Extinction per Hydrogen Atom and the Visual Opacity of Dust: What Growth and Composition Do to a Canonical Number

The ratio of visual extinction to hydrogen column is among the most used numbers in interstellar astronomy, and the opacity it implies for dust is less often stated. This paper converts the canonical ratio into a visual mass absorption coefficient across factors representing grain growth and a range of dust-to-gas ratios. At the canonical ratio and a dust-to-gas ratio of 0.01, the visual opacity is 21442.8 square centimetres per gram; combining doubled extinction per hydrogen with a halved dust-to-gas ratio raises it to 85771.2, while halved extinction per hydrogen with a dust-to-gas ratio of 0.015 lowers it to 7147.6, a spread whose ratio is 12.0. Extinction-based and emission-based dust masses agree only when the same opacity family is used in both, and much of the apparent tension between them in the literature is a mismatch of adopted opacities rather than of measurements. All values are computed, not measured.