Posted on August 31 2015
When someone has farsightedness or hyperopia, an eye-care professional will normally treat the condition with the prescription of a plus lens. The plus lens compensates for an eye that does not have enough refractive power. How exactly does it do this, though? You may also wonder what the measurements in a plus lens mean. This article will cover both of these topics.
To review, hyperopia is caused by a few different factors. Either the eyeball is too short or it does not refract light enough. With any of these causes, the result is that the focal point of incoming light waves misses the retina and converges behind it. The solution for this problem is to add more refractive power to the eye. This is usually done by prescribing plus lenses in glasses or contacts. A plus lens is a convex lens that causes light waves to converge, or move towards each other. Therefore, the light waves are already moving closer together before they even reach the cornea. The refractive power of the plus lens combines with the reduced power of the cornea and crystalline lens of the eye to make the light waves converge exactly at the retina, where the eye’s photoreceptors will have a perfectly focused image.
To understand how plus lenses work, it can be useful to think of a prism. A prism is a transparent object that bends light. The two main parts of a prism are the base, which is the widest part, and the apex, which is the narrowest part. Imagine a triangular mountain: the lowest, widest part is the base while the top peak, where there is the least amount of surface, is the apex. When light passes through a prism, it will be bent towards the thickest part (the base). It works the same way with lenses. In a plus lens, the thickest part is in the middle and the thinnest parts are at the top and bottom. The light waves hitting the lens will be bent towards the thickest part. Consequently, light waves at both the top and bottom of the lens will be bent towards the middle, meaning that the light waves are being bent towards each other.
Not all people with farsightedness need the same amount of vision correction, though. One hyperopic person may have a cornea that refracts light less well than another hyperopic person. That’s where the measurement of the power of a plus lens becomes important. Lens power is measured in diopters. A higher diopter means the lens bends light more. So a +3.00 diopter lens will have more refractive power than a +2.00 diopter lens. Diopters are measured in regular intervals, too, so a +4.00 diopter lens is twice as powerful as a +2.00 lens. Likewise, a +5.00 diopter lens is the same strength as a +3.00 and +2.00 lens combined. In our above example, the person with more severe hyperopia will need a higher number plus lens.