  The 0th order of affine perturbation series of the deflection angle of a ray
near a photon sphere is more accurate than a deflection angle in a strong
deflection limit, which is used often, because the later has hidden error
terms. We investigate gravitational lensing by using 0th order affine
perturbation series of the deflection angle in a general asymptotically-flat,
static, and spherical symmetric spacetime with the photon sphere. We apply our
formula to Schwarzschild black hole, Reissner-Nordstr\"om black hole, and
Ellis-Bronnikov wormhole spacetimes as examples. By comparing observables by
using the deflection angles, we show that we can ignore the effect of the
hidden error terms in the deflection angle in the strong deflection limit on
the observables in a usual lens configuration with the photon sphere since the
hidden error terms are tiny. On the other hand, in a retro lensing
configuration, the deflection angle in the strong-deflection-limit analysis
have error of several percent and the 0th order of affine perturbation series
of the deflection angle has almost half of the error. Thus, in the retro
lensing configuration, we should use the 0th order of affine perturbation
series of the deflection angle rather than the deflection angle in the
strong-deflection-limit analysis. The 0th order of affine perturbation series
of the deflection angle can give a brighter magnification by a dozen percent
than the one by using the deflection angle in the strong-deflection-limit
analysis.
