1. A method of optical sighting implemented in an optical sighting apparatus that comprises: a camera unit comprising a lens system arranged to focus images of respective portions of a scene within the field of view of the lens system, and an image sensor arranged to capture images focused by the lens system; and a viewing system arranged to provide viewing by a user of the images focused by the lens system, the method comprising: detecting features within each of plural images captured by the image sensor, being the current image captured at the current time and one or more earlier images captured at earlier image-capture times; generating descriptors in respect of each of the plural images from patches of the respective images at the positions of each detected feature; detecting corresponding descriptors generated in respect of the plural images; deriving the geometric calibration of the camera unit at the current time and at the image-capture time of one of the one or more earlier images, referred to as a designator image, with respect to the scene from the positions in the respective images of the features corresponding to the detected, corresponding descriptors generated in respect of the plural images; in respect of at least one target location in the designator image, identifying, from the derived geometric calibration of the camera unit at the current time and at the image-capture time of the designator image, a corresponding location relative to the current image that corresponds to a target location in the scene that itself corresponds to the target location in the designator image; and indicating in the viewing system where the corresponding location lies relative to the current image.