Under clear skies, strong fluctuations in the downwelling irradiance, Ed, prevail in shallow water as a result of the focusing and defocusing of sunlight by surface waves. Such temporal fluctuations were measured in the Black Sea, usually at a depth of 1 m, from a fixed platform located 600 m off the coastline. A method of thresholding analysis was applied to 109 time-series records of Ed (525 nm), each of which lasted 10 min. The frequency of occurrence of flashes of intense foci (intensity exceeding the time-averaged irradiance, Ed, by > 50%) decreased exponentially with increasing flash intensity. The frequency and intensity of flashes, hence the slope of the exponential relationship, all varied with wind-wave conditions and atmospheric lighting conditions. The best conditions for wave focusing were characterized by light winds of 2 to 5 m s-1, solar elevation > 40 degree(s), and diffuseness of surface irradiance < 40%. Then, at a depth of 1 m, the flashes > 1.5 Ed occurred at rates as high as 6 Hz. The most intense flashes exceeded Ed 5-fold at rates of 10-3 Hz. These results, which are consistent with our previous observations, substantially improve the database on still poorly documented wave focusing effects.