When a flash system fires, although it may appear so to humans, the light emitted does not come on and go off instantaneously. In reality, the flash tube ‘turns on’ very quickly and then dims gradually as the flash capacitors discharge. Short flash durations are important to prevent blurring when the subject is in rapid motion such as a pirouetting ballerina or a Michael Jordan slam-dunk. Flash manufacturers do not rate flash duration as the entire time the flash tube is emitting light however, because the end of the slowly dimming ‘tail’ does not contribute much to the overall exposure (or to blur). t.1 is defined as the time during which the flash lamp output is above 0.1 (or 1/10) it’s peak intensity. t.3 you may guess is the time the output is greater than 0.3 (about 1/3) peak, and t.5 is how long its above 0.5 (half) the peak value.