When your optometrist asks you to read the Snellen Chart, that chart with lines of letters of increasingly smaller size, what they are testing is the clarity of your vision–also known as your visual acuity. This can be impacted by several factors including how sensitive your brain’s interpretive faculty is, the functionality of the retina, and how sharp is the retinal focus of the eye.
How your eyesight is measured is important for determining whether you have vision concerns currently or may develop any in the future. Here’s more on what you need to know about the measurement of your eyesight.
You may have to identify optotypes in order to have your visual acuity measured
As in the above mentioned Snellen Chart, the use of black symbols on a white background is effective for determining the clarity of your vision. The symbols are known as optotypes, and they can include the following: letters that have been stylized, Landolt broken rings (rings that have a gap in them–the person being tested must state on what side the gap is located) and other symbols used for those who are illiterate. The symbols will typically be placed on a printed chart, though other formats are also used.
Charts with optotypes must be placed at a specific distance away depending on the type of test
Your optometrist can test your near visual acuity, by placing the chart at a reading distance, as well as your acuity for how far away you can see.
The measurement of your eyesight is a psychophysical procedure
The results of a test on your visual acuity is ultimately determined by your perception of the physical characteristics of the symbols and the resulting responses that you make.
It is essential that various factors in the room are controlled to ensure the most accurate results. This means that the chart and room must have the right lighting, the person being tested must be given an appropriate amount of time for responding, and an allowance should be made for error.