I examine a lot of older people most of whom have stopped driving. However, a few do still drive. Occasionally an older driver comes back to my practice after having had a new distance prescription from me complaining that they can’t see road signs in good time when they are driving. What am I doing wrong?
Most consulting rooms have a visual acuity system where letters a read at a distance of 6 metres. Some rooms are set up so that letters are read at 3 metres and the chart is calibrated for this shorter distance.
If you are conducting subjective refraction to get the best possible visual acuity in each eye with maximum plus or minimum minus then this is where you might be going wrong.
Some optometrists forget that if they aim for the best possible visual acuity in each eye with maximum plus or minimum minus then this sets the optimum focal range at around 6 metres (or 3 metres for shorter rooms). This is okay for an older person who spends a lot of their time in the home or indoors. A 6-metre optimum focal distance will work for them in an indoor environment.
However, for younger people with a more outdoor lifestyle and who drive or older people who drive, a 6-metre working distance will be too short. Distant objects such as road signs will be beyond the optimum focal range of your distance prescription and will be blurred.
What I do in practice when using a 6-metre room and working with younger people and older drivers is determine the least plus maximum minus for best visual acuity at 6 metres and then add -0.25 DS to the prescription I have found. So I reduce plus by 0.25 DS or I increase minus by 0.25 DS. This moves the optimum focal range beyond 6 metres. In my experience, this is enough for most drivers to be able to see road signs. Very occasionally some drivers prefer a -0.50 DS adjustment. I make any adjustments I’ve made to the prescription very clear in the records. The final prescription I issue is the one including this adjustment.
I find that this technique reduces the number of complaints I receive from drivers about not being able to see road signs clearly at a reasonable distance.