Quite often patients will ask what 20/20 means. The explanation of the Snellen visual acuity fraction doesn’t often satisfy their need to understand. A patient once mentioned that with her reduced vision from macular degeneration, the furthest she could see was to the end of her arm. Having just received a 4 power bioptic telescope, she remarked that it in effect it made her arm 4 times longer. She no longer needed to walk up so close to see her friends and family, read signs, watch TV, and shop at the supermarket.
Her experience suggests an alternate way to define distance vision—the furthest distance one can see an object of regard, which we can call the “Visual Radius.” There is also a range of distances in which we engage in our normal day-to-day activities, often from 3-20 feet, which we can call the “Social Range.” Getting closer than 3 feet to recognize an individual is considered socially aggressive behavior—we’re invading their personal space. They don’t feel comfortable having it done to them, and we don’t feel comfortable doing it. Rooms are rarely bigger than 20 feet, so if we can see to the far end of a room, we can pretty much see what we need to see to be socially engaged.
So, for many situations, the goal in prescribing telescopic low vision aids for better distance seeing might be to extend the “Visual Radius” far enough in to the “Social Range” to be helpful for the individual. If, for instance, we can only see faces as far as 4 feet away, a 4 power telescope will extend the distance to 16 feet, far enough into the social range to be helpful. If, in another example, one can only see a face from 1 foot away, a 4x telescope would only extend the “Visual Radius” to 4 feet-- insufficient to see far enough into the “Social Range” to provide a functional benefit.
So, knowing the distance at which an individual can normally see well, can offer a method for us to describe visual acuity in functional terms in a way that the patient can appreciate. It can also be helpful in determining the proper telescope power needed to be helpful to an individual.