The size and shape of the spectacle lenses that conform to the shape of the spectacle frame after the processing of the wool lenses naturally depend on the curve of the inner edge of the spectacle frame. The lens should indicate the horizontal direction and nose side, otherwise the asymmetric shape of the lens will become a completely different shape after rotation. The size of the lens is expressed by both the length of the reference line and the height of the reference line (reference line method), that is, AA '× b'b, in mm (for example, 46 × 40); or by the horizontal dimension and the vertical dimension of the circumscribed rectangle (square frame method).
The lens shape is represented by shape difference. The so-called sheet shape difference is the difference between the length of the reference line and the height of the reference line (reference line method), or the difference between the horizontal and vertical dimensions of the circumscribed rectangle (square frame method). The unspecified is the baseline method. For example, for a 46 × 40 size lens, the sheet shape difference is 6 (i.e., 46-40). The different shapes of some lens series are expressed by shape difference. The shape difference is 3 or 4, which belongs to high shape; the shape difference is 10, which belongs to short shape. The early lenses were symmetrical - round or oval. The shape difference of round lens is 0, the short ellipse is 4, and the long ellipse is 9. The semicircle is joined under the semicircle to form a "panoramic round oval" (pro). There are many kinds of sheet shape differences, and the one with large sheet shape difference is called "wide field" shape. In addition, there are "contour", "square", "boundless" and "kinematic" and so on. The change of lens shape is mainly due to the constant renovation of modern glasses frame. Mirror frames with different reference line lengths (e.g. 40, 42, 44, 46) can have the same sheet shape difference 4 (e.g. 40 × 36, 42 × 38, 44 × 40, 46 × 42), but they are not in proportion to each other, so the sheet shapes are also different.
