Optical sighting is performed using plural optical sighting apparatuses that each comprise: a camera unit and a viewing system for viewing images. Geometric calibration of the camera units with respect to the imaged scene is performed by detecting features within captured images, generating descriptors from respective patches of the image at the features; detecting corresponding descriptors from different images, and deriving the geometric calibration from the positions in the respective images of the corresponding descriptors. A target location in a designator image captured at an earlier image-capture time is designated and a corresponding location relative to the current image captured at the current time is identified from the geometric calibration. A viewing system indicates where the corresponding location lies relative to the current image.