Chromatic Aberration

Focal length (green, 550nm): 150 px Lens aperture: 80 px Abbe number V: 35

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f red (656nm)

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f green (550nm)

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f blue (486nm)

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Axial CA (px)

V = (n_d − 1) / (n_F − n_C)

Δf ≈ f / V

f_λ = f_d · [1 − (n_d − 1)⁻¹·Δn]

Crown glass: V ≈ 60−70
Flint glass: V ≈ 25−40
Achromat doublet cancels CA

■ C: 656 nm (red Hα)
■ D: 589 nm (Na yellow)
■ e: 546 nm (Hg green)
■ F: 486 nm (blue Hβ)
■ g: 436 nm (violet)