Interesting Video

[gman]

I never hear much talk of flash duration and this video really made sense of it among other things.

Flash sync and duration

 

[Garry Edwards]

That's a good explanation, but in fact, with the vast majority of studio flash heads, it works the other way round - flash duration is longer at low power settings than at high ones.

 

[gman]

Dare I ask but do you know why that is?

 

[Garry Edwards]

Dare I ask but do you know why that is?

Yes. All good quality studio flash heads have a number of capacitors (cheaper ones may only have 1 or 2)
Power is reduced in one of 2 ways, either switching out capacitors or reducing the voltage to the capacitors. Switching out capacitors is by far the best method but people expect to be able to set the flash power to any setting they want, switching out capacitors doesn't allow this level of fine control so, even if the flash head switches out capacitors, heads typically have a voltage reduction control (potentiometer) in place too.

So, when the power is reduced the voltage is reduced too, and it takes longer for the electrical current to flow through the flash tube. Cheaper more cheaply made flash heads, with a small number of capacitors, rely on a potentiometer heavily, so the flash duration can increase dramatically at low power settings. There are other problems too, with a dramatic shift in colour temperature and inconsistent power, but I won't go into those in detail as your question is only about flash duration.

BTW, I didn't get the point of using a half second exposure in that video, although I only watched it the once.

 

[gman]

Cheers for the reply.

Regarding the half second exposure, he was proving that it is the flash duration which freezes the action and not the shutter speed. To make his point, he was using a shutter speed that was obviously too slow and would without doubt blur a dancing girl.

I feel the video demonstrates things very well as I understood first time! lol

 

[Wail]

This has been very in-lighting, thank you for posting this.

Yes. All good quality studio flash heads have a number of capacitors (cheaper ones may only have 1 or 2)
Power is reduced in one of 2 ways, either switching out capacitors or reducing the voltage to the capacitors. Switching out capacitors is by far the best method but people expect to be able to set the flash power to any setting they want, switching out capacitors doesn't allow this level of fine control so, even if the flash head switches out capacitors, heads typically have a voltage reduction control (potentiometer) in place too.

So, when the power is reduced the voltage is reduced too, and it takes longer for the electrical current to flow through the flash tube. Cheaper more cheaply made flash heads, with a small number of capacitors, rely on a potentiometer heavily, so the flash duration can increase dramatically at low power settings.

As always, a very detailed and much appreciated explanation. Thank you very much.

There are other problems too, with a dramatic shift in colour temperature and inconsistent power, but I won't go into those in detail as your question is only about flash duration.

Dare I ask, please, to explain how that happens; and how can it be avoided?

BTW, I didn't get the point of using a half second exposure in that video, although I only watched it the once.

For my little understanding of lights, I think he was trying to show how to get the subject in motion, not exactly motion blur, but movement (with the hair and skirt)?

I am not sure if half a second is too much, or too little, to achieve that .. or even at all possible?

 

[Barrysprout]

The one part I am not sure of is that he initially said he was using a shutter speed of 1/250 ,ISO100 and that he had metered at f8. Does that mean he metered with the studio light on? How would he have done this?
Sorry if these are stupid questions (i'm assured there is no such thing) but I would rather it ask and know for sure.

 

[Garry Edwards]

The one part I am not sure of is that he initially said he was using a shutter speed of 1/250 ,ISO100 and that he had metered at f8. Does that mean he metered with the studio light on? How would he have done this?
Sorry if these are stupid questions (i'm assured there is no such thing) but I would rather it ask and know for sure.

He means that he metered the flash, using a separate, hand held meter. The meter would also measure the ambient light, but unless the ambient light levels are very high, or a very long shutter speed is used, the ambient light doesn't contribute enough to make any difference.

 

[Barrysprout]

Thanks for that Garry

 

[gman]

For my little understanding of lights, I think he was trying to show how to get the subject in motion, not exactly motion blur, but movement (with the hair and skirt)?

I am not sure if half a second is too much, or too little, to achieve that .. or even at all possible?

I don't think it was the motion he was trying to show but 'freezing' the motion. He was explaining that when using studio flash, freezing the motion has nothing to do with shutter speed but the flash duration.

He had his settings at a level so that no ambient light would be picked up (including the model). When the flash fires only the model will be lit because of the way the lights are setup.

Therefore, it doesn't matter what duration his shutter is open for because it's the light from the flash which captures the action because when the flash isn't lit the sensor is just seeing black. (unless you've got it open for a long enough time to start picking up the ambient light).

Here's how I see it, it's not technically accurate but just the theory side of it (not sure if this is correct but it's how I understand it! lol)

Therefore if the flash duration is say 1/1250 then this is essentially your shutter speed and is fast enough to freeze most forms of motion (as it will most likely always be higher than your sync speed). It is something I don't hear much of on these forums but came across it when researching my first set of studio lights. Now that I understand flash duration I can see how important it is as I'm guessing cheap lights will have a slower flash duration and therefore make it more difficult to capture motion in the studio and reducing your creative possibilities.

 

[Wail]

Graham,

Thank you for the diagram and explanation. I kind of understood what was going on, but seem to have phrased it wrongly ...

<snip>

For my little understanding of lights, I think he was trying to show how to get the subject in motion, not exactly motion blur, but movement (with the hair and skirt)?

<snip>

Freezing the motion, as you've put it, is what I was trying to say.

And thank you for that diagram too, makes it easy to "see" and understand this scenario.

 

[gman]

No problem, glad it helped and didn't hinder as this is all new to me. I'm digging in for the long haul now with lighting!