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A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders .
When printed out at this size, the E on line one will be 88.7 mm tall and when viewed at a distance of 20 ft, you can estimate your eyesight based on the smallest line you can read.}} |Source=Own work by uploader, Based
For 6/6 = 1.0 acuity, the size of a letter on the Snellen chart or Landolt C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree), which is a 43 point font at 20 feet. [10] By the design of a typical optotype (like a Snellen E or a Landolt C), the critical gap that needs to be resolved is 1/5 this value, i.e., 1 arc min.
used for examining the anteriorly placed structures the eye; video link: Charts for vision - •Distant vision: to determine visual acuity of distant vision ••Snellen's distant vision chart-do-; for those who can read in English ••Regional language charts-do-; for those who can read in their local language ••E Chart
An example of the Landolt C eye chart (also known as the Japanese eye chart). Numerous types of eye charts exist and are used in various situations. For example, the Snellen chart is designed for use at 6 meters or 20 feet, and is thus appropriate for testing distance vision, while the ETDRS chart is designed for use at 4 meters. [16]
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Visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size . The diagram on the right shows an observer's eye looking at a frontal extent (the vertical arrow) that has a linear size S {\displaystyle S} , located in the distance D {\displaystyle D} from point O ...