T.1 vs T.5 Flash Duration: What the Specs Actually Mean

T.5 and T.1 are two ways of measuring how long a flash fires. T.5 clocks the time until output drops to 50% of peak; T.1 measures all the way to 10%. Most manufacturers publish the T.5 figure, which is the less useful of the two if you need to freeze motion. This SLR Lounge piece explains the difference and why tracking down T.1 specs takes more digging than it should.