The visual standard in use in the early twentieth century was the sharpest vision that could be obtained by use of a minus lens (20/20). During World War I, this standard was determined to be excessively high and therefore unreasonable; a great many soldiers would be wearing minus glasses if this standard were enforced. It was decided to not use the 20/20 standard and to choose 20/40 as reasonable and acceptable.
If you are working to achieve a standard of vision, I suggest using the 20/40 standard rather than using a strong minus lens to get to 20/20. The exception would be for driving a car at night. This is a reasonable and practical way to meet requirements and not have your natural eyes adapt to the minus lens any more than is necessary.
Keep in mind: The 20/40 standard was done under room illumination. That is the legal requirement. In deep darkness you will get readings of 20/70 or so, but the Department of Motor Vehicles does not test for deep dusk conditions. Strong consistent work with a plus lens can help you clear your “dusk” vision to 20/40 or better—providing you have the motivation to do so. Be wise and be careful.
More information is available at www.myopiafree.com.