1. Blurry Vision Happens When Light Focuses in the Wrong Place
The eye works like a camera. The cornea and lens bend light to form an image on the retina at the back. If the focus lands exactly on the retina, you see clearly. Nearsightedness is a state where, when the eye is not accommodating, parallel light rays from a distance form an image in front of the retina. Because the light has already started to spread out by the time it reaches the retina, distant objects appear blurry [1].
When looking at something close, the eye can change the shape of the lens to bring the focus forward. It is thought that this is why people with nearsightedness can often read nearby text but cannot read the blackboard. Since the focus is already shifted forward, it can match the focus of nearby objects.
2. How Small Differences in Eye Depth and Cornea Curve Shift the Focus
There are two main reasons why light focus shifts forward: the curve of the cornea is too steep, or the axial length (the depth of the eye) is too long. Both can occur together [2].
A normal axial length is about 23 to 24 millimeters. In cases of high nearsightedness, it can extend to more than 30 millimeters [1]. Going from 24 to 30 millimeters is an increase of about one-quarter. However, exceeding 30 millimeters is only part of high nearsightedness; not all nearsightedness is this severe.
Even a few millimeters of extra axial length can shift the image well in front of the retina. This small difference affects how clearly you see things like the text on a blackboard.
3. Farsightedness: When Light Focuses Behind the Retina
Farsightedness occurs when the focus falls behind the retina because the axial length is too short or the cornea is too flat [2]. Convex lenses are used to correct this [3].
When you are young, the lens can adjust its shape to compensate for the shift in farsightedness. Therefore, you might not notice any difficulty seeing. As you age and the ability to adjust weakens, it first becomes hard to see things up close [3].
In nearsightedness, the focus is in front of the retina; in farsightedness, it is behind. Although the direction of the shift is different, the core issue is the same: the focus is not landing on the retina.
4. How Concave and Convex Lenses Bend Light Differently
A concave lens for nearsightedness is thin in the center and thick at the edges. It spreads the light entering the eye outward, which pushes the focus slightly backward so it lands on the retina rather than in front of it [4]. The stronger the nearsightedness, the stronger the negative lens required.
A convex lens for farsightedness bulges in the center and works to gather light. By gathering light earlier for an eye where the focus is shifted backward, it pulls the focus forward [3].
The unit for lens strength is the diopter (D). It is the reciprocal of the focal distance in meters. Convex lenses have a plus sign, and concave lenses have a minus sign. For example, a focal distance of 0.5 meters is 2D [5]. A larger number means a stronger lens.
There is also astigmatism. This is a condition where the surface curves that focus the eye are irregular. It is corrected with a cylindrical lens, which changes refraction power in only one direction [6].
5. Why Age-Related Far Vision Problems Differ from Farsightedness
Presbyopia (often called age-related farsightedness) is a state where the ability to change the shape of the lens to focus on near objects decreases due to aging. It is said to become noticeable in the early to mid-40s [2].
Farsightedness is a shift in focus caused by the structure of the eye. Presbyopia is a decline in the power to adjust focus to near objects. While similar symptoms may appear, the causes are different phenomena, and some people have both.
People with nearsightedness also lose their adjustment power as they age. In the history of glasses, convex lenses for seeing nearby were made first, as explained in the next section.
6. The History of Glasses: Convex Lenses First, Concave Lenses Later
The first glasses in Europe are estimated to have been invented in Italy around 1280. In 1306, the monk Giordano from Pisa stated in a sermon that the technology for making glasses had been found less than 20 years prior, and it is said he met the inventor [7].
Regarding the origin, there are records of glasses being made in 13th-century Italy. Theories include a nobleman from Florence or a Chinese origin, but it is not settled [8].
Early glasses were convex lenses for looking at things up close (for presbyopia). Concave lenses for nearsightedness appeared later [8]. The date of their appearance is debated in sources. Encyclopedias say the mid-16th century, but research suggests they existed in the 15th century, based on documents where the Milan court ordered concave lens glasses from Florence in the 1460s [9].
In Raphael’s painting *Portrait of Leo X with Two Cardinals*, painted around 1518 to 1520, Pope Leo X is holding gold-rimmed lenses. He is said to have been nearsighted, but sources do not clarify if those lenses were concave or convex [10].
7. Why Glasses Are Used to Correct Children's Vision
Refractive errors in children are mainly corrected with glasses. The MSD Manual Home Edition explains that leaving them untreated can lead to permanent vision loss known as amblyopia [6].
Children may not notice that they have trouble seeing. If the blackboard is hard to see or you find yourself squinting, consult an eye doctor.
8. How to Investigate Lenses Further with a Magnifying Glass
You might find it interesting to verify how lenses work with a magnifying glass and then read materials based on questions like these. If you keep the distance between a magnifying glass (convex lens) and text constant, but move the glass slowly away from your eye, how does the image change? You can verify the light-gathering property of the lens with your own eyes (do not point the magnifying glass at the sun).
Whether concave lenses for nearsightedness appeared in the 15th or mid-16th century is debated in sources [8][9]. What kind of evidence would settle this?
If you convert the diopter numbers written on glasses or contacts into focal distance, what distance would they be [5]?
This note explains how eyes work and is not advice for diagnosis or treatment. If you have symptoms like blurry vision, please consult an ophthalmologist.