A system for determining a value of a magnitude associated with a subjective value of an optical feature of at least a corrective lens adapted to an eye of a subject, by implementing a subjective test where the subject is asked to describe his visual performance in a plurality of real optical conditions by choosing one descriptive answer among a set of possible descriptive answers. The system includes one or more processor programmed to collect the results of the subjective test, performed by asking the subject to describe his visual performance in each real optical condition of the plurality of real optical conditions by choosing, for each real optical condition, one descriptive answer, the results comprising each descriptive answer chosen and the corresponding real optical condition, and determine the value of the magnitude by taking into account the initial model and each descriptive answer collected.