I work in an examination room that is 3 m long. My visual acuity chart has been calibrated for 3 m so I know my visual acuity values are correct. One of the other eye specialists in the practice told me I have to adjust my final prescription to take into account the 3 m working distance. He explained why but I couldn’t understand his explanation. Why do I need to adjust the final prescription and how do I do this?

In most business space is expensive. Eye care is no exception. Some practices will try to squeeze as many examination rooms as possible. The more rooms there are the more patients can be seen and the greater the chance of a prosperous business. I know a 3 m room can work as a 6 m room with the addition of a mirror at the far end of the room and a visual acuity chart mounted on the wall behind the patient. This is fine. However, some practitioners I know consider this to be old fashioned and prefer to mount a computer monitor on the 3 m wall and use software to generate properly calibrated visual acuity charts.

However, it is important to recognise that when refracting in a shorter room there is an accommodation effect and this needs to be accounted for.

To calculate the accommodation, use 1/distance to visual acuity chart in metres. Therefore, in a 3 m room this would be 1/3 giving 0.33 DS. This means the patient is effectively getting an extra -0.33D of refracting power from the shorter room. With this in mind, for every patient refracted and focused at infinity in a shorter exam room, additional minus power needs to be added to what was found in the trial frame or phoropter. For example, add –0.33 D for a 3 m visual acuity testing distance and –0.50 DS for a 2m visual acuity testing distance.

This means that when the patient wears this prescription in the real World, they will have optimum visual acuity when dealing with far distance visual tasks.

One more point to consider. With respect to visual acuity testing, it is important to understand that when a patient leans in to see the chart better, the testing distance can be 5 to 10 cm less. A lean in of around 7 cm inches in room with a 3 m visual acuity testing distance is equivalent to a one-line improvement in visual acuity. Some patients do this without realising. They are trying as hard as they can to give you accurate results and sometimes lean in. It is up to you to look out for this and to make sure the patient is sitting back in the chair. For accurate visual acuity values and an accurate prescription, it is important that there should be no leaning forward in an attempt to see the chart better.