More and more people need presbyopia glasses to see clearly in the near. And those who are farsighted, need to wear farsighted glasses to correct their vision. Whether they are presbyopes, farsighted glasses, or reading glasses, the lenses are essentially lenses with a diopter greater than 0.00D. (For ease of description, they are collectively referred to as presbyopic lenses below.)
Commonly available ready-made lenses for presbyopia are mostly 70-diameter and 65-diameter. When assembling, there may be some special reasons for dissatisfaction, such as the lenses may be too small to fit, the assembled lenses may look too thick, the weight difference between the left and right lenses may be too great, etc.
Theoretically, we cannot make the edges of the lenses too thin in order to have a solid fit and safe use. However, increasing the thickness of the edge also increases the thickness of the entire lens, making it significantly heavier. From the patient’s point of view, they need lighter lenses to reduce the pressure on the nasal bridge (this is especially true for patients who need highly orthoptic lenses to correct their vision). From an aesthetic and visual point of view, sometimes we also need to increase the thickness of one lens on purpose to equalize the weight of the left and right lenses to ensure that the glasses do not tilt during wear.
So how to deal with the contradiction between these two be reasonable? You can start from the following aspects.
1, choose the right frame.
2. choosing the right lens material.
3. choosing the right type of lens design.
So what is the right frame to choose?
The weight of the frame also depends on the volume and specific gravity of the material, so it is beneficial to choose a frame with a small specific gravity and volume to reduce the weight of the frame itself. In terms of style, in general, the size of the frame is: full frame > half frame > rimless. However, in terms of the current rimless glasses, most of them are inevitably loose after wearing them for a long time, so I personally prefer half-frame and full-frame frames. The more popular titanium frames have made a greater contribution to the reduction of the weight of the frames themselves.
However reducing the weight of glasses is not only about choosing a lighter frame, but also seeing the effect of the frame on the weight of the lenses. Rimless frames usually require thicker lens edges due to the greater strength required, and half-frame frames require slotting, and the edge thickness cannot be too thin, so the lenses required for full-frame frames can instead be thinned to gain weight advantages.
In addition, the size of the rim determines the size of the lens, and the size of the lens determines the weight of the lens, so a smaller rim means that smaller and thinner lenses can be chosen, thus making the weight of the assembled glasses much lower.
How to choose the material of the lenses in a rational way?
There are many material parameters of lenses, among which 3 parameters – refractive index, Abbe number, and specific gravity – have a great impact on the wearer. In general, a high refractive index means that the lenses can be made thinner, but at the same time the Abbe number will be smaller and the specific gravity will be higher.
A smaller Abbe number means a greater chromatic aberration. How much does chromatic aberration affect imaging? This can be calculated by the out. Simply put, if the Abbe number is 32, then most people who look through a lens over 5.50D will notice a rainbow of colors appearing at the edges.
The specific gravity will have an effect on the weight of the lens. Anyone who has studied physical forces knows that weight = volume x density x acceleration of gravity. So a smaller specific gravity will give a smaller weight.
From the point of view of several common materials, the specific gravity does not have a great impact on the weight of the lenses processed, while the refractive index has a very significant impact on the volume of the lenses, so to reduce the weight of the lenses, choosing lenses with high refractive index is a good way to go. However, when choosing a high refractive index, it is important to pay attention to the Abbe number and to avoid choosing PC and 1.74 for lenses over 5.50D. Also, high refractive index lenses mean a higher price. However, for lenses up to 1.50D, the contribution of the high refractive index material to the weight of the lens itself is not very significant, but the price is much higher, and at the same time, the smaller refractive index has a larger Abbe number, higher transmission rate, and clearer vision, so it is wise to try to choose a CR39 material with a low refractive index (1.499 refractive indexes) at this time.
How to choose the right type of lens design?
Aspheric lenses will have a thinner center thickness than spherical lenses when they have the same refractive index, the same minimum edge thickness, and the same diameter. Usually, lenses with astigmatism and posterior aspheric lenses with free-form technology are thinner than normal single-sided aspheric lenses because they are usually aspheric for specific astigmatism as well.
Of course, choosing custom-made glasses or those with higher requirements for frames and lenses will be relatively more expensive. You can get cheap glasses online at this time. The reason for this relatively cheap price is that online glasses stores have flexible access to resources and fewer costs, and the price can be lower than offline stores without compromising on quality.