Category Archives: research

Calculating Landolt C ring size based on the visual acuity

Landolt Ring

Tiny Python script to calculate the size (5d) in meters of a Landolt C ring based on the visual acuity belonging to that ring.


# determine Landolt C ring size based on decimal visual acuity scales
# argument: visual acuity [0.1 ... 3]
# result: size of Landolt C ring (5d in the image) (in meters)
# http://upload.wikimedia.org/wikipedia/de/2/27/Landoltring.jpg

import math, sys

# distance from ring in meters
distance = 5

print 2*distance*math.tan(math.radians(1/(float(sys.argv[1])*120)))*5

landolt C Chart

Row 8 in the picture to the left shows Landolt rings to determine a decimal visual equity of 1. Use 1 as argument in the Python script and it will returnĀ 0.00727220526792, which means the size of that ring when viewed from 5 meters is 7.27 millimeters.