Compensator selection considerations for a zoom lens

摘要

The selection of compensators for a cam-driven zoom lens is more complex than for a prime lens, because tolerances cause the back focal distance to shift by different amounts in different zoom positions, i.e, the system loses parfocality. Adjustment of the back focal distance can bring one, but not all, of the zoom positions back into focus. Furthermore, compensator selection is more complex because it is usually desirable to avoid adjustments within the moving groups. In this paper, we examine the effects of tolerances and compensators on a photographic-format zoom lens. We begin by assigning reasonable tolerances to all surfaces, materials, and groups, and then examine in detail how these tolerances affect the image quality. We determine the relative amount of degradation caused by transverse tolerances (decenters and tilts) compared to rotationally symmetric tolerances (power, index, thicknesses and spacings). For the rotationally symmetric tolerances, we examine the efficacy of shifting the detector, shifting the fixed groups, and respacing elements within the fixed groups. Similarly, for the transverse tolerances, we examine the efficacy of implementing decenter compensators within the fixed groups.