Flash Duration – How Fast Can We Go

Flash duration – how long the burst of photons from flash actually lasts, does seem to get a lot of people confused.

Earlier this year I posted an article on using flash HERE where the prime function of the flash was as a fill light. As a fill, flash should not be obvious in the images, as the main lighting is still the ambient light from the sun, and we’re just using the flash to “tickle” the foreground with a little extra light.

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography,Red Squirrel

Flash as “fill” where the main lighting is still ambient daylight, and a moderate shutter speed is all that’s required. 1/800th sec @ f8 is plenty good enough for this shot.

Taking pictures is NEVER a case of just “rocking up”, seeing a shot and pressing the shutter; for me it’s a far more complex process whereby there’s a possible bucket-load of decisions to be made in between the “seeing the shot” bit and the “pressing the shutter” bit.

My biggest influencers are always the same – shutter speed and aperture, and the driving force behind these two things is light, and a possible lack thereof.

Once I make the decision to “add light” I then have to decide what role that additional light is going to take – fill, or primary source.

Obviously, in the shot above the decision was fill, and everything was pretty straight forward from there on, and aperture/shutter speed  selection is still dictated by the ambient lighting – I use the flash as a “light modifier”.

The duration of the flash is controlled by the TTL metering system and it’s duration is fairly irrelevant.

Let’s take a look at a different scenario.

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

The lovely Jo doing her 1930’s screen icon “pouty thing”. Flash is the ONLY light source in this image. 1/250th @ f9 ISO 100.

In this shot the lighting source is pure flash.  There’s very little in the way of ambient light present in this dark set, and what bit there is was completely over-powered by the flash output – so the lighting from the Elinchrom BX 500 monoblocks being used here is THE SOLE light source.

Considerations over the lighting itself are not the purpose of this post – what we are concerned with here are the implications for shutter speed due to flash synchronization.

The flash units were the standard type of studio flash unit offering no TTL interface with the camera being used, so it’s manual everything!

But the exposure in terms of shutter speed is capped at 1/250th of a second due to the CAMERA – that is it’s highest synch speed.

The focal length of the lens is 50mm so I need to shoot at around f8 or f9 to obtain workable depth of field, so basic exposure settings are dictated.  This particular shot was achieved by balancing the light-to-subject distance along the lines of the inverse square law for each light.

But from the point of view of this post the big consideration is this – can I afford to have movement in the subject?

At 1/250th sec you’d think not.  Then you’d think “hang on, flash durations are a lot faster than that” – so perhaps I can…..or can I ?

Flash Duration & Subject Movement

Flash duration, in terms of action-stopping power, is not as simple or straight forward as you might think.

Consider the diagram below:

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

Flash Power Output curve plotted against Output duration (time).

The grey shaded area in the diagram is the “power output curve’ of the flash.

Most folk think that a flash is an “instant on, instant off” kind of thing – how VERY wrong they are!

When we set the power output on either the back panel of our SB800/580EX etc, or on the power pack of a studio flash unit, or indeed any other flash unit, we are setting a peak output limit.

We might set a Nikon SB800 to 1/4 power, or we might set channel B output on a Quadra Ranger to 132Watt/sec, but either way, we are dictating the maximum flash output power – the peak output limit. The “t 5 time” – or to be more correct the “t 0.5 time” is the total time duration where the flash output is at 50% or above of the selected peak output limit we set.

Just to clarify: we set say, 1/4th power output on the back of a Canon 580EX – this is the selected peak output limit. The t5 time for this is the total time duration where the light output is at or above 50% of that selected 1/4th power – NOT 50% of the flash units full power output – do not get confused over this!

So when it comes to total “light emission duration” we’ve got 3 different ways of looking at things:

  1. Total – and I mean TOTAL – duration; the full span of the output curve.
  2. T 0.5 – the duration of the flash where its output is at 50% or above that level set by the user – the peak output limit.
  3. T 0.1 – the duration of the flash where its output is at 10% or above that level set by the user.

Anyone looking at the diagram above can see that the total output emission time/flash duration is A LOT LONGER than the t5 time.  Usually you find that t5 times are somewhere around 1/3rd of the total emission time, or flash duration.

Getting back to our shot of Jo above, if my memory serves me correctly the BX heads I used for the shot had a t5 time of around 1/1500th sec.  So the TOTAL duration of the flash output would be around 1/500th sec.

So I can’t afford to have any movement in the subject that isn’t going to be arrested by 1/500th sec flash duration, let alone the 1/250th shutter speed.

