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Carefully select the lens processing section of glasses (3)
After the introduction of the previous two articles "optometry for heavy glasses (I)" and "Selection of frames for heavy glasses (II)", after obtaining reliable optometry data and suitable frames, the final step is to process the lenses and get the final myopia glasses.

Key words: refractive index, dispersion coefficient, aspheric surface, transmittance, material, coating, discount.

The last step-lens processing-needs to be divided into two small steps.

Choosing lenses is as important as buying frames, because short-sighted lenses do have technical content. The choice of myopia lenses is the most dazzling, and the publicity copy of major brands is varied, making it difficult to see which ones are real black technology and which ones are just gimmicks. Faced with this situation, it is relatively simple to abandon the rhetoric on the packaging of the merchants, pay attention to the technical indicators of the lenses themselves, and then refer to the data and reputation of major brands, and finally choose a suitable pair of myopia lenses.

Therefore, the myopia lens is divided into two parts: optical lens and coating. By comparing two first-line famous brands, Zeiss in Germany and Essilor in France, as well as domestic Yueming glasses, we can basically sum up a set of methodology for selecting myopia lenses.

It should be noted that this paper only discusses monocular lenses, that is, lenses with only one focus (also called monocular lenses). Monolens is the most mass-produced lens type by lens manufacturers, and there are usually existing lenses with various parameters (also called stock lenses), so generally speaking, the lenses we use are monocular lenses. It can be seen why the interval between myopia and astigmatism is adjusted by 25 degrees during optometry, which is in line with the lens parameters of current lens manufacturers. If the optometry result is 153 degrees, it must be a film with no corresponding degree; Of course, if the degree of myopia or astigmatism is particularly high, for example, the degree of myopia reaches 1000, then there is usually no lens with corresponding parameters in the current film, so we can only customize the lens (also called garage film), and the price will naturally rise. Similarly, those bifocal and progressive multifocal lenses whose official listing price is more than 5000 yuan are also beyond the scope of this article.

Optical mirror sliding

The optical lens (concave lens) of myopia lens is the main body of the lens, and its technical indicators are as follows.

1. refractive index

Refractive index affects the thickness of myopia lens. Common refractive indexes are 1.50, 1.56, 1.60, 1.67, 1.74. The exact number of digits after the decimal point of different brands may be different, but the basic law is that the greater the refractive index, the thinner the lens and the higher the price. Refractive index is a parameter that merchants are most willing to advocate to customers. For example, eternity gives different names to lenses with different refractive indexes.

From the perspective of eTV, the reason for doing this is to let customers know the difference of each shot, but in fact it is only to emphasize the importance of one of the parameters, which is one of the typical business routines. In fact, due to the existence of the next parameter-dispersion coefficient, we can't blindly pursue large refractive index when choosing myopia lenses.

2. Dispersion coefficient (√e)

Dispersion coefficient affects the imaging quality of myopia lenses. Because the concept of dispersion coefficient was put forward by Ernst Abbe, one of the founders of Zeiss, the dispersion coefficient is also called Abbe number. Generally, the Abbe number of myopia lenses is 30 ~ 60, and the greater the Abbe number, the better the imaging quality.

The relationship between refractive index and Abbe number can be considered to be inversely proportional. Taking Zeiss, Essilor and Yue Ming as examples, the Abbe number reaches the highest value of 58 when the refractive index is 1.50, and decreases with the increase of refractive index.

When the refractive index is 1.60, it can be found that the Abbe number of Zeiss and Essilor lenses is 40. This is because the lens with refractive index of 1.60 is made of MR-8 material of Mitsui Chemical Co., Ltd., Japan, which can have both refractive index and Abbe number, and can basically be the best choice for refractive index parameters of low myopia lenses. Actually, the refractive index is 65438+. On the other hand, when the refractive index is 1.60, the Abbe number of domestic Yue Ming lens is only 33. Although it is impossible to guess the reason, it may be the difference of market strategy, but it is still difficult not to regard this as the gap between domestic and imported technical strength.

