Nasvacometer - a distance visual acuity calculator

Dr Alan Smith has spent considerable time listening to students' responses when questioned about calculating distance visual acuity in practical situations. He gradually became concerned that the current system of using a nomogram (or alignment chart) has limitations. He subsequently decided to change the situation

Many students refer to a chart known as a 'nomogram', believing it to be the most convenient and perhaps the only way to determine 'on site' distance visual acuity. Rather disappointingly, not all of those who are aware of the nomogram are familiar with how it works. Furthermore, many students openly admit to struggling with the rudiments of basic trigonometry and seem quite unashamed at being incapable of calculating distance visual acuity from first principles. 

Before the advent of the computer and associated electronic calculating devices, slide rules were used extensively. These came in two constructional arrangements (linear and circular) and were used for both general mathematical functions (for example multiplication and division), and also in specific applications where 'special' rules were constructed eg for central heating calculations and for the calculation of electrical cable sizes.

This formed the basis of a special calculator to carry out the same functions as the nomogram, but which would simultaneously have added benefits. In theory the use of the nomogram may still be valid but in practice the shelf life of the paper and its inability to withstand constant handling led me to believe that there could be a much more suitable alternative. In addition, the current nomogram only gives acuity in the Snellen notation and additional scales