1. A method of collaborative sighting using plural optical sighting apparatuses that each comprise: a camera unit arranged to capture an image of a portion of a scene within the field of view of the camera unit; and a display device arranged to display the image captured by the camera unit, the method comprising: detecting features within images captured by the camera units of the optical sighting apparatuses; generating descriptors in respect of each image from respective patches of the image at the position of each detected feature; detecting corresponding descriptors generated from different images; deriving the geometric calibration of the camera units with respect to the scene from the positions in the respective images of the features corresponding to the detected corresponding descriptors; in respect of at least one target location in the image captured by the camera unit of a first one of the optical sighting apparatuses, identifying, from the derived geometric calibration of the camera units, a corresponding location relative to the image captured by the camera unit of a second one of the optical sighting apparatuses that corresponds to a target location in the scene that itself corresponds to the target location in the image captured by the camera unit of the first one of the optical sighting apparatuses; and indicating on the display device of the second optical sighting apparatus where the corresponding location lies relative to the displayed image.