The effect of viewing distance on subjective refraction assessment

Accurate detection of subjective refraction is important to provide patients with the best visual quality. One of the factors affecting detection of subjective refraction is viewing distance. Lack of requirements for optometric office arrangement and required space dimensions leads to smaller optometric offices that provide shorter distance between the patient and the optotype chart. However, the effect of decreased viewing distance on detected subjective refraction remains unknown. The aim of this study was to evaluate the effect of viewing distance on the assessment of subjective refraction. Two types of subjective refraction of the dominant eye were determined in 45 participants at five viewing distances (6 m, 5 m, 4 m, 3 m, and 2.5 m): subjective refraction that ensured visual acuity 1.0 (in decimal units) and maximum subjective refraction that ensured the best corrected visual acuity. The results demonstrated that viewing distance significantly affected the outcome of subjective refraction detection; there were hyperopic shifts in all types of refractions that increased as the viewing distance decreased. To conclude, the most appropriate viewing distance for subjective refraction detection is 5 m or 6 m. If viewing distance is reduced to 4 m or less, a negative spherical lens of the corresponding proximity should be added to the obtained subjective refraction as it cannot be reliably stated that ocular accommodation is fully relaxed. Since this is not always achievable by 0.25 D steps, mirror systems should be used in smaller (3 m and closer viewing distances) optometric offices to provide more appropriate subjective refraction detection.