Why? Well that 1/250th sec the shutter is open will comprise of 1/500th sec of flash photons entering the lens, and 1/500th sec of NOTHING entering the lens but AMBIENT LIGHT photons.

Let us break flash output down a bit more:

In the previous article I mentioned, I quoted a table of Nikon SB800 duration times.  At the top of the table was the SB800 1/1 or full output power flash duration.  All times quoted in that table were t5 times.

The one I want to concentrate on is that 1/1 full power t5 time of 1/1050th sec.

Even though Nikon try to tempt you into believing that the flash only emits light for 1/1050th sec it does in fact light the scene for a full 1/350th sec – most flash manufacturers units are quoted as t5 times.

Now in most cases when you might employ flash – which let’s face it, is as some sort of fill light in a general ambient/flash mixed exposure, this isn’t in reality, a big problem.  Reduced power multiple pulse AutoFP/HSS also makes it not a problem.

But if you are trying to stop high speed action – in other words “freeze time”, then it can become a major headache; especially when you need all the flash power you can get hold of.

Why? Let’s break the diagram above down to basics.

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

The darker shaded area represents the “tail” of the flash output – the area that can cause many problems when trying to stop high speed action.

  • The first 50% of the total light output is over and finished in the first 1/1050th of the total flash duration.
  • The other 50% of the total light output takes place over a further 1/525th sec, and is represented by the dark grey area – let’s call this area the flash “output tail”.  Some publications & websites refer to this tail as after-glow.  I always thought that ‘after glow” was something ladies did after a certain type of energetic activity!
  • The light will continue to decay for a full 1/525th sec after t5, until the output of light has died down to 0% and the full “burn time” of 1/350th sec has been reached.

That’s right – 1/1050th + 1/525th = 1/350th.

So, if our shutter speed is 1/350th sec or longer we are going to see some ghosting in our image caused by the movement of the subject during that extra 1/525th sec post t5 time.

I need to point out that most speedlight type flash units are “isolated-gate bipolar transistor” devices – that’s IGBT to you and me. Einstein studio flash units are also IGBT units – I’ll cover the implications of this in a later post, but for now you just need to know that the IGBT circuitry works to eliminate sub t5 output BUT doesn’t work if your speedlight is set to output at maximum power.  And if you need access to full 1/1 power with your speedlights for any reason then IGBT won’t help you.

Let’s see the problem in action as it were:

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

A bouncing golf ball shot at 1/250th sec using full power output on an SB800.
The ball is moving UPWARDS.
The blur between points A & B are caused by the “tail” or “after-glow” of the flash.

And the problem will be further exacerbated if there is ANY ambient light in scene from a window for instance, as this will boost the general scene illumination during that “tail end” 1/525th sec.

We might be well advised, if using any form of non-TTL flash mode, to use a shutter speed equal to, or shorter in duration to the t5 time, as in the shot below:

flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

A bouncing golf ball shot at 1/2000th sec using full power output on an SB800.

All I’ve done in this second shot is go -3Ev on the shutter speed, +1Ev on the aperture and +2Ev on ISO speed.

Don’t forget, the flash is in MANUAL mode with a full power output.

With the D4 in front-curtain synch the full power, 1/350th sec flash pulse begins as the front shutter curtain starts to move, and it “burns” continuously while the 1/2000th sec “letter-box” shutter-slot travels across the sensor.

In both shots you may be wondering how I triggered the exposure. Sitting on the desk you can see a small black box with a jack plug sticking out the back – this is the audio sensor of a TriggerSmart audio/light/Infra Red combined trigger system.  As the golf ball strikes the desk the audio sensor picks up the noise and the control box triggers the camera shutter and hence the flash.

Hardy, down at the distributors,Flaghead, has been kind enough to send me one of these systems for incorporation into some long-term photography projects, and in a series of high speed flash workshops and training tutorials.  And I have to say that I’m mighty impressed with the system, and at the retail pricing point ownership of this product is a no-brainer.  The unit is going to feature in quite a few blog post in the near-future, but click HERE to email Hardy for more details.

Even though I constantly extol the virtues of the Nikon CLS system, there comes a time when its automatic calculations fight AGAINST you – and easy high speed photography becomes something of a chore.

Any form of flash exposure automation makes assumptions about what you are trying to do.  In certain circumstances these assumptions are pretty much correct.  But in others they can be so far wide of the mark that if you don’t turn the automation OFF you’ll never get the shot you want.

Wresting full control over speed lights from the likes of Nikons CLS gives you access to super-highspeed flash durations AND high shutter speeds without a lot of the synching problems incurred with studio monoblocks.

