The two CALIBRATION values are the ONLY values that influence the display size of the chart lines. The RECORDING values above only change the VA labels on the chart lines, and have no bearing on the chart lines themselves.
- Distance: Enter the distance from the patient's eye to the letter chart in mm. Eg 6m is 6000mm. You can work in feet (see Notation above) but you MUST enter the Distance in mm. If using a mirror to double the test distance, measure BOTH from the patient's eye to the mirror AND from the mirror to the chart and ADD the two distances (and don't forget to check the Mirrored/Direct setting - see below). If you are using Notation value of "Feet", Distance will be converted to the nearest foot, or to 20 depending on the value in Numerator.
Length of the line below: Calibrates the chart for the pixel density of the display being used. Measure the ruler line as accurately as possible. Measure two more times, and if the values vary, enter an average. You will need to re-check this value if you change display hardware, graphics adapter settings, or if you turn your display from landscape to portrait or vice versa. Take particular care here if you are using extended display from a laptop as pixel density may be different between the laptop screen and the external display.