Method for characterizing an optical focusing defect of an image capture instrument, wherein the instrument comprises: - an image formation optical system suitable for forming an image of a scene in at least one focal plane; - at least two image sensors (1, 2), each of which comprises a set of elements for detecting pixels arranged at intersections of at least one line and columns of a sensor matrix, both sensors being placed parallel to said at least one focal plane and oriented so that directions of respective lines of the sensors are identical or optically conjugated by a focal plane duplication system, and wherein the two sensors are offset relative to one another parallel to the directions of the lines with a partial overlap of said sensors along the directions of the lines, so that each sensor has a length segment overlapping the other sensor and a complementary segment that does not overlap the other sensor; - a control unit suitable for controlling an image capture operation coordinated between the two sensors (1, 2); and - an optical focusing system suitable for shifting said at least one focal plane of the image formation optical system with respect to the sensors in a direction perpendicular to each sensor; wherein the method comprises the following steps: /1/ capturing two images of a same portion of the scene respectively with both of the two sensors (1, 2) in the respective overlapping length segments of said sensors; /2/ calculating two values of a contrast separately for the two images of the same scene portion; and /3/ deriving a direction of the optical focusing defect along the direction perpendicular to each sensor, based on a comparison of the contrast values calculated respectively for the two images of the scene portion; and characterized in that: - the two sensors (1, 2) are mounted within the instrument with respective average heights that are different in the respective overlapping length segments, said heights being measured along the direction perpendicular to each sensor with respect to a shared reference plane (PRef) or reference planes respectively associated with each sensor and conjugated with one another by the focal plane duplication system; and the two images are captured in step /1/ for one and same adjustment of the focusing system, so that said at least one focal plane of the image formation optical system is situated for said images at respective defocusing distances (df 1, df 2) of the two sensors (1, 2) that are different. - an image formation optical system suitable for forming an image of a scene in at least one focal plane; - at least two image sensors (1, 2), each of which comprises a set of elements for detecting pixels arranged at intersections of at least one line and columns of a sensor matrix, both sensors being placed parallel to said at least one focal plane and oriented so that directions of respective lines of the sensors are identical or optically conjugated by a focal plane duplication system, and wherein the two sensors are offset relative to one another parallel to the directions of the lines with a partial overlap of said sensors along the directions of the lines, so that each sensor has a length segment overlapping the other sensor and a complementary segment that does not overlap the other sensor; - a control unit suitable for controlling an image capture operation coordinated between the two sensors (1, 2); and - an optical focusing system suitable for shifting said at least one focal plane of the image formation optical system with respect to the sensors in a direction perpendicular to each sensor; /1/ capturing two images of a same portion of the scene respectively with both of the two sensors (1, 2) in the respective overlapping length segments of said sensors; /2/ calculating two values of a contrast separately for the two images of the same scene portion; and /3/ deriving a direction of the optical focusing defect along the direction perpendicular to each sensor, based on a comparison of the contrast values calculated respectively for the two images of the scene portion; - the two sensors (1, 2) are mounted within the instrument with respective average heights that are different in the respective overlapping length segments, said heights being measured along the direction perpendicular to each sensor with respect to a shared reference plane (PRef) or reference planes respectively associated with each sensor and conjugated with one another by the focal plane duplication system; and