JR materials have very high rates of index change with temperature. These index changes cause thermal defocusing that is an order of magnitude worse than that seen in the visible spectral band, and result in focus shifts usually too rapid to be compensated by housing material selection. The alternative is to actively compensate focus using motorized motions of a lens subgroup. Compensation of a zoom lens is more complex since the required thermal refocusing varies with zoom position. This paper presents a method for thermal compensation that is done in software for computer controlled, stepper motor driven zoom lenses. Compensation can be done to within an arbitrarily small temperature increment.

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Strategies for selecting or creating starting designs vary depending on the designer and the problem. We describe several such techniques and include the basic approach, its advantages, and its limitations. Starting from previous designs is the simplest approach if relevant designs can be found. Most designs come from the literature, including in-house lens design libraries. When the literature is inadequate, the design must be created from scratch. Scratch design approaches vary from "minimum spherical bending" for the simplest cases to application of the designer's knowledge of past designs to provide a "guess" as to the overall configuration type, number of elements, shape, and materials. Experienced designers are often able to sketch the general form they think may apply and then use paraxial ray tracing to adjust the design to the desired values for a starting point. The y–y bar diagram is a tool for synthesizing the first-order properties of optical configurations that are limited by packaging requirements (e.g., periscope and laser beam delivery systems). One can directly specify on the y–y bar diagram such parameters as pupil and image locations, magnification, minimum beam size, and lens barrel diameter constraints. The diagram may then be modified to satisfy these requirements and packaging constraints; detection of system infeasibility is also possible. The diagram does not directly provide aberration performance, but an experienced user can usually infer this and other similar attributes.

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