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Near-field (Fresnel) length

λ = 0.3080 mm

N=D24λ=D2f4cN=\frac{D^2}{4\lambda}=\frac{D^2 f}{4c}

N = D² / (4 λ) = D² f / (4 c). The last axial maximum of an unfocused circular piston.

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