Place four planes
Measure z from the object toward the image. Put the object at 0 mm, lens 1 at 50 mm, lens 2 at 150 mm and the image plane at 200 mm. Both lenses have positive focal length 50 mm and operate in air. The 100 mm lens spacing is the sum of the focal lengths.
Follow two rays from one point
Start at object height +2 mm. Choose initial slopes of 0 and −0.04 rad in the paraxial approximation. Over a distance L, add L times the slope to the ray height. At each lens, subtract height divided by focal length from the slope. Both rays arrive at height −2 mm in the image plane.
| z (mm) | Ray 1 height (mm) | Ray 2 height (mm) |
|---|---|---|
| 0 | 2.0 | 2.0 |
| 50 | 2.0 | 0.0 |
| 150 | -2.0 | -4.0 |
| 195 | -2.0 | -2.2 |
| 200 | -2.0 | -2.0 |
Change the image plane
Move the observation plane 5 mm toward the second lens, to z = 195 mm. The two ray heights become −2.0 mm and −2.2 mm. This first-order model shows imaging and defocus. It omits finite apertures, aberrations and diffraction.