Blur equivalence – precision in blurriness

Blur equivalence is a theoretical concept which assigns a precise numerical value to an inherently vague quantity – blur in refractive defocus. The basic premise is simple: overall blur is gauged in terms of equivalence to blur in simple myopia. Thus, if the blurriness of an ametropic eye is no more or no less than an otherwise similar eye requiring -1.00 DS correction, one could state the blur has a unit value of 1.00. In purely spherical refractive errors, this unit is equal to the absolute lens power required to neutralise the refractive error. Hence, the scale for blur equivalence extends from zero in positive numbers only.