We investigate strong gravitational lensing by a marginally unstable photon sphere in a Reissner-Nordström naked singularity spacetime. Using the Picard-Fuchs equation, we derive full-order power series expressions for the deflection angle in various regimes, including the strong deflection limits from both outside and inside the photon sphere. We show that the deflection angle diverges nonlogarithmically in both cases, refining existing asymptotic formulas. Comparing truncated approximations with numerical results, we find that higher-order corrections are essential to achieve comparable accuracy to logarithmic divergence cases. Using these improved formulas, we also derive precise approximations for image positions that are not restricted to the almost perfectly aligned cases.

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• Classical black holes
• Gravitational lenses