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 a field of view of the camera unit; and a display device arranged to display images 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 a position of each detected feature by the optical sighting apparatuses whose camera units capture the images; detecting corresponding descriptors generated from different images; deriving geometric calibration of the camera units with respect to the scene from positions in respective images of the features corresponding to the detected corresponding descriptors, wherein the geometric calibration is derived in real time as the corresponding descriptors are detected; in respect of at least one target location in the image captured by the camera unit of a first optical sighting apparatus, 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 optical sighting apparatus 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 optical sighting apparatus; and indicating on the display device of the second optical sighting apparatus where the corresponding location lies relative to the displayed image, wherein detecting the features and generating the descriptors are performed in respect of each image by the optical sighting apparatus whose camera unit captures that image.