As seen from the formula for spot size, the diffraction contribution to spot size is independent of lens shape, while the aberration contribution is dependent on lens shape through the parameter k. Thus, it is mainly when the aberration contribution becomes significant, which occurs at low f-numbers, that lens shape becomes important.
II-VI offers best form meniscus, plano-convex, and aspheric lenses. The prime advantage of plano-convex lenses is lower cost, whereas meniscus lenses can provide better performance. Thus, determining which lens shape is appropriate for a specific application is a tradeoff between the cost and performance factors. To make this evaluation, formulas are used to calculate the spot size for the two different lens shapes, shown in the example of the 5.00” focal length lens in Choosing the Right Focusing Lens.In some cases, calculating exact spot size is not possible.
This is true when the laser contains higher order modes, which can be difficult to accurately detect and analyze as to their effect on lens performance. Under these circumstances, use the general rule that when operating below f/5, the meniscus lens will yield demonstrably better performance. For f-numbers between 5 and 10, the performance differences between the two lens shapes are marginal. Above f /10, it is unlikely there is any significant difference in lens performance.