Liquid in Motion,flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

Liquid in Motion – arrested at 1/8000th sec shutter speed using SB800’s at full 1/1 power.

Liquid in Motion,flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

Liquid in Motion – arrested at 1/8000th sec shutter speed using SB800’s at full 1/1 power. A 100% crop from the shot above.

Liquid in Motion,flash duration,fill flash,flash,shutter speed,photography,Andy Astbury,digital photography,wildlife photography

“Scotch & Rocks All Over The Place”
Simple capture with manual speed lights at full power and 1/8000th shutter speed.

The shots above are all taken with 2x SB800s lighting the white background and 1 heavily defused SB800 acting as a top light.

One background light is set at 1/1 manual FP, the other to manual 1/1 SU-4 remote.  The top light is set to 1/8 power SU-4 remote.

The majority light in the shot is in fact that white background – it’s punching light back through the glass and liquid splash – the subject is backlit.

So, that background is being lit for a full 1/350th of a second.

But shooting in front curtain synch I’m using 1/8000th sec as a shutter speed, an exposure duration 3 stops shorter than the flash unit t5 time for full power. So in effect I’m using the combined background flash units as a very short-term continuous light source which lasts for 1/350th of a second, but the camera is only recording the very first 1/8000th sec – in other words, photons are still leaving the flash AFTER the rear shutter curtain has closed and the exposure is finished.

Finally, the shutter and flash are triggered by dropping the faux crushed ice through the IR sensor beam of the TriggerSmart unit.

This is very much along the lines of what’s termed HYPERSYNCH – a technique you can use with conventional slow burn studio flash units and certain types of 3rd party trigger units such as Pocket Wizards – but that’s yet another story, and is fraught with synch problems that you have program out of the system using the Pocket Wizard utility.

So, there’s more to come from me about flash in future posts, but for now just remember – there’s not a lot you can’t do with speed lights – as long as you’ve got enough of the little darlings!

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Flash Photography

Flash Photography

 

Red Squirrel,Andy Astbury,Flash,flash photography,fill flash,photography techniques

Really Cute Red Squirrel

 

On Sunday myself and my buddy Mark Davies made a short foray up to the Lake District and our small Red Squirrel site.  The weather was horrible, sleet, sun. rain, cloudy, sunny then rain again – in other words just not conducive to a half-descent session on the D4.

The one Achilles Heal with this site is the fact that it’s hard to get a descent background for your shots – it’s in the middle of a small wooded valley and you just can’t get away from tree trunks in the background.

This is further complicated by the fact that the “Squidgers” have a propensity for keeping in the ‘not so sunny’ bits, so frequently you end up with a scenario where backgrounds are brighter than foregrounds – which just won’t DO!

So what’s needed is some way to switch the lighting balance around to give a brighter foreground/subject AND a darker background.

Now that sounds all very well BUT; how do we achieve it?

Reflectors perhaps?  They’d do the trick but have one big problem; they rely on AMBIENT light  – and in the conditions we were shooting in the other day the value of the ambient light was up and down like a Yo-Yo.

Wouldn’t it be cool if we could have a consistent level of subject/foreground illumination AND at the same time have some degree of control over the exposure of the background?

Well with flash we can do just that!

Let’s look at a shot without flash:

 

No FLASH

No FLASH, AMBIENT light only – 1/320th @ f7.1

 

I don’t suppose this shot is too bad because the background isn’t strongly lit by the sun (it’s gone behind a cloud again!) but the foreground and background are pretty much the same exposure-wise.  For me there is not enough tonal separation between the two areas of the image, and the lighting is a bit flat.

If we could knock a stop or so out of the background; under expose it, then the image would have more tonal separation between foreground and background, and would look a lot better, but of course if we’re just working with ambient light then our adjusted exposure would under expose the foreground as well, so we’d be no better off.

Now look at the next image – we’ve got a background that’s under exposed by around  -1.5Ev, but the subject and foreground are lit pretty much to the same degree as before, and we’ve got a little more shape and form to the squirrel itself – it’s not quite so flat-looking.

 

With FLASH

With FLASH added – 1/800th @ f7.1

 

The image also has the slight sense that it’s been shot in more sunny conditions – which I can promise you it wasn’t !

And both images are basically straight off the camera, just with my neutral camera profile applied to them on import.

 

The Set Up

The Setup - shocking iPhone 3 quality!

The Setup – shocking iPhone 3 quality!

 

The first secret to good looking flash photography OF ANY KIND is to get the damn flash OFF the camera.

If we were in a totally dark studio with the sexiest looking model on the planet we’d NOT be lighting her with one light from the camera position now would we?

