Fig. 1. Schematic diagram of secondary refraction imaging of object point Q on optical axis by biconvex symmetric thick lens

Fig. 2. Curve of image distance versus object distance of biconvex symmetric lens with 2.5 cm actual 2.7 cm focal length

Fig. 3. Curve of image distance versus object distance of biconvex symmetric lens with 5.0 cm actual 5.2 cm focal length

Fig. 4. Curve of image distance versus object distance of biconvex symmetric lens with 10.0 cm actual 10.2 cm focal length

Fig. 5. Curve of absolute error sB'-sH' of image distance of ideal thin convex lens changing with “the ratio of object distance to focal length” α (>2)