Of course, if the degree of myopia is high, in order to prevent the lens from being made into the bottom of a beer bottle, the imaging quality can only be sacrificed in exchange for the thinness of the lens. However, when Abbe number is close, for example, compared with lenses with refractive index of 1.67 and 1.74, Abbe number is only one unit, which is almost imperceptible to naked eyes, so we can make a trade-off: choose lenses with refractive index of 1.67, which is cheap but heavy; Choose a lens with a refractive index of 1.74, which is expensive but thin.

Then, in the case of low myopia, in order to pursue high imaging quality, can we directly choose lenses with refractive index of 1.50? Unfortunately, the answer is no at present, because at present, myopia lenses with refractive index 1.50 are the low-end series in the whole market. For example, Yueming glasses do not make myopia lenses with refractive index 1.50; In addition, another reason is that myopia lenses with refractive index of 1.50 are mostly spherical lenses, which will be explained later.

3. Aspheric surface

As we all know, there are two kinds of myopia lenses: spherical lenses and aspherical lenses. The advantages of aspheric mirror have long been popularized by major opticians: small aberration, undistorted edge imaging of lens, flatter, thinner and lighter than spherical mirror. At present, almost all first-line brands have introduced double-sided aspherical mirrors with better performance, and spherical mirrors can be said to be close to the edge of elimination. You can choose an aspherical mirror directly, regardless of the spherical mirror. On the premise of having a budget of several thousand yuan, you can also choose a double-sided aspheric mirror.

4. Transparency

Transmittance refers to the light transmittance of lenses, which is generally divided into five categories: 0 ~ 4. The larger the number, the lower the light transmittance. Relatively speaking, the transmittance parameter is more important for sunglasses. The transmittance category of most myopia lenses is 0, which means that the transmittance is above 80%.

5. Materials

At present, the common materials of myopia lenses on the market are glass, PC and resin. For the material of myopia lenses, the market basically gives the basis for selection. If there is no special reason, it is often not worthwhile to sacrifice other advantages in order to deliberately pursue a lens of a certain material.

(1) glass

The earliest glasses used for myopia are all glass lenses, which have the advantages of high hardness, wear resistance and high refractive index for patients with high myopia. Disadvantages are heavy, fragile and difficult to coat, and most of them are spherical mirrors. Glass lenses are rarely used now. For example, the high refractive index customized glass lenses provided by Zeiss are often aimed at customers with myopia exceeding 1000 degrees.

(2) computer

PC is made of plastic, which is light in weight and easy to coat. And tough and not easy to break. After PC lens is processed to a certain thickness, it can be used as bulletproof glass, porthole of space shuttle, mask of spacesuit, etc. So PC lens is also called space film and cosmic film. For myopia lenses, PCs is widely used in lenses with refractive index of 1.60. For example, eTV named its PC lens eTV Universe Film with a refractive index of 1.60.

However, PC also has many shortcomings, such as not wearing, and it needs coating to increase the hardness of the lens; Easy to deform, not suitable for plastic and frameless frames; The most fatal thing is that the Abbe number is too low. For example, the Abbe number of the above-mentioned Essilor cosmic film is only 3 1, so coating is needed to improve this problem.

(3) resin

Resin is also a plastic material, which has the advantage of lightness and thinness. More importantly, the application range of resin is so wide that myopia lenses are made of resin by default without special instructions. The above MR-8 material with both refractive index and Abbe number belongs to a kind of resin. The disadvantages of resin are similar to those of PC, such as abrasion resistance and fragility. Therefore, resin lenses often need to be coated, which is why this paper can divide myopia lenses into optical lenses and coated lenses for analysis.

