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, each real optical condition of said plurality of real optical conditions comprising a value of an optical feature of an optical component placed in front of the eye of the subject, comprising a computer with one or more memory and one or more processor, wherein: - an initial model giving the probability of each descriptive answer of the set of descriptive answers for each theoretical optical condition of a plurality of theoretical optical conditions is stored in one of said one or more memory of said computer, - said one or more processor of said computer is programmed to: a) collect the results of said subjective test, performed by asking the subject to describe his visual performance in each real optical condition of said plurality of real optical conditions by choosing, for each real optical condition, one descriptive answer among said set of possible descriptive answers, said results comprising each descriptive answer chosen and the corresponding real optical condition, b) determine (800, 900) the value of said magnitude by taking into account said initial model and each descriptive answer collected, characterised in that said initial model comprises an initial model curve and said computer is programmed to perform, in step b), the following substeps: - plotting at least one collected descriptive answer chosen by the subject against the corresponding real optical condition, and superposing said initial model curve on the plot thus obtained so that the initial model curve fits the plot, - determining said value of said magnitude taking into account the relative positions of said initial model curve and said plot. - an initial model giving the probability of each descriptive answer of the set of descriptive answers for each theoretical optical condition of a plurality of theoretical optical conditions is stored in one of said one or more memory of said computer, - said one or more processor of said computer is programmed to: a) collect the results of said subjective test, performed by asking the subject to describe his visual performance in each real optical condition of said plurality of real optical conditions by choosing, for each real optical condition, one descriptive answer among said set of possible descriptive answers, said results comprising each descriptive answer chosen and the corresponding real optical condition, b) determine (800, 900) the value of said magnitude by taking into account said initial model and each descriptive answer collected, characterised in that said initial model comprises an initial model curve and said computer is programmed to perform, in step b), the following substeps: a) collect the results of said subjective test, performed by asking the subject to describe his visual performance in each real optical condition of said plurality of real optical conditions by choosing, for each real optical condition, one descriptive answer among said set of possible descriptive answers, said results comprising each descriptive answer chosen and the corresponding real optical condition, b) determine (800, 900) the value of said magnitude by taking into account said initial model and each descriptive answer collected, characterised in that said initial model comprises an initial model curve and said computer is programmed to perform, in step b), the following substeps: - plotting at least one collected descriptive answer chosen by the subject against the corresponding real optical condition, and superposing said initial model curve on the plot thus obtained so that the initial model curve fits the plot, - determining said value of said magnitude taking into account the relative positions of said initial model curve and said plot.