Fig. 68. Effect of the position of a spectacle lens on retinal image size in 5-D refractive myopia. Object subtends 0.1 radians. a. Emmetropic eye forms an image 1.72 mm long. b. Chief ray height of uncorrected eye is 1.72 mm. Power of eye is 63 D. c. Fitting the correction lens at the first focal point does not affect power but shifts second principal plane to within 15.9 mm of the retina. Image height is 1.59 mm. d. Fitting the correction lens at the refracting surface results in a normal power of 58 D, and since the principal plane remains at the vertex, the image height is 1.72 mm, the same as in emmetropia.