Flash Sync Speed

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Steve
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I understand flash sync speed - in that the focal plane shutter moves so that over a certain speed the curtain moves together as a slit, hence seeing a black bar if you go over the sync speed. However given that the flash itself has a certain duration, lets say 1/1000th of a second, what will happen if I shoot a 1/2000th of a second. Surely the duration of the flash exceeds the exposure so the slit would move whilst the flash is active so I would get a properly exposed photo?

I'm sure that can't be that simple otherwise I would have heard about it. What made me think is that canon has a high speed sync mode and I was wondering how the did it - just a long flash duratation or a series of very close flashes?
 
You've worked out how HSS and other methods work, here's a little more detail.

The reason you need smarter cameras and triggers to take advantage of the flash duration is that the flash has to fire early, at normal X sync, the first curtain has finished moving when the flash fires. So we need to trigger the flash early so it burns through the exposure.

True HSS works by pulsing the flash for the shutter duration, which obviously costs flash power.

The other method takes advantage of the flash duration, but some flashes have quick durations, which doesn't suit the technique. Once you have a longer duration, you run into the problem of the shape of the flash output, if you can imagine the light output measured on an oscilloscope, you'd see a sharp rise to maximum output with a longer tail towards no output (this technique is often described as long tail sync). So manufacturers are actually trying to create a flash burn that suits this technique with a flatter output.

It seems odd that the answer to faster flash sync speeds is actually better slower flashes.

I've probably missed something...
 
The max possible sync speed is the shutter speed at which the first shutter curtain has fully opened and the second one hasn't started to close, therefore the sensor (or film) is fully uncovered.

What that speed actually is depends on the camera, but generally it's around 1/200th - 1/250th second.

If that speed is exceeded, the second shutter curtain will have started to close, and this will show as an out of focus black bar across the bottom of the image.

If you go back into history, focal plane shutters on cameras moved laterally, not vertically, which of course took longer, they were also usually made of cloth, not metal, and because of this the maximum sync speed was generally just 1/30th. In 1953 Zeiss then brought out a clever mod to their camera shutters, called FP (focal plane) that created a timing delay, and flashbulb makers brought out a special bulb (looked a bit like a standard modlling lamp of today, very long, and filled with magnesium foil) that took much longer to burn. This long burning time allowed any shutter speed to be used, right up to the maximum, which was usually 1/1000th. Those flash bulbs were, effectively, continuous lights. They were very expensive, they got extremely hot and until the makers started coating them with clear or blue plastic, they used to explode too. They also produced a light that seemed to be blindingly bright but didn't provide much effective power.

Move forwards to today's technology, and we have the same principle with different technology, and in HSS mode, flashguns produce a large number of flashes which emulate the old FP flashguns of the 60's and 60's, but in terms of hype -v- effectiveness, nothing much has changed.
Please see these links
Jargon buster
Setting up your camera for indoor flash
Setting up your camera for outdoor flash
 
Thanks Phil/Garry.

Makes sense with the flash firing when the first curtain is in the open position - hence the second curtain is already on the move when over the sync speed. And the flash duration tails off significantly so if you could manage to fire on the second curtain moving the "tail" would cause exposure problems.
 
However given that the flash itself has a certain duration, lets say 1/1000th of a second, what will happen if I shoot a 1/2000th of a second. Surely the duration of the flash exceeds the exposure so the slit would move whilst the flash is active so I would get a properly exposed photo?
That's called "tail sync," "hyper sync," or even (erroneously/confusingly) "high speed sync." But since the bulb has to rise to a certain brightness and then fall back to darkness you tend to get a bit of a gradient to the exposure. And because each potion of the sensor is only exposed to a portion of the light, there is a loss of effective power.

True HSS (pulsed) doesn't have the gradient issue. But in order to flash that many times w/o recharging it has to put out less light for each pulse. The loss in power is roughly equivalent in general practice.
 
Your image was taken with the flash pulsing to cover the path of the shutter. The actual output is reduced, so as it has sufficient reserve to flash repeatedly throughout the whole exposure. Hence why your image is under exposed.

This article covers a number of flash terms and the physics involved.
my manual says the max is 1/2000sec max so thats probably why its dark can't remember who was showing me but i was a bit sceptical when they said the maximum shutter speed was as fast as your camera can go? the only limitation here was the maximum aperture of the lens f4
 
my manual says the max is 1/2000sec max so thats probably why its dark can't remember who was showing me but i was a bit sceptical when they said the maximum shutter speed was as fast as your camera can go? the only limitation here was the maximum aperture of the lens f4

Your actual x sync ceiling is 1/250th sec. Once you go above this, your speedlight then pulses to cover the window of the shutter as it travels across the sensor.
The gentleman is correct insofar as you can use flash at faster shutter speeds, but he failed to warn you of the tradeoff in power.
 
