Reading magnifiers : variation in magnification, image distance and field width

Three quantities determine the usefulness of a loupe or reading magnifier as a reading aid: the angular magnification, the distance of the virtual image to the eyes and the reading field width. It will be shown in this paper how these quantities depend theoretically on eye-to-loupe distance, object-to-loupe distance and focal distance. In doing so, three possible reading fields are distinguished: the monocular reading field, the binocular one and the so-called composite reading field, which is a mixture of both. The relation between magnification and these three reading fields is discussed.