Exam Room Length
The room in which the refraction is carried out is important in achieving satisfactory results. For optical purposes, a 20-foot (6-M) distance from the patient to the vision chart approximates infinity (−0.17 diopters [D]). Many exam rooms are much shorter than this. If the vision chart is placed so that the distance to the patient is only 10 ft (3.05 m) the refraction will be overplussed by 0.33 D. Mirrors are used to extend the viewing distance to the standard 20 ft (6.09 m) in such rooms. Some patients (including many children) are not able to effectively fixate into a mirror and need to be examined in longer rooms or hallways. They may also tend to accommodate in a short room, even if it has a mirror to optically extend it. Simply correcting for the shorter working distance by decreasing the plus or increasing the minus in the measured refraction is not recommended because proper accommodative fogging techniques cannot be performed in such a situation.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.