  This paper presents the electron and photon energy calibration achieved with
the ATLAS detector using about 25 fb$^{-1}$ of LHC proton--proton collision
data taken at centre-of-mass energies of $\sqrt{s}$ = 7 and 8 TeV. The
reconstruction of electron and photon energies is optimised using multivariate
algorithms. The response of the calorimeter layers is equalised in data and
simulation, and the longitudinal profile of the electromagnetic showers is
exploited to estimate the passive material in front of the calorimeter and
reoptimise the detector simulation. After all corrections, the $Z$ resonance is
used to set the absolute energy scale. For electrons from $Z$ decays, the
achieved calibration is typically accurate to 0.05% in most of the detector
acceptance, rising to 0.2% in regions with large amounts of passive material.
The remaining inaccuracy is less than 0.2-1% for electrons with a transverse
energy of 10 GeV, and is on average 0.3% for photons. The detector resolution
is determined with a relative inaccuracy of less than 10% for electrons and
photons up to 60 GeV transverse energy, rising to 40% for transverse energies
above 500 GeV.
