Why contact lenses have a lower prescription than frames, an article explains why

Why are contact lenses usually lower than frames? The key lies in the “lens-to-eye distance”, i.e. the distance from the lens to the eye.

To better understand this concept, let's do a little experiment: look at a tree or a building in the distance and push your glasses closer to the base of your nose, which is the close lens-to-eye distance. Then, move the glasses forward and place them on the bridge of the nose, which is the distance between the lens and the eye. Comparing these two states, do you notice a considerable difference in clarity? The higher the lens prescription, the more the lens-to-eye distance affects the clarity of vision.

The reason for the difference in prescription between contact lenses and frames is that frames hang from the bridge of the nose, while contact lenses are directly attached to the cornea. This difference in distance makes it necessary to choose contact lenses with a slightly lower prescription. Optometric formulas usually refer to a frame prescription that needs to be converted to a corresponding contact lens prescription.

So, are contact lenses worse than frames because of their lower prescription? On the contrary, contact lenses usually have better visual quality than frames.

First of all, the world seen through contact lenses is closer to the natural field of vision. Many people will find that their eyes become bigger and brighter immediately after wearing contact lenses. This is because the concave lens of myopia has a visual narrowing effect, where the outside world we see through the lens is narrowed, and the outside world looks at our eyes through the lens is also narrowed. Contact lenses, on the other hand, fit snugly over the eyeball and do not have this shrinking effect.

In addition, the prescription of frames is accurate only in the center of the lens, and there is a “prismatic effect” when looking through the rest of the lens. When frames are distorted for other reasons, the eye is not aligned with the center of the lens, and the prismatic effect can lead to visual fatigue, such as eye swelling, soreness, headache, nausea and other symptoms, and even artificial strabismus.

The figure below shows the position of the center of the frame glasses in relation to the human eye in different states.

Contact lenses are fixed to the eye without these problems and are more suitable for sports and other occasions.

For some special populations, contact lenses offer unparalleled visual advantages. For example, people with more than 1,000 diopters of nearsightedness, people with more than 75 diopters of astigmatism, or people with a large difference in diopters between two eyes. These special populations are characterized by the fact that frames are uncomfortable to wear or do not provide a good quality of vision, and contact lenses can solve these problems. For example, when there is a large difference in prescription between the two eyes, frames cause the difference in the size of objects transmitted to the brain to be too large to be blended into a three-dimensional image. With contact lenses, there is little difference in the size of the images and the brain can better integrate the information from both eyes.

Contact lenses are used on the same principle as frames and myopia surgery, where the desired degree of myopia is placed in front of the eye (frames), applied to the eye (contact lenses) or refracted into the eye (myopia surgery). Contact lenses combine efficiency, simplicity, aesthetics and practicality. Its various subcategories, such as rigid contact lenses, keratoplasty (OK lenses), bandage lenses, progressive multifocal soft contact lenses, and scleral lenses, represent high-tech advances for many patients with complex corneal diseases.

Proper and sensible use of contact lenses can greatly enhance the quality of life.