Thin Lens Objective Lens Design Examples
The following thin lens objective lens design examples illustrate some of the above points presented in “Thin-lens Design Goals”.
The three different lens types illustrated have identical optical system parameters: equivalent focal length = 100, image size = 20 and aperture = 25. These yield an F/4.0 lens having about 11 degrees field of view.
The physical properties are quite different, however, as can be seen in the diagram where the back focal distances and lens apertures are seen to vary greatly. Generally, if the aperture size and field of view are appropriately limited, the three designs can all yield good imagery in the real lens design effort. These limitations are known to the designer from a knowledge of the performance of existing lenses and from his or her actual design experience in real lens designs.
Each design has the aperture stop located at the first lens. This gives rise to the ray envelopes shown. By adjusting the stop location in the thin lens design process the ray bundle paths can be made to vary markedly from the ones shown. This has great significance in terms of aberration control in the subsequent real lens design as well as control of lens element diameters and ray bundle vignetting.
Note that the telephoto and reverse telephoto lenses are one and the same thin lens pair, simply reversed in application. Lens element focal lengths and back focal lengths have been determined by iterative calculations using spreadsheet solutions to the equations shown in the above illustration.
Thin Lens Periscope Example
In the example given below, the lens focal lengths (f) and apertures (Φ) are presented as well as the apertures of the prisms and diameters of the images. Prisms are modeled as glass blocks during the “straight line” design process but are converted back to prisms for the illustration. In the case of a periscope, ray bundle vignetting is a key trade-off parameter owing to the need to stretch the optical path to satisfy a length requirement. Some vignetting (~20%) has been permitted in this example, as indicated in the diagram which shows just four, not, five rays in the off-axis ray bundle.