What is the equivalent of a Reduced Snellen 20/50 optotype in metric notation assuming a working distance of 40 cm?

To determine the equivalent metric notation of a Reduced Snellen 20/50 optotype at a working distance of 40 cm, it's important to understand the relationship between Snellen visual acuity and metric notation.

In the Snellen system, 20/50 means that a patient can see at 20 feet what a person with normal vision can see at 50 feet. This translates to a vision level of approximately 0.4 when converted to a decimal fraction (20 divided by 50).

When converting this visual acuity to metric notation, the convention is to use the 6-meter distance as the standard reference. In this context, the visual acuity is often expressed in meters, thus giving rise to factors such as "1M," which corresponds to a visual acuity of 6/60 (or worse), and other notations scaling from there.

For a visual acuity of 20/50 at a distance of 40 cm, the metric equivalent can be derived as follows: the equivalent visual acuity of 0.4 in metric notation would be calculated based on the working distance.

At 40 cm, converting that distance to meters results in 0.4 meters. When applying the calculation based on Sn