So we use basic studio lighting layouts where ever we can.

There are two other things to consider too:

  •   It’s broad daylight, so our exposure will contain both FLASH and an element of AMBIENT light – so we are working along the premise of ADDING to what’s already there.
  •   If we put the flash closer to the subject (off camera) then the output energy has less distance to travel in order to do its job – so it doesn’t have to have as much power behind it as it would have if emanating from the camera position.

 

You can see in the horrible iPhone 3 shot I took of the setup that I’m using two flash guns with white Lambency diffusers on them; one on a stand to the left and slightly in front of the log where the squirrels will sit, and one placed on the set base (Mr. Davies old knackered Black & Decker Workmate!) slightly behind the log and about the same distance away from where I anticipate a squirrel will sit on the log as the left flash.

The thing to note here is that I’m using the SIDE output of these Lambency diffuser domes and NOT the front – that’s why they are pointed up at the sky. The side output of these diffusers is very soft – just what the flash photography doctor ordered in terms of ‘keeping it real’.

The left light is going to be my MAIN light, the right is my FILL light.

The sun, when & if it decides to pop its head out, will be behind me and to my left so I place my MAIN light in a position where it will ‘simulate’ said ball in the sky.

The FILL light basically exists to ‘counter balance’ the ‘directionality’ of the MAIN light, and to weaken any shadows thrown by the MAIN light.

Does this flash bother a subject? For the most part NOT SO YOU’D NOTICE!

Take a look at the shot below – the caption will be relevant shortly.

This SB800 has just fired in "front curtain synch" and the balance of the exposure is from the ambient light - the shutter is still open after the flash has died. Does the squirrel look bothered?

This SB800 has just fired in “front curtain synch” and the balance of the exposure is from the ambient light. Does the squirrel look bothered?

Settings & The Black Art!

Before we talk about anything else I need to address the shutter curtain synch question.

We have two curtain synch options, FRONT & REAR.

Front Curtain (as in the shot above) – this means that the flash will fire as the front curtain starts to move, and most likely, the flash will be finished long before the rear curtain closes. If your subject reacts to the flash then some element of subject movement might be present in the shot due to the ambient light part of the exposure.

Rear Curtain Synch – my recommended ‘modus operandi’ – the ‘ambient only’ part of the exposure gets done first, then the flash fires as the rear curtain begins to close the exposure. This way, if the subject reacts to the flash the exposure will be over before it has chance to – MOSTLY!

The framing I want, and the depth of field I want dictates my camera position and aperture – in this case f7 or f8 – actually f7.1 is what I went for.

 

I elect to go with 2000 iso on the D4.

So now my only variable is shutter speed.

Ambient light dictates that to be 1/320th on average, and I want to UNDER EXPOSE that background by at least a stop and a bit (technical terms indeed!) so I elect to use a shutter speed of 1/800th.

So that’s it – I’m done; seeing as the light from the flashes will be constant my foreground/subject will ALWAYS be exposed correctly. In rear curtain synch I’ll negate the risk of subject movement ‘ghosting’ in the image, and at 1/800th I’ll have a far better chance of eliminating motion blur caused by a squirrel chewing food or twitching its whiskers etc.

 

Triggering Off-Camera Flashes

 

We can fire off-camera flashes in a number of ways, but distance, wet ground, occasional rain and squirrels with a propensity for chewing everything they see means CORDS ain’t one of ’em!

With the Nikon system that I obviously use we could employ another flash on-camera in MASTER/COMMANDER mode, with the flash pulse deactivated; or a dedicated commander such as the SU800; or if your camera has one, the built-in flash if it has a commander mode in the menu.

The one problem with Nikon CLS triggering system, and Canons as far as I know, is the reliance upon infra-red as the communication band. This is prone to a degree of unreliability in what we might term ‘dodgy’ conditions outdoors.

I use a Pocket Wizard MiniTT1 atop the camera and a FlexTT5 under my main light. The beauty of this system is that the comms is RADIO – far more reliable outdoors than IR.

Because a. I’m poor and can’t afford another TT5, and b. the proximity of my MAIN and FILL light, I put the SB800 FILL light in SU mode so it gets triggered by the flash from the MAIN light.

What I wouldn’t give for a dozen Nikon SB901’s and 12 TT5s – I’d kill for them!

The MAIN light itself is in TTL FP mode.

The beauty of this setup is that the MAIN light ‘thinks’ the TT5 is a camera, and the camera ‘thinks’ the miniTTL is a flash gun, so I have direct communication between camera and flash of iso and aperture information.

Also, I can turn the flash output down by up to -3Ev using the flash exposure compensation button without it having an effect on the background ambient exposure.

