Thin Lens Design Rationale
The lens designer is generally presented with the optical system requirements, the lens physical constraints including size, weight, space envelope, and, of course, the image quality requirements.
The optical system requirements include equivalent focal length, field of view, F-number, entrance and exit pupil locations, vignetting and image relative illumination. The entrance pupil location is important for mating with additional optics, such as those introduced for field of view change, i.e., magnification or focal length change. Similar remarks apply to the exit pupil, a special case being when it is located at infinity (telecentric).
The physical requirements of the system include the overall length of the train of lens elements, the back focal length, i.e., the distance from the last lens element to the image plane, and the total track, the distance from the first lens element to the image plane. There is also generally a limit on the diameters and number of lens elements and, in some cases, the minimum or maximum separation between them.
Finally, there are, very importantly, the image quality requirements. Image quality in the subsequent real lens design will depend greatly on design decisions made in the thin-lens design phase. Some guidelines for these decisions follow.
Aberrations vis-a-vis Thin-lens Layout
While it is not possible in the thin-lens design process to make definitive predictions on the image quality attainable in the real lens design certain guidelines will be of use to the designer.
First, the aperture size-to-focal length ratio of the lenses should not be too great, i.e., the F/number not too small. A “speed” of of F/1, for example, would make aberration correction an extreme challenge in most cases. This is an obvious fact to lens designers when the lens is employed as an objective lens. A multi-element camera objective seldom operates faster than F/1.4. But even when the lens element use is nearer to being a “field lens”, the resultant extreme refractions caused by the high lens speed lead to off=axis image quality issues, including distortion.
Secondly, the sum of the optical powers of the lens elements should be targeted at zero. The Petzval curvature of the system, which is proportional to the sum of the powers of the lens elements, i.e., ∑ N/f, should be minimized in order to minimize field curvature aberration.
Thirdly, the aperture stop should be located to not only satisfy the required entrance and exit pupil locations but also to reduce the burden of correcting off-axis aberrations. Symmetry in chief ray refraction on either side of the stop eases the task of correcting coma and lateral color aberrations since there is then inherent aberration cancellation.
An optimal thin-lens design considers the optical system parameters, physical constraints and image quality requirements simultaneously. It provides the means to create a solid foundation for a real lens design which can then focus on aberration correction. The table below summarizes the thin-lens design goals.