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Rise (and Fall) of The Machines: Understanding t.5/t.1 Times

We are gonna get our Lighting Geek on today, and take a moment to understand two measurements which are very important to know if you are shopping for flashes: t.5 and t.1.

When measuring the length of a flash pulse, the duo of t.5 and t.1 times are the industry standard metrics. Understanding those numbers -- and the difference between them -- can help you make better purchasing decisions on your lighting gear.

What are t.5 and t.1 Times?

The first thing you should know is that the numbers are not the same thing and are not interchangeable. If one manufacturer is giving you a t.5 number and the other a t.1, you should not compare them as equals.

At the t.5 time, the flash pulse has dropped down to 50% of its peak. At t.1, the flash has dropped 90% from its peak. Essentially, they are giving you a feel for the shape of the back slope of the pulse.

So of course, flash manufacturers would prefer you think in terms of the t.5 measurement. But the t.1 measurement is much more of a "all-in" number, even though it is not perfect either.

Take this graphical representation of a typical flash pulse, which is from a good explanation of t.5 and t.1 times on Paul Buff's website.

In it, he also gets into the differences between regular flashes and IGBT-controlled flashes, the latter being the special sauce in an Einstein unit. (IGBT circuits just chop off the pulse on the back side when the power has reached its required level, which obviously makes for very flattering t.5 and t.1 times.)

The graph shown here is that of a variable voltage controlled flash, at full power. The vertical axis represents energy intensity and the horizontal axis represents time.

So, what does this graph tell us?

First, you can see that the flash pulse rises very quickly and it decays more slowly, over time. This is typical.

Second, the t.5 (1/2000 sec) time is way less than half of the amount of time needed for t.1 (1/666 sec). This is because of the slower decay on the back side of the pulse.

Why not just list the time it takes for an entire pulse? Look at that curve as it heads to 100% discharge, which is what you'd need. It's getting pretty flat, right? That's a lot of time needed to get to 100%. Pretty much nobody would look good if they gave you that measurement.

And frankly, that last bit of energy does not make a lot of difference in your exposure, either. PW even cheats the edges of the pulse a little bit to give you higher sync times with the new TT5/TT1 units. Lots of time savings there with very little energy loss.

Okay, back to t.5/t.1 times.

Obviously, a t.5 time is going to be way more flattering. First, because your flash pulse is not even near finished yet, and second because the graph of that energy shows that it attacks faster than it decays.

And speaking of energy, the total area under the curve (seen here in yellow) represents the total energy given out by the flash up to any one point in time, marking possibly the first time basic calculus rears its head for us photogs.

Why should you care about pulse times?

Well, certainly if you are an action shooter, you'll want as short a t.1 time as possible -- at the power level you will most likely be shooting at. Short pulse time = action-stopping power. If you do not shoot action with flash, maybe you are more concerned about color consistency, and you can skip the pulse-length worries.

But for some really slow flashes, that t.1 time may even put a roof on your ability to sync at your normal sync speed, let alone stop action. If only 75% of that full-power pulse is being delivered in a 250th of a sec, your camera really cannot sync that particular flash at a 250th, now can it?

That can be a deal breaker. Which is why you should study those numbers and understand what you are getting, depending on how you will use it.

And fortunately, the numbers usually get better the further you walk down the power scale. So this is more of a problem for you watt-second hogs who like to hang out at full output while shooting action photos.

Compromises, Compromises

Here is your formula:

1. Fast Pulse
2. Color Correctness / Consistency
3. Reasonable Price

Pick any two.

Which is to say that you can have color consistency and fast pulses at the same time, but just be prepared to dig deep into your wallet.

Fortunately, speedlights do not really bump up against these physical restrictions. At full power, most ~60ws speedlights will deliver a color consistent full pop in about t.5=1/1000th and t.1=1/500th of a sec. And it gets better from there very quickly as you drop the power level. So the above compromises are mostly for the people considering big lights.

And that is one of the cool things about the Einsteins -- that you can have low prices and fast pops or low prices and color consistency, depending on the mode you select. Just not all three at once.

A pretty good compromise, if you are a schizophrenic photog who goes for fast action on some days and tight color tolerances on others.

But no matter what your needs, make sure you understand the t.1/t.5 numbers that are being thrown around, and how they relate to you. And don't be snowed by a deceptively pretty t.5, either -- it is the t.1 that counts.

Read more:

:: SportsShooter on Flash Duration ::
:: Paul C. Buff on Flash Duration ::


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