T.1 and T.5 surely pertain to Terminator movies, no? I must confess that I’m not terribly well versed in the current state of action movies, but I do know about photography stuff. It turns out that T.1 and T.5 are designations for the time it takes a flash to output a burst of light. The duration of a flash, as you know, is really short. But not all flashes are equal. In fact, some flashes are less powerful than others even though they can deliver the same amount of total illumination. They do this by lasting longer—say a 500th of a second instead of a 1000th of a second. The strobe pulse duration may not mean much in many situations, but sometimes it can make the difference between a sharp shot and a blurry one.
For example, if you’re photographing a dancer in motion and you’re relying on a flash to freeze her in midair, you’d be much better served by a shorter flash duration—say 1/2000th of a second—than you would a flash that lumbers along taking all of 1/200th of a second. There’d be a big difference in motion blur there, just as there would be with the same sort of shutter speed changes. (In fact, I always suggest to folks who are looking to stop motion with their flash to set the unit for its lowest power output and make that exposure work in camera—because the lower power translates into a shorter flash duration that’s much better at stopping fast action.) Now back to those Terminator numbers.
Manufacturers publish T.1 and T.5 numbers that can be used to compare different flashes. The T.1 number represents the time it takes for a flash to output almost all (90%) of a strobe pulse, whereas the T.5 number is in indication of the time it takes to output 50% of a pulse. Generally faster is better, but the most important thing when you’re comparison shopping is to compare apples to apples—if you’re looking at a T.1 number for one unit, be sure to compare the T.1 (as opposed to the T.5) number of another. Think of the T.1 as the time it takes to pump out the entire flash, and you’re closer to understanding the overall speed a flash is capable of delivering.
All of this photo-geekery comes to mind courtesy of the Strobist blog, which is the place to geek out on obscure flash technical information like this. I recommend heading over there asap to read the recent post all about T.1 and T.5 numbers to get a better understanding of how they work and how to put that knowledge to use.