English: Lorentzian linewidth and finesse versus Airy linewidth and finesse of a Fabry-Pérot resonator. ^[1] (top) Relative Lorentzian linewidth ${\displaystyle \Delta \nu _{c}/\Delta \nu _{FSR}}$ (blue curve), relative Airy linewidth ${\displaystyle \Delta \nu _{Airy}/\Delta \nu _{FSR}}$ (green curve), and its approximation (red curve), and (bottom) Lorentzian finesse ${\displaystyle {\mathcal {F}}_{c}}$ (blue curve), Airy finesse ${\displaystyle {\mathcal {F}}_{Airy}}$ (green curve), and its approximation (red curve) as a function of reflectivity value ${\displaystyle R_{1}R_{2}}$. The exact solutions of the Airy linewidth and finesse (green lines) correctly break down at ${\displaystyle \Delta \nu _{Airy}=\Delta \nu _{FSR}}$, equivalent to ${\displaystyle {\mathcal {F}}_{Airy}=1}$, whereas their approximations (red lines) incorrectly do not break down. Insets: Region ${\displaystyle R_{1}R_{2}<0.1}$.