System for determining the level of difference between two ophthalmic lenses characterized by one or more comparison parameters, the comparison parameters being selected from parameters reflecting one or more of the following properties: geometric properties, optical properties, mechanical properties, and properties in respect of the comfort and aesthetics of the ophthalmic lenses, comprising: - an interface for acquiring parameters corresponding to an ophthalmic lens intended to be worn by a future wearer and to an ophthalmic reference; - a converter configured to associate the comparison parameters corresponding to the first ophthalmic lens with a first position in a stimuli space and the comparison parameters corresponding to the ophthalmic reference with a second position in the stimuli space, and connected to the acquiring interface, each point in the stimuli space corresponding to one level of sensory stimulus, the sensory stimulus being a visual stimulus and the stimuli space being a colour space, preferably chosen from CIE RGB, CIE XYZ, CIE xyY, CIE UVW, CIE U'V'W', CIE L*a*b*, CIE LCH, and CIE L*u*v*; - a computer configured to determine a distance between the comparison parameters of the first ophthalmic lens and the comparison parameters of the ophthalmic reference, and connected to the acquiring interface and to the converter, where the comparison parameters are determined from definition parameters of the first ophthalmic lens and ophthalmic reference, and where the definition parameters of the first ophthalmic lens are determined from design parameters measured on the wearer; the converter being configured to associate the one or more comparison parameters of an ophthalmic lens with a point in the stimuli space so that: - if a first position and a second position in the stimuli space correspond to two levels of sensory stimulus that a user for whom the stimuli are intended is able to distinguish between sensorily, then the distance between the comparison parameters on the basis of which the first position is determined and the comparison parameters on the basis of which the second position is determined is greater than a local threshold and the user is capable of discerning a difference in the geometric properties of the first ophthalmic lens (V 1) with respect to the ophthalmic reference (V ref) through the lens, so that the more the sensory stimuli are perceived as being different by the user the greater the difference between the first ophthalmic lens and the ophthalmic reference; and - otherwise the distance between the comparison parameters on the basis of which the first position is determined and the comparison parameters on the basis of which the second position is determined is less than a local threshold and the user is not capable of discerning a difference in the geometric properties of the first ophthalmic lens (V 1) with respect to the ophthalmic reference (V ref). - an interface for acquiring parameters corresponding to an ophthalmic lens intended to be worn by a future wearer and to an ophthalmic reference; - a converter configured to associate the comparison parameters corresponding to the first ophthalmic lens with a first position in a stimuli space and the comparison parameters corresponding to the ophthalmic reference with a second position in the stimuli space, and connected to the acquiring interface, each point in the stimuli space corresponding to one level of sensory stimulus, the sensory stimulus being a visual stimulus and the stimuli space being a colour space, preferably chosen from CIE RGB, CIE XYZ, CIE xyY, CIE UVW, CIE U'V'W', CIE L*a*b*, CIE LCH, and CIE L*u*v*; - a computer configured to determine a distance between the comparison parameters of the first ophthalmic lens and the comparison parameters of the ophthalmic reference, and connected to the acquiring interface and to the converter, where the comparison parameters are determined from definition parameters of the first ophthalmic lens and ophthalmic reference, and where the definition parameters of the first ophthalmic lens are determined from design parameters measured on the wearer; - if a first position and a second position in the stimuli space correspond to two levels of sensory stimulus that a user for whom the stimuli are intended is able to distinguish between sensorily, then the distance between the comparison parameters on the basis of which the first position is determined and the comparison parameters on the basis of which the second position is determined is greater than a local threshold and the user is capable of discerning a difference in the geometric properties of the first ophthalmic lens (V 1) with respect to the ophthalmic reference (V ref) through the lens, so that the more the sensory stimuli are perceived as being different by the user the greater the difference between the first ophthalmic lens and the ophthalmic reference; and - otherwise the distance between the comparison parameters on the basis of which the first position is determined and the comparison parameters on the basis of which the second position is determined is less than a local threshold and the user is not capable of discerning a difference in the geometric properties of the first ophthalmic lens (V 1) with respect to the ophthalmic reference (V ref).