Yes, you used HSS (auto FP). I didn't follow the link to check the gear/settings...
Hi Steven thanks for that, i sort of knew that the inverse square law came into it but thought (at the time) that the maximum speed was 1/320fp but now understand it a little better, would i be right in thinking HSS has limited use?
 
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Hi Steven thanks for that, i sort of new that the inverse square law came into it but thought (at the time) that the maximum speed was 1/320fp but now understand it a little better, would i be right in thinking HSS has limited use?
Yes, in that it has limited power... but when you don't need much power it can be a good choice.
It's not really the ISL, it's just power. FP uses flash pulses... basically, think of it as a full charge 1:1 output divided into many smaller pops, which is why many flashes max out at 1/4 power when set to HSS. Another way to think of FP/HSS is that the flash is pulsing very fast in order to *become* a constant light source (same as led's/fluorescent's do). And therefore, the faster the SS the less light you get from it.
It does have the advantage of not having the gradient to it that tail sync does.


(FP actually stands for "focal plane," but I think it should stand for "flash pulse" ... at least with Nikon)
 
Hi Steven thanks for that, i sort of new that the inverse square law came into it but thought (at the time) that the maximum speed was 1/320fp but now understand it a little better, would i be right in thinking HSS has limited use?
Yes, as I said in the first answer, the power starts to reduce significantly when you use HSS.

I believe the calculations are roughly 1 stop for HSS use and a stop for every halving of shutter speed. So if your X sync speed is 1/320, for HSS at 1/500 you'll lose a stop and a half, 2 1/2 at 1/1000 etc. So your flash loses 4 and a half stops of power at 1/4000, there's no 'free lunch'.
 
Leaf shutters are the way forward, you can sync a flash at any speed in a single burst up to 1/1600. The whole sensor area is exposed at the same time unlike the lag of the focal plane shutter. I just wish the modem lens manufacturers would start producing them.
 
Leaf shutters are the way forward, you can sync a flash at any speed in a single burst up to 1/1600. The whole sensor area is exposed at the same time unlike the lag of the focal plane shutter. I just wish the modem lens manufacturers would start producing them.
Theoretically yes, but in reality sync speeds on leaf shutters rarely gets close to the high shutter speed a FP shutter reaches.

It's not unusual for it to be less than 1/1000 for med format lenses
 
Theoretically yes, but in reality sync speeds on leaf shutters rarely gets close to the high shutter speed a FP shutter reaches.

It's not unusual for it to be less than 1/1000 for med format lenses

I have heard people report this Phil but personally I've never had any issues with my Bronicas.
 
Camera make and model would help ;)
Hi phil missed this post camera was a nikon d700 with sb900 flash using fpmode and shutter speed taken to 4000 more to prove the point (the person who was showing me) was making than getting the lighting right hope that makes sense?
 
What reasons are there to set the sync speed other than at max? Is there any advantage if shooting at 1/160 to set sync speed to 1/160 as against 1/250?

Generally speaking, it would be when the ambient would cause an overexposure at maximum sync when a greater aperture is desired.
 
What reasons are there to set the sync speed other than at max? Is there any advantage if shooting at 1/160 to set sync speed to 1/160 as against 1/250?

By reducing the camera speed you open up a whole world of possibilities to start recording the ambient light atmosphere of the background subject.
 
By reducing the camera speed you open up a whole world of possibilities to start recording the ambient light atmosphere of the background subject.
Im not sure I understand you, I understand that you control ambient light with shutter speed but not sure if your comment is referring to that or with regards to my question which was why would you use other 'in-camera' sync speed settings than the max? In other words why would you bother to set sync speed at 1/30 meaning 1/60 for example would be unusable when you can just set it at max sync speed and be done?

I thought I understood Mikes answer but actually I dont think I do. If ambient caused overexposure at say 1/250 at f4 and one needed f4, why would it help to reduce max sync speed to 1/125 for example?
 
Im not sure I understand you, I understand that you control ambient light with shutter speed but not sure if your comment is referring to that or with regards to my question which was why would you use other 'in-camera' sync speed settings than the max? In other words why would you bother to set sync speed at 1/30 meaning 1/60 for example would be unusable when you can just set it at max sync speed and be done?

I thought I understood Mikes answer but actually I dont think I do. If ambient caused overexposure at say 1/250 at f4 and one needed f4, why would it help to reduce max sync speed to 1/125 for example?

I'm not sure if I understand you now, are you talking about balancing the flash with ambient light now or using the flash as a primary light source ? If it's the former then the ambient light on the subject shouldn't be over exposed in the first place. However, If you use the flash as the primary light source then reducing the shutter speed will allow the background to be recorded.
 
Generally speaking, it would be when the ambient would cause an overexposure at maximum sync when a greater aperture is desired.
I think wires got crossed... the original post/thread was about using SS's higher than max x-sync. But the question you responded to was about using SS's *slower* than max x-sync.

Using slower SS's is primarily for letting in more ambient (less flash/battery), using a lower ISO, or with radio transmitters that can't quite keep up.
 
