Single-focus lenses, – the most widely used
A monofocal lens, also known as a unifocal or monofocal lens, is a lens that has the same dioptric power across its entire surface. It is the oldest and most widely used lens type. Monofocal glasses can be made for all ages, although in many cases they are not the most ideal solution.
Single-focus lenses emphasise natural vision and correct the following visual defects:
- Nearsightedness (myopia): distant objects are blurred, while near objects are sufficiently sharp; correction is done with a “minus” concave lens.
- Distance vision (hypermetropia): things far away are sufficiently sharp, while things close up are blurred; correction is done with an “extra” convex lens.
- Astigmatism: Our eyes are not a perfectly regular, ideally shaped optical body, so everyone’s eyes are more or less distorted to some degree. This defect in vision is corrected with so-called cylindrical or toric lenses. These also distort, but in the opposite direction to the eye wearing the glasses. Therefore, the angle of the axis of the prescription lens is important.
- Presbyopia: over the age of 40, sooner or later, everyone has problems reading, as the lens loses its flexibility, making it harder to focus on close objects. In the single-focus lens category, reading lenses are the solution. Different dioptres are needed for different distances.
Monofocal lenses are available in a variety of materials, which not only ensures the aesthetic thinness and durability of the lenses, but also determines the weight of the lenses. Depending on the size of the correction, the wearer’s lifestyle and needs, the following materials can be chosen:
Production technology
Spherical surface lenses
Their surfaces are derived from spherical surfaces. These are the most common, simplest lenses. They are made of semi-finished material, the front surface is given, the back surface is shaped according to the dioptre. For this reason, they perform less well than aspherical or freeform lenses, both optically and aesthetically. They are the most distorting and significantly thicker.
Aspherical surface lenses
An aspherical lens is an optical lens whose surface profiles are not spherical or cylindrical. Its more complex surface profile can reduce or completely eliminate various optical errors and distortions compared to spherical lenses. They are much flatter, thinner, and the glasses are less distorting to the wearer’s eyes (for outside observers), giving a more aesthetic appearance.
FreeForm lenses
The biggest advantage of FreeForm surface design technology is that it treats the lens surface as a collection of very small, individual points, and these points are ground in a very precise way along multiple axes. Because the grinding is so precise, each lens can be manufactured individually to the wearer’s prescription. This allows the thinnest and most accurate lens possible to be made.
Plastic or glass lenses?
This basic question is always asked when buying glasses: do you want plastic or glass lenses? When deciding, remember that your glasses should be strong, beautiful, unbreakable, comfortable and last but not least, easy to wear. The following statement applies to both plastic and glass lenses: the choice of the most suitable material depends on individual factors such as visual acuity and personal taste.
Glass lenses – that is, lenses made of natural glass, according to the professional classification – were once the norm. Thanks to their exceptional scratch resistance, they still have a place in optometry today. For high dioptres, they can be thinner than plastic lenses and this aesthetic aspect should not be underestimated.
Glass has a clear advantage: its refractive index ranges from 1.5 to 1.9, while the refractive index of the organic material (= plastic) ranges only from 1.5 to 1.74. Glass has a higher density than plastic. The result is that glass lenses are thinner than plastic lenses, even at the same refractive index, but also significantly heavier.
Plastic lenses, also known as organic lenses, are still used in all types of glasses and are the best for sports and children’s glasses. These lenses are very lightweight and therefore comfortable to wear. They also have excellent resistance to breakage. In this respect, the plastic used performs up to 100 times better than glass, depending on the type. In fact, plastic lenses also offer greater protection against flying sparks (e.g. during fireworks, campfires, welding and grinding) and do not shatter, which can be a significant safety factor in many everyday situations.
Their disadvantage: compared to glass, plastics have low scratch resistance. This makes them more fragile and requires extra care. The solution is to apply a special coating, such as one that repels dirt and hardens the material.
Another advantage of plastic is that while natural glass can only be tinted to a few shades at relatively high cost, plastic lenses can be easily tinted to virtually any shade. Plastic is the best choice for those who want to wear coloured lenses as a fashion accessory.
Thinned lenses
The refractive index is one of the most important optical properties of a spectacle lens. The higher the refractive index, the thinner the lens. This is particularly important for glasses ordered with a dioptric lens of 2 or more. This allows a higher dioptric lens to fit more aesthetically, even in a metal frame, and the lens will not or only slightly protrude beyond the frame.
The basic, i.e. normal thickness plastic lenses have a refractive index of 1.49, while thinned lenses have a refractive index of 1.53, 1.56, 1.59, 1.6, 1.67, 1.74 for plastic and 1.6, 1.7, 1.8, 1.9 for glass.
Special material is polycarbonate plastic with a refractive index of 1.59. Polycarbonate material was initially only used in the military industry, but is now widely used (e.g. as a raw material for CDs and DVDs). After safety spectacles, dioptric versions started to be produced in the 1980s. Although this material is not the most optically pleasing, its mechanical properties can be very advantageous. It is lightweight (less than half the weight of glass lenses) and unbreakable, with the disadvantage that without surface treatment it has a lower scratch resistance. Its resistance to breakage is 12 times greater than that of plastic lenses, making it particularly suitable for today’s fashionable drilled and dovetailed frames. It is an ideal material for mechanical safety and sports eyewear.