As you've probably noticed during your eye exams, one of the first tests you get is the Snellen chart reading, which is made up of letters that gradually get smaller and smaller as you move to the bottom lines. In this way, the doctor can measure your visual acuity and estimate the order of magnitude of the defect that he should detect during the refraction examination. If you can't read the letters on the 10/10 line, the ophthalmologist will likely ask you to try again, this time looking through a very small hole (pinhole) to make sure that a simple optical correction with lenses is sufficient. improve your visual skills. This article describes how to measure your own visual acuity at home using a few simple calculations and without the need for an optotype.
- Remember that this test does not replace the visit performed by the doctor and the purpose of this article is purely informative to help the reader better understand the concepts related to visual acuity. The results obtained may not be accurate due to factors that should only be checked in a professional environment.
- Visual acuity is only one of the elements that play a role in visual abilities, and a full eye exam performed by an ophthalmologist involves a number of other tests; an acuity equal to 10/10 is not synonymous with perfect vision or healthy eyes!
Steps
Step 1. Get a sheet of white printer paper, a ruler, a tape measure, a black marker and some clear tape
Step 2. Using the ruler and marker, draw a series of 2mm long segments, starting from one of the top corners of the paper down along each edge
Identify at least 10 segments and repeat the process on the other edge, always starting with the corresponding top corner. This way, you can define perfectly parallel lines that cross the paper from side to side.
Step 3. Draw horizontal lines by resting the ruler to connect each pair of points
Color the space between the first and second line with the felt-tip pen, making it completely black; repeat the process for the spaces between the third and fourth lines, between the fifth and sixth lines, keeping this order until you reach the last line. At this point, you should have a page with horizontal black lines, 2 mm thick, always 2 mm apart from each other.
Step 4. Hang the paper vertically on a wall so that the medial part of the striped section is approximately at eye level and in the center between your eyes
Also make sure that the edges of the paper are parallel to the sides of the wall and that the room is well lit with a good light source.
Step 5. Put away the items you used except the ruler and stand in front of the hanging sheet
Cover your left eye and gradually retreat while keeping the right eye line of sight in the center of the paper. As you move further away, you find that it is increasingly difficult to distinguish black segments from white spaces until you reach a distance where the page appears to be a solid gray without any lines; at this point, stop and advance slightly until you can just make out the stripes. Find the position by placing the ruler on the ground in front of your toes and parallel to the wall.
Step 6. Take the tape measure and measure the distance from the base of the wall in front of you to the ruler
Remember that to get the most accurate data possible, the tape measure must be perpendicular to both the wall and the ruler. Mark the value obtained with the letter "d", you will need it for the calculations that are described below.
Step 7. Now you need to do the calculations that involve simply doing division 138 / d
The number you get becomes the denominator of the fraction 20 / x. At this point, solve the fraction obtained and find your visual acuity expressed with a decimal value. To transform this number into the classic fraction that expresses visual acuity (3/10, 5/10, 10/10 and so on), simply multiply it by 10 and thus obtain the numerator. For example, if "d" is equal to 3.45 m, the quotient of the first division is 40 (138/3, 45 = 40), consequently the second division is 20/40 = 0, 5. Transforming the decimal value in a fraction with a denominator equal to 10, a visual acuity of 5/10 is obtained. The shorter the distance "d" and the higher the first quotient you find, the worse the view. Note that the acuity of 10/10 is obtained when d = 6.9 m.
Step 8. Repeat the last three steps covering the right eye and measuring the visual acuity of the left one
You can also do a third test with both eyes open to check your binocular vision.
Step 9. Now that you have calculated your visual acuity, you understand the mechanism behind the calculations
When calculating vision, you are actually measuring the minimum angular distance between two points that the eye can distinguish as two separate entities and not as a single point. This angle is called the "minimum resolution angle," or MAR, and is standardized to the value of 1.0 minute of arc (one sixtieth of a degree) for a normal eye. Consequently, if a person with a visual acuity equal to 1.0 ′ can distinguish two points on the wall that are 2 mm apart, it means that they cannot be at a distance greater than {(2/2) / [tan (0, 5/60)]} = 6900 mm = 6.9 m from the wall. If the MAR is equal to 2, 0 ′ (acuity of 5/10), it means that the distance between the points must be double or that the reading distance (6, 9 m) must be reduced by half to allow the individual to identify the two points as two distinct elements. This is the method that has been applied in this article.
