The size and shape of the goggles lens that conforms to the shape of the frame after processing the raw film naturally depends on the curve of the inner edge of the frame. The lens should indicate the horizontal direction and the nasal side, otherwise the asymmetrical lens will become a completely different shape after being rotated. The size of the lens is expressed by both the reference line length and the reference line height (reference line method), namely AA’×B’B, in mm (for example, 46×40); or expressed by the horizontal and vertical dimensions of the circumscribed rectangle ( Box method).
The shape of the lens is represented by the 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 (frame method). What is not specified is the baseline method. For example, a lens with a size of 46×40 has a sheet shape difference of 6 (that is, 46-40).
The different lens shapes of some lens series are represented by the lens difference. The shape difference is 3 or 4, which is a tall shape; the shape difference is 10, which is a short shape. Early lenses were symmetrical-round or oval. The shape difference of the round lens is 0, the short ellipse is 4, and the long ellipse is 9. Connecting the semicircle under the semi-ellipse becomes a “Pantoscopic Round Oval” (Pantoscopic Round Oval, PRO), there are many kinds of sheet shape difference, the sheet shape difference is larger is called “wide field of view” shape.