Stellar intensity scintillation on short and very short time scales (≃ 100 ms - 100 ns) was studied using an optical telescope on La Palma (Canary Islands). Photon counting detectors and real-time signal processing equipment were used to study atmospheric scintillation as function of telescope aperture size, degree of apodization, for single and double apertures, in different optical colors, at different zenith distances, times of night, and seasons of year. The statistics of temporal intensity variations can be adequately described by log-normal distributions, varying with time. The scintillation timescale (≃10 ms) decreases for smaller telescope apertures until ≃5 cm, where the atmospheric ‘shadow bands’ apparently are resolved. Some astrophysical sources may undergo very rapid intrinsic fluctuations. To detect such phenomena through the turbulent atmosphere requires optimized observing strategies.