To sum up, for the optical lens part of myopia lens, there are mainly five technical indicators: refractive index, Abbe number, aspheric surface, transmittance and material, but only need to weigh the thickness and imaging quality according to their myopia degree and budget range, and other indicators will naturally surface. For example, my myopia in my left and right eyes is not more than 300 degrees, and my lenses should not be too thick. So considering the lens thickness and light weight, I can pursue high imaging quality. So I chose the most mainstream resin lens, refractive index 1.60, Abbe number 40. Considering the limited budget, we chose the ordinary aspheric mirror instead of the expensive double-sided aspheric mirror.

Coated film

As mentioned above, myopia lenses, especially resin and PC lenses, usually make up for some physical defects of optical lenses or enhance the performance of lenses by coating; Moreover, mainstream brands of myopia lenses are usually coated with multi-layer films, which make them have various characteristics at the same time. For example, Zeiss diamond cubic platinum film, as shown in the following figure, the bottom lens substrate is the above-mentioned optical lens, and the other nine layers are coated with various functions.

If choosing the optical lens part of myopia lens is to weigh the thickness and imaging quality of the lens, then choosing the coating part of the lens is basically choosing the brand. This is because, at present, there is no uniform standard to measure the coating performance of major brands, and those words mentioned by merchants in promotional copy can only make people more dizzy. As an ordinary consumer, there is not much you can do here. You can only know in advance which films are usually necessary and which films are often gimmicks, then make subjective judgments according to the word-of-mouth of different brands, and finally choose the necessary films according to your own budget.

1. Antireflective film

Anti-reflection film is also called anti-reflection film. Its function is to reduce the reflection of light and increase the transmittance of light. As shown in the figure below, the left lens has no antireflection film effect, and the right lens has antireflection film.

Add dura mater

For PC, resin and other plastic lenses, hard film is essential to make up for the physical defects of lens materials and improve the wear resistance of lenses.

3. Anti-fouling film

The antifouling film is the most mysterious. Different brands adopt different technologies, so it is impossible to measure their effects, but most of them claim to be waterproof, fog-proof, dust-proof and fingerprint-proof. Due to the wide application of antifouling film, here is a brief introduction.

At present, in addition to the above three films, the functions of other films, such as anti-blue film, anti-ultraviolet film and anti-radiation film, are not easy to judge. If the budget is limited, these extra films can be ignored. For example, after inquiring about various series lenses of Zeiss and Essilor, I found the lens of Zeiss Diamond Cube and Essilor Diamond A4 respectively. The following table lists their introduction to coatings. They all have the above three essential films and additional anti-pornography films, which meet my needs.

After choosing the desired lens, the next step is to find the supplier of the required lens. For first-line well-known brands such as Zeiss and Essilor, their official quotations for myopia lenses are very high. For example, the above-mentioned Zeiss Diamond Cube Blu-ray lens, the official price of two pieces is 1980 yuan; So there is a skill that you have to know, that is, go to Taobao or the glasses wholesale market to find the seller of discounted lenses. For example, the Tmall flagship store in Essilor will give customers who provide their own frames a 30% discount. On the premise of ensuring the authenticity, which lens to choose to see which store offers lower discounts. Generally speaking, Taobao stores where lens factories of various brands are located can provide lower discounts, even original assembly services, such as Zeiss' factory in Guangzhou and Essilor's factory in Shanghai.

On Taobao, I inquired about several shops' Zeiss Diamond Cube anti-blue lens and Essilor diamond A4 lens, and finally found a Taobao shop in Guangzhou (the physical store is in Guangzhou Glasses City) that can assemble Zeiss Diamond Cube anti-blue lens in the original factory, and the price is around 750 yuan. So in the end, I chose to install glasses in this store, put the glasses frame in the glasses box (from the boss who sold the glasses frame) and sent it to the seller. I waited for a week or so to receive the final myopia glasses.

After receiving the glasses, try them on first to see if there is any discomfort, then check the authenticity of the glasses on the official account of "carl zeiss Optics" WeChat, and call Zeiss customer service to check the original assembly number. If there is no problem, the journey of carefully matching glasses is over and a pair of myopia glasses is obtained.

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