1. A method for determining at least one geometric/physiognomic parameter associated with the mounting of an ophthalmic lens in a spectacle frame ( 30 ) worn by a user, comprising the following steps: a) placing an accessory ( 20 ) on the user's head ( 10 ), said accessory comprising at least one brightness calibration element ( 21 D, 21 G, 22 H, 22 B, 23 , 24 ) including at least two contrasted zones (ZS, ZC); b) capturing an image (I 1 ) of the user's head ( 10 ) on which said accessory ( 20 ) has been placed, using an image capture device; c) determining the position of the image of the brightness calibration element (I 21 D, I 21 G, I 22 H, I 22 B, I 23 , I 24 ) of said accessory ( 20 ) on said image (I 1 ) captured in step b); d) determining a measured value of the brightness of at least one part of the image of this brightness calibration element (I 21 D, I 21 G, I 22 H, I 22 B, I 23 , I 24 ); e) modifying a parameter for adjusting the optical image-acquisition conditions of said image capture device as a function of this measured value in such a way as to improve the contrast of the image of the contrasted zones (ZS, ZC) of the calibration element; f) capturing another image with the modified parameter for adjusting the optical acquisition conditions; and g) determining said looked-for geometric/physiognomic mounting parameter on the basis of this other image.