04 DQ 03 Positive Achromatic Doublets

These doublets easily outperform equivalent singlets even in monochromatic light, being corrected for spherical aberration and coma, as well as for chromatic aberration. Their performance and uses depend on the relative aperture. All but the largest apertures are essentially diffraction-limited on axis and give excellent imaging over a small field (say 5°) as required for microscopes, telescopes, etc. The performance of the largest-aperture lenses (listed as not diffraction limited) is inevitably affected by higher-order aberrations, and they are typically used for laser or fibre collimation or focusing onto detectors, etc. For better imaging at these apertures in monochromatic light see doublet/meniscus combinations; for white light, seemicroscope objectives. All positive doublets are designed for an infinite conjugate (parallel light) on the more steeply curved side.