Diffractive Apochromatic Double-Gauss Lens
Large-aperture double-Gauss lenses normally employ high-index lanthanum crown glasses in the positive elements in order to minimize field curvature and spherical aberration. However, the partial dispersion characteristics of these glasses is similar to the short flint types (KZF, KZFS, etc.), and thus departs from the "normal" glass line in a direction opposite to that required for secondary color correction. The resulting exaggerated secondary spectrum limits the axial performance in otherwise well-corrected large aperture double-Gauss photographic lenses.
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