The accuracy of ground-based astronomical photometry is limited by two factors: photon statistics and stellar scintillation arising when starlight passes through the Earth's atmosphere. This paper examines the theoretical role of the outer scale LO of the optical turbulence (OT), which suppresses the low-frequency component of the scintillation. It is shown that for typical values of LO ∼ 25–50 m, this effect becomes noticeable for telescopes of diameter about 4 m. On extremely large, 30–40 m, telescopes with exposures longer than a few seconds, the inclusion of the outer scale in the calculation reduces the scintillation power by more than a factor of 10 relative to conventional estimates. The details of this phenomenon are discussed for various models of non-Kolmogorov turbulence. In addition, a quantitative description of the influence of the telescope central obscuration on the measured scintillation noise is introduced and combined with the effect of the outer scale. Evaluation of the scintillation noise on future TMT and E-ELT telescopes predicts an amplitude of approximately 10 μmag for a 60-s exposure.