Scale (from German Maßstab <Maß - measure, degree + Stab - staff, stick)
~~ images: the ratio of the size of the image to the size of the object.
For shooting lenses, if the subject is located at a considerable distance (at "infinity"), then the scale of the image is determined by the ratio of the focal length lens to the distance from the front principal plane lens to the subject. It is indicated by a ratio, for example: 1: 10000; 1: 25000, etc.
When projecting, when reproducing images of objects located at a close distance from lens, for example, in microscopes, - the scale of the image is determined by linear magnification.
For optical systems of observing instruments, such as a microscope system and a telescopic system, the scale of the image is determined by the apparent magnification.