In the Lighthouse example where 4M near VA is used to read 1M print at 25 cm, what is the measured working distance?

In the Lighthouse example where 4M near VA is used to read 1M print at 25 cm, what is the measured working distance?

• A. 10 cm
• B. 15 cm
• C. 25 cm
• D. 40 cm

Answer: (C)

The concept is that 1M print is defined at a standard near testing distance of 25 cm. In the Lighthouse example, using 4M letters to read 1M print at 25 cm means the chart is placed at 25 cm from the eye and the patient can recognize the 1M size at that distance. Therefore, the measured working distance—the actual distance from eye to chart at the moment of reading—equals 25 cm. The fact that a larger 4M letter size is used to reach that result doesn’t change the distance; it simply shows why the patient can read the smaller, standard 1M print when the chart is at 25 cm.

The concept is that 1M print is defined at a standard near testing distance of 25 cm. In the Lighthouse example, using 4M letters to read 1M print at 25 cm means the chart is placed at 25 cm from the eye and the patient can recognize the 1M size at that distance. Therefore, the measured working distance—the actual distance from eye to chart at the moment of reading—equals 25 cm. The fact that a larger 4M letter size is used to reach that result doesn’t change the distance; it simply shows why the patient can read the smaller, standard 1M print when the chart is at 25 cm.