indicator to measure the clarity of lens imaging.
The smaller the Abbe number, the greater the dispersion and the worse the clarity of the image. The Abbe number of the lens has a certain relationship with the refractive index. The larger the refractive index of the material, the more the dispersion, that is, the lower the Abbe number.
Therefore, for this single indicator, the lower the refractive index, the better. But how to judge it is be based on the overall situation of vision. Which is the most factor is whether it is appropriate, and it can not be mechanically limited to the Abbe number.