I do hate having to reply via the browser on my phone. Age and eyesight are mutually exclusive.
That, coupled with the fact I'm currently in the middle of a good shoot, with very little signal for communication with the outside world, means I shouldn't court disaster by responding until I'm back at home or the office.

Meh!
 
I'm not sure if I understand you now, are you talking about balancing the flash with ambient light now or using the flash as a primary light source ? If it's the former then the ambient light on the subject shouldn't be over exposed in the first place. However, If you use the flash as the primary light source then reducing the shutter speed will allow the background to be recorded.
I think wires got crossed... the original post/thread was about using SS's higher than max x-sync. But the question you responded to was about using SS's *slower* than max x-sync.

Using slower SS's is primarily for letting in more ambient (less flash/battery), using a lower ISO, or with radio transmitters that can't quite keep up.

In my case, and sorry if I have detracted from the OP. it is simply to understand why we have so many sync speed settings in camera? I dont understand the need for any other than max sync speed, hss is not relevant to my question nor (I think is ambient light)

So, if I set my camera at max sync, say 1/250 I can use any shutter speed below this. Let us say I decide to shoot at 1/125 to let in more ambient light, my sync is still set at 1/250 afaik, the shot will not suffer in any way?
So why would I bother to set my sync speed to 1/125? There must be a catch I am missing somewhere? At sync speed of 1/250 am I losing any power in my flash than at 1/125?
 
So why would I bother to set my sync speed to 1/125? There must be a catch I am missing somewhere? At sync speed of 1/250 am I losing any power in my flash than at 1/125?

According to Thom Hogan, the reason for a slower sync setting is to improve compatibility as some external flash units might have a slower maximum speed. The other scenario is when working with 2 different cameras. If the sync speeds are different, then the images might look slightly different if the subject is moving. By setting a slower sync speed, shots taken from either camera should look consistent.
 
According to Thom Hogan, the reason for a slower sync setting is to improve compatibility as some external flash units might have a slower maximum speed. The other scenario is when working with 2 different cameras. If the sync speeds are different, then the images might look slightly different if the subject is moving. By setting a slower sync speed, shots taken from either camera should look consistent.
Sounds a likely reason to me, thanks Andy
 
In my case, and sorry if I have detracted from the OP. it is simply to understand why we have so many sync speed settings in camera? I dont understand the need for any other than max sync speed, hss is not relevant to my question nor (I think is ambient light)

So, if I set my camera at max sync, say 1/250 I can use any shutter speed below this. Let us say I decide to shoot at 1/125 to let in more ambient light, my sync is still set at 1/250 afaik, the shot will not suffer in any way?
So why would I bother to set my sync speed to 1/125? There must be a catch I am missing somewhere? At sync speed of 1/250 am I losing any power in my flash than at 1/125?

The power of your flash is not effected by the camera speed.

Perhaps forget the word sync for the moment although its usage is correct but think of it as the operating speed (camera speed) for the flash to work. This will work from an exposure of a couple of minutes right up to your maximum sync speed of 1/250.

Whenever a flash is used essentially two exposures of your subject are recorded: (1) the light from the flash falling on the subject. (2) The ambient light recorded whilst the shutter remains open. In essence, the longer you leave your shutter open the more ambient light your camera will record. Using flash at 1/250, 1/125 will not record much ambient light resulting only in the subject being lit by the flash which will not record the background. Whereas the shutter speed of 1/60, 1/30 or less will allow a lot more light on the sensor and resulting expose will lighten to the background to show scene detail whilst the flash has illuminated the foreground subject.
 
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The power of your flash is not effected by the camera speed.

Perhaps forget the word sync for the moment although its usage is correct but think of it as the operating speed (camera speed) for the flash to work. This will work from an exposure of a couple of minutes right up to your maximum sync speed of 1/250.

Whenever a flash is used essentially two exposures of your subject are recorded: (1) the light from the flash falling on the subject. (2) The ambient light recorded whilst the shutter remains open. In essence, the longer you leave your shutter open the more ambient light your camera will record. Using flash at 1/250, 1/125 will not record much ambient light resulting only in the subject being lit by the flash which will not record the background. Whereas the shutter speed of 1/60, 1/30 or less will allow a lot more light on the sensor and resulting expose will lighten to the background to show scene detail whilst the flash has illuminated the foreground subject.
I think you are mis-interpreting my question Nick, I was not asking about shutter speed/ambient light relationship, I undestand this. I was purely asking why have so many sync settings and what do they actually do? This I think has now been answered or at least partially so.
Thanks for going to the trouble though
 
I think you are mis-interpreting my question Nick, I was not asking about shutter speed/ambient light relationship, I undestand this. I was purely asking why have so many sync settings and what do they actually do? This I think has now been answered or at least partially so.
Thanks for going to the trouble though

Ok no problem but I don't understand what you mean by 'so many sync settings' perhaps if I did I could answer your question more fully.
 
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