X-Sync explained

Robinivich

After reading some talk about flash exposure earlier in cameras forum I started a reply. It was to do with indoor sports, I thought I'd pipe in with a few thoughts on FP sync, max X-sync speed, shutter speeds, and flash, for those among us making complicated decisions with flash in low light, high speed situations. This is part for the benefit for those who don't know, and part for me to check my understanding against the knowledge of others. I was going to reply in that thread, but as you can see, it soon had a life of it's own, and I now submit to the almighty technique forum, that it may be of use either to others, or to myself if I'm way off! :bow



Xenon flashbulbs (basically all current ones) have a VERY VERY short duration flash, it can be on the order of microseconds for some applications :huh. Because of this, flashes work with shutters in very special ways.

The X-sync speed of a given camera is the fastest shutter speed where the two shutter curtains leave the sensor completely uncovered for long enough for the flash to fire completely, thus exposing the sensor to one single burst of light that lasts an extremely short duration (ie, waaaay shorter than 1/200th of a second). In some camera's with CCD sensors, the shutter effect can be acheived electronically, which allows higher X-Sync, but the effects are otherwise pretty similar.

As concerns high speed sync, normally a focal plane shutter at very high shutter speeds never exposes the entire surface of the sensor at any one time. Example, 1/4000th of a second is really the shutter moving so fast that there's just a tiny slit of light moving across the sensor, exposing any given part of the sensor to light for only 1/4000th of a second as it moves top to bottom (or bottom to top, etc, as the case may be). This still happens so fast that we don't notice any weird motion blurring, but the point is, no single burst of light will expose the whole sensor. So FP sync is the flash firing many many pulses of light, all so fast that few things in nature could ever notice the difference between pulses and the one long flash we imagine. Because the flash is bursting, it's working much much harder.


What does this mean when comparing camera's with 1/200th X-Sync to those with 1/500th+ X-Sync? And moreover, should you choose your max X-sync speed? Or go higher?

A higher X-sync means the camera will be able to make better use of flash at high shutter speeds. Basically, the shutter mechanism is more efficient, working faster, so the sensor still has a moment where it's fully exposed even at very high shutter speeds.

The aspects of exposure that we need to think about are:

A. The balance between flash and ambient light.

B. The speed of the action you need to freeze

If there is no ambient light (it's perfectly dark) and the only light being used is the flash, then you could have a 10 minute exposure, and still freeze even hummingbird-speed action with that one, milli-or-even-micro-second flash burst. Of course, this would be a bit silly, since you'd need the hummingbird to be very cooperative for the 10 minutes, at least with second-curtain sync enabled. In this situation (ZERO ambient light), any shutter speed below X-Sync is probably an inconvience, and any speed above is a waste of energy for the flash.

If it's bright out, then a 10 minute exposure is going to leave you with a purely blown out image, flash or no flash. So, of course, we use shorter shutter speeds to cut down on that. The less ambient light, and the less sensitive the sensor, the less important the shutter speed to freezing action. The biggest difference between 1/200 X-sync and 1/500 X-sync is that with 1/500 you have the choice of having 1.5 stops less ambient light, if you want to. This way you have to be more reliant on the flash, it will be working 1.5 times as hard, but the action will be "more frozen".

Here's an extreme example, a chickadee at a feeder near dusk, taken with flash as the primary (but not only!) source of illumination:


Have a look at the tips of the wing, the streaking you see is the ambient light illuminating the wing, then the flash freezes the image of the wing. Because of the ambient light, the image is not terribly sharp, but just think! Here's the frantic movement of a small songbird, mostly frozen, by a shutter speed of 1/200th of a second. If I'd dropped the ISO and stopped down the aperture to reduce the exposure by a couple of stops, forcing E-TTL to compensate with more light from the flash, the wing might have been perfectly frozen, and at a shutter speed of 1/200 (the maximum X-sync of my rebel XT)

The reeaaaally strange part is, if I'd switched to FP flash and upped my shutter speed to 1/250 of a second (the natural reflex when one wants to stop action) it would have blurred the image more, because the actual flash burst would have been longer, which is the critical variable in this image since it relies on the flash almost entirely (the flash would also have to work much harder)


Alrighty, well, that about sums it up for me, now I'd like to know if I nailed it! :scratch

rwells

Robinivich wrote:

After reading some talk about flash exposure earlier in cameras forum I started a reply. It was to do with indoor sports, I thought I'd pipe in with a few thoughts on FP sync, max X-sync speed, shutter speeds, and flash, for those among us making complicated decisions with flash in low light, high speed situations. This is part for the benefit for those who don't know, and part for me to check my understanding against the knowledge of others. I was going to reply in that thread, but as you can see, it soon had a life of it's own, and I now submit to the almighty technique forum, that it may be of use either to others, or to myself if I'm way off!



Xenon flashbulbs (basically all current ones) have a VERY VERY short duration flash, it can be on the order of microseconds for some applications . Because of this, flashes work with shutters in very special ways.

The X-sync speed of a given camera is the fastest shutter speed where the two shutter curtains leave the sensor completely uncovered for long enough for the flash to fire completely, thus exposing the sensor to one single burst of light that lasts an extremely short duration (ie, waaaay shorter than 1/200th of a second). In some camera's with CCD sensors, the shutter effect can be acheived electronically, which allows higher X-Sync, but the effects are otherwise pretty similar.

As concerns high speed sync, normally a focal plane shutter at very high shutter speeds never exposes the entire surface of the sensor at any one time. Example, 1/4000th of a second is really the shutter moving so fast that there's just a tiny slit of light moving across the sensor, exposing any given part of the sensor to light for only 1/4000th of a second as it moves top to bottom (or bottom to top, etc, as the case may be). This still happens so fast that we don't notice any weird motion blurring, but the point is, no single burst of light will expose the whole sensor. So FP sync is the flash firing many many pulses of light, all so fast that few things in nature could ever notice the difference between pulses and the one long flash we imagine. Because the flash is bursting, it's working much much harder.

If by "working much much harder", you simply mean the bulb is powered on then off many times, OK.

But remember: The bulb ALWAYS receives the same