Don’t forget, seeing as my exposure is always going to 1/800th @ f7.1 at 2000 iso the CAMERA is in MANUAL exposure mode. So as long as the two flashes output enough light to expose the subject correctly at those settings (which they always will until the batteries die!) I basically can’t go wrong.

When shooting like this I also have a major leaning towards shooting in single servo – one shot at a time with just one AF point active.

 

Flash Photography – Flash Duration or Burn Time

Now here’s what you need to get your head around. As you vary the output of a flash like the SB800 the DURATION of the flash or BURN TIME of the tube changes

Below are the quoted figures for the Nikon SB800, burn time/output:

1/1050 sec. at M1/1 (full) output
1/1100 sec. at M1/2 output
1/2700 sec. at M1/4 output
1/5900 sec. at M1/8 output
1/10900 sec. at M1/16 output
1/17800 sec. at M1/32 output
1/32300 sec. at M1/64 output
1/41600 sec. at M1/128 output

On top of that there’s something else we need to take into account – and this goes for Canon shooters too; though Canon terminology is different.

Shutter Speed & The FP Option

35mm format cameras all have a falling curtain shutter with two curtains, a front one, and a rear one.

As your press the shutter button the FRONT curtain starts to fall, then the rear curtain starts to chase after it, the two meet at the bottom of the shutter plane and the exposure is over.

The LONGER or slower the shutter speed the greater head-start the front curtain has!

At speeds of 1/250th and slower the front curtain has reached the end of its travel BEFORE the rear curtain wakes up and decides to move – in other words THE SENSOR is FULLY exposed.

The fastest shutter speed that results in a FULLY EXPOSED film plane/sensor is the basic camera-to-flash synch speed; X synch as it used to be called, and when I started learning about photography this was usually 1/60th; and on some really crap cameras it was 1/30th!

But with modern technology and light weight materials these curtains can now get moving a lot faster, so basic synch now runs at 1/250th for a full frame DSLR.

If you go into your flash camera menu you’ll find an AUTO FP setting for Nikon, Canon refer to this as HSS or High Speed Synch – which makes far more sense (Nikon please take note, Canon got something right so please replicate!).

There’s something of an argument as to whether FP stands for Focal Plane or Flash Pulse; and frankly both are applicable, but it means the same as Canon’s HSS or High Speed Synch.

At speeds above/faster than 1/250th the sensor/film plane is NOT fully exposed. The gap between the front and rear curtains forms a slot or ‘letter box’ that travels downwards across the face of the sensor, so the image is, if you like, ‘scanned’ onto the imaging plane.

Obviously this is going to cause on heck of an exposure problem if the flash output is ‘dumped’ as a single pulse.

So FP/HSS mode physically pulses or strobes the flash output to the point where it behaves like a continuous light source.

If the flash was to fire with a single pulse then the ‘letterbox slot’ would receive the flash exposure, but you’d end up with bands of under exposure at the bottom or top of the image depending on the curtain synch mode – front or rear.

In FP/HSS mode the power output of each individual pulse in the sequence will drop as the shutter speed shortens, so even though you might have 1:1 power selected on the back of the flash itself (which I usually do on the MAIN light, and 1/2 on the FILL light) the pulses of light will be of lower power, but their cumulative effect gives the desired result.

By reviewing the shot on the back of the camera we can compensate for changes in ambient in the entire scene (we might want to dilute the effect of the main light somewhat if the sun suddenly breaks out on the subject as well as the background) by raising the shutter speed a little – or we might want to lighten the shot globally by lowering the shutter speed if it suddenly goes very gloomy.

We might want to change the balance between ambient and flash; this again can be done from the camera with the flash exposure compensation controls; or if needs be, by physically getting up and moving the flash units are little nearer or further away from the subject.

All in all, using flash is really easy, and always has been.

Except nowadays manufacturers tend to put far more controls and modes on things then are really necessary; the upshot of which is to frighten the uninitiated and then confuse them even further with instruction manuals that appear to be written by someone under the influence of Class A drugs!

 

"Trouble Brewing.." Confrontation over the right to feed between two Red Squirrels.

“Trouble Brewing..” Confrontation over the right to feed between two Red Squirrels.

 

The whole idea of flash is that it should do its job but leave no obvious trace to the viewer.

But its benefits to you as the photographer are invaluable – higher shutter speeds, more depth of field and better isolation of the subject from its background are the three main ones that you need to be taking advantage of right now.

If you have the gear and don’t understand how to use it then why not book a tuition day with me – then perhaps I could afford some more TT5s!

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