A method for calibrating a stereo imaging system by using at least one camera, each carried by a respective camera device, and a planar mirror, the method comprising the steps of: - obtaining at least two images by means of said at least one camera, each of the images being captured from a different camera position and containing pictures of a mirrored view of the camera used to capture the image, and a mirrored view of an object, thereby obtaining multiple views of said object, - finding the center of the picture of the mirrored view of the image-capturing camera in each of the images, - obtaining a focal length in pixels of the at least one camera as the ratio of the focal length (f) and the pixel size (s) of the camera, - determining the direction of the mirror's normal vector from the center of the mirrored view of the image-capturing camera for each of the images - determining the distance between the camera and the mirror for each of the images by using (i) a reference point on the camera device, said reference point having known coordinates in the camera coordinate system, and (ii) the coordinates of the corresponding point of the mirrored view of the camera device, - determining the mirror plane equation in the coordinate system of the at least one camera by using the direction and the distance of the mirror plane's normal and the focal length in pixels of the camera, - defining an up-vector in the mirror's plane, - selecting a reference point in the mirror's plane, - defining a reference coordinate system with said reference point as its origo and said up-vector as its vertical y-axis, - for each image, separately determining the coordinate transformation from the coordinate system of the image-capturing camera into said reference coordinate system, and - for any pair of images, determining the coordinate transformation from the camera coordinate system of the first image-capturing position into the camera coordinate system of the second image-capturing position. - obtaining at least two images by means of said at least one camera, each of the images being captured from a different camera position and containing pictures of a mirrored view of the camera used to capture the image, and a mirrored view of an object, thereby obtaining multiple views of said object, - finding the center of the picture of the mirrored view of the image-capturing camera in each of the images, - obtaining a focal length in pixels of the at least one camera as the ratio of the focal length (f) and the pixel size (s) of the camera, - determining the direction of the mirror's normal vector from the center of the mirrored view of the image-capturing camera for each of the images - determining the distance between the camera and the mirror for each of the images by using (i) a reference point on the camera device, said reference point having known coordinates in the camera coordinate system, and (ii) the coordinates of the corresponding point of the mirrored view of the camera device, - determining the mirror plane equation in the coordinate system of the at least one camera by using the direction and the distance of the mirror plane's normal and the focal length in pixels of the camera, - defining an up-vector in the mirror's plane, - selecting a reference point in the mirror's plane, - defining a reference coordinate system with said reference point as its origo and said up-vector as its vertical y-axis, - for each image, separately determining the coordinate transformation from the coordinate system of the image-capturing camera into said reference coordinate system, and - for any pair of images, determining the coordinate transformation from the camera coordinate system of the first image-capturing position into the camera coordinate system of the second image-capturing position.