model. (The front of the rim is a convenient place to measure to and marks the center of the eye model's twolens system.) Record this distance, which is the near point of the eye model when equipped with the +62 mm lens. The optics of a twolens system can be simplified looking at the combined effect of the lenses and the total effective focal length of the system. Measure the image distance (d), from the model's rim to the handle of the retina . Calculate the total effective focal length ( f) of the two-lens system using the thin lens in equation. Add the +400 mm lens to slot B. How close can the eye focus now? Keep the +400 mm lens in slot B and replace the lens in the SEPTUM slot with the +120 mm lens (see Figure 11.5a). At what distance does the model eye focus now? What does a real human eye do to change the focal length of its crystalline lens? Remove both lenses and place the +62 mm lens in the SEPTUM slot. Adjust the eye-source distance to the "near poin \" distance for this lens (which you found in step iv) so that the image is in focus. While looking at the image, place the round pupil in slot A. Move the light source several centimeters closer to the eye model. Is the image still in focus