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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Blogger Brian Lance said...

Thanks for posting this, David! I've been looking at strobes and these numbers were somewhat confusing.

June 22, 2010 12:51 AM  
Blogger Hipporage said...

I started to doze but yiur summary kept things in order. Great post!

June 22, 2010 1:40 AM  
Blogger Chris said...

This is really quite an eye opener to me. This really seems like good information when I get into shopping for bigger lights. I really appreciate it. I recently got into the off camera lighting scene just because of this blog (Thanks David.)(Only got a Lumopro lp120 though at the moment) Following the guidelines in this blog has made the learned much less steep (as sometimes it seems kind of like that graph up there.) as a side note you misspelled "pulse" about halfway through the post, it is in its own line.

June 22, 2010 2:45 AM  
Blogger svnsn said...

hey david!

first off and most important: THANX A TON for all the effort, work and passion you put into your blog!!! there sure isn't anything comparable to this... *bow*

second: sorry for probably being too geeky, but this might be a little confusing: you mixed up the figures in the speedlight section:
"At full power, most ~60ws speedlights will deliver a color consistent full pop in about t.5=1/500th and t.1=1/1000th of a sec."
As i get it, the approx. figures should be t.5=1/1000th and t.1=1/500th.

as always: i'm looking forward for your next posts - keep 'em coming!! :-)
thx & cheers from switzerland - sven

June 22, 2010 2:56 AM  
Blogger Desmond said...

Hi Dave like the post, but i was still a little confused about the whole t1 & T5 thing. went to pauls site a little more confused. then I came across this link I just wanted to share kinda cleared things up


June 22, 2010 3:53 AM  
Blogger jason anderson said...

good info but you kinda lost me ,pretty much near the top. dont stop teaching us master :)

June 22, 2010 4:24 AM  
Blogger Eric Duminil said...

Hi David.

Thanks a lot for the explanation.

Not to be too picky here, but the vertical axis represents power, not "energy intensity".

The power output is expressed in W.
The total area beneath the curve is expressed in Ws, which, as you said it, represents the total energy delivered by the flash pulse.

Some calculus on my favorite blog. I'm in heaven! :D

June 22, 2010 4:44 AM  
OpenID damnuglyphotography said...

Wow Dave...you really geeked out today with this one.....brings back all those photo tech classes I took 30+ years ago! Stuff at the time I figured I would never have to know, until I started working for Sports Illustrated and had to actually FREEZE something! Way to go!


June 22, 2010 4:49 AM  
Blogger Evan said...

Thanks for clearing that up mate, now I understand it all! Now, to save up and get me some Einsteins...*says to missus* Oh but dear, it's like TWO flashes in ONE :)

June 22, 2010 5:30 AM  
Blogger erik said...

Heres some more nerd pornography for you. Its a chart of different well known flashes and their flash duration (among alot of other info). It's not my site, but it may be in your interest anyway.


June 22, 2010 5:38 AM  
Blogger theChipmunk said...

Well, that's just bizarre (and slightly scary) - I woke up this morning and, while reading at breakfast, saw t.5 and t.1 times referred to. I have a basic idea of what they are, but decided to do a bit more research...and, first site I visit, the answer is waiting.

Moral of the story? David Hobby is inside your head. In a good way.

Thanks for the article!


June 22, 2010 8:11 AM  
Blogger 60/40 said...

Very informative article. Great stuff as always.

June 22, 2010 8:31 AM  
Blogger David said...

AHHHHH... nothing like dropping in a post, going to sleep and waking up to 63 (literally) people letting you know you swapped two time values in a post.

Yes, the t.1 and t.5 times in the speedlight paragraph were reversed. Thanks for the heads-up(s). And fixed it.

I spare everyone the comment marathon. :)


June 22, 2010 9:37 AM  
Blogger Adrian Henson Photgraphy, Inc. said...

Hey, you mention the pulse times get longer as the power increases. This is true for small strobes but for studio strobes the pulse time gets shorter as the power increases. This has huge implications on deciding which lights to purchase as well as what lights can be mixed together on action work.

June 22, 2010 9:41 AM  
Blogger Larry said...

