The difference between these three vision-impairing problems lies in the place in the eye where light rays converge to form the image. “In a normal person, light rays pass through the cornea, which is the first lens of our eye, and when they reach the other lens, the retina, they converge – that is, they come together at the same point to form the image” , says ophthalmologist Canrobert Oliveira, from the Hospital Oftalmológico de Brasília.
As you might already be guessing, this process doesn’t work well with those who have nearsightedness or farsightedness. The first see poorly from afar, while the farsighted suffer to see up close. People with astigmatism cannot see things that are close or far away. This is because the horizontal or vertical lines that make up an image do not form in the right place. “If a person with astigmatism tries to see a cross, for example, the vertical line may be right on top of the retina, but the horizontal one will form before or after it. The result is blurred vision”, says another ophthalmologist, Joel Edmur Boteon, from the Federal University of Minas Gerais (UFMG).
To fix each of the three defects, the key is to get the light rays to converge in the right place, on the retina. You can do this by wearing glasses and contact lenses or by modifying the curvature of the cornea with corrective surgery.
What is the degree?
Value indicates how well you can see
Popularly, the power of glasses and lenses to change the point of convergence of light rays is called “degree”. For myopic people, the calculation is simple: degree = 1 / d, where “d” is the distance in meters to which the person has clear vision. Someone who can only see well up to 0.5 meters, for example, needs to wear glasses with two degrees (1 / 0.5 = 2). For farsightedness and astigmatism, the degree depends on the eye’s ability to adjust to the problem or on the plane that is seen most clearly.
Point of view
Compare the three vision difficulties
MYOPIA
Problem: Difficulty seeing far away. The myopic eye is long and the image forms in front of the retina
Solution: Use the so-called negative concave lenses, which make the rays converge further back on the retina
HYPERMETROPIA
Problem: Difficulty seeing up close. The eye is small and the image forms behind the retina
Solution: Use so-called positive convex lenses, which make the rays converge forward—again, on the retina
ASTIGMATISM
Problem: Blurred vision with more than one focus point. You can’t see what’s vertically or horizontally
Solution: Use the so-called cylindrical lenses, which make the rays of these two planes converge at the same point