You just made my day (for a couple of reasons). I primarily shoot multiple speedlights and, until last month, never owned a studio strobe. I got on the Einstein preorder list back in January and got my light in May. The reason I liked this post so much is:

1) You said it was "advanced/geeky" and I understood it anyway

2) You had relatively nice things to say about the Einstein and I feel even better about my purchase (though I just bought one, I want a couple more, and the waiting list is months long)

David, I want to work toward occasionally doing some part-time paid work and your blog has been immensely helpful. Thanks!

June 22, 2010 10:08 AM  
Blogger Mr said...

Just out of curiosity, are there any copyright issues with using the terminator image in your blog?

June 22, 2010 1:30 PM  
Blogger David said...


1. The Transformative Factor: The Purpose and Character of Your Use

In a 1994 case, the Supreme Court emphasized this first factor as being a primary indicator of fair use. At issue is whether the material has been used to help create something new, or merely copied verbatim into another work. When taking portions of copyrighted work, ask yourself the following questions:

Has the material you have taken from the original work been transformed by adding new expression or meaning?
Was value added to the original by creating new information, new aesthetics, new insights and understandings?


June 22, 2010 1:59 PM  
Blogger John said...

Excellent stuff, who knew?! Thanks much. Went to Paul Buff site as directed to read in more detail, saw "IGBT" many times but no expansion of the acronym and finally had to go looking, so lemme save you the trouble: "Insulated-Gate Bipolar Transistor". Big letdown, thought sure it was some cool marketing name for his technology.

Enuf tomfoolery, back to work, thanks again.

June 22, 2010 2:09 PM  
Blogger David said...


I probably missed your comment, thinking it was one of the people noticing the typo in the speedlight paragraph. Sorry about that.

I reworded the first reference to frame it in terms of "percentage off of the peak," which granted is more accurate. Area (i.e., total illumination) under the curve works out to be very close, actually. And I think of it as how much of the cumulative exposure has happened by the two different times, which is easier for me to understand in terms of what flash will freeze a subject in what amount of time.

Thanks for the catch.


True. The important thing is that people look at those listed numbers and know how to interpret them into real-world shooting, IMO.

June 22, 2010 2:38 PM  
OpenID javiergarciarosell said...

David, what can I say...I´ve been looking at this data for a while and did n´t really understood it till now. Thanks once again for all.
Regards from Sevilla!

June 22, 2010 5:47 PM  
Blogger RobertKills said...

Dave, your just the man.

June 22, 2010 6:43 PM  
Blogger Greg said...

Hi David, I'm sure you get this all the time but I've just done a video on basic stuff needed to be a strobist, it's basic but I hope informative... I make sure to give you a huge plug because if it were not for you I would not be doing off camera flash the way I do!



June 23, 2010 2:58 PM  
Blogger Granjow said...


Sorry for attacking you, this was quite impolite of me. This time I will try to think before typing. (I've even read that you had to go through >60 comments.) Shame on me.

50% of the peak is fine.
Linking the area below the curve and the exposure this way is quite descriptive!

As you are speaking of action photography, there are some interesting things to mention.

The first one, which is kind of obvious, is that lowering the flash power (usually) results in a shorter t.1 time. I think this is something one should be aware of, that shooting at lower flash power freezes moves «better». (Well, at the cost of the aperture which has to be openend.) The Nikon flashes (and, likely, most other as well) also cut off the power, as visible in this measurements (not mine):

SB-800 at different power levels
(Google Translate might be your friend)
(The second image illustrates the transistor control mechanism, IGBT, which you mentioned above.)

The second interesting thing is FP-Sync or High-Speed Sync, because it works quite different; There is not really a t.1 time anymore, and the peak power is reduced.

Nikon: FP-Sync

There is a good document from canon as well, but it is … Ah, amazing. Just found it in my bookmarks.

Canon: Flash Work
Page 5.

End of my knowledge for this evening ;)
Greetings from Switzerland,

June 23, 2010 5:33 PM  
Blogger Jaromir said...

There is a couple of mistakes here, but the most important is that lowering the power level will not shorten the flash duration for the most of conventional (e.i. voltage controlled) studio strobes. Another thing is t.1 for popular speedlights - if t.5 for 1/1 pulse is 1/1000-1/800s (which is true for the most popular models) then t.1 is approx. 1/3 t.5 so action stopping ability is ~1/300-1/250s, not 1/500s as you wrote.

June 28, 2010 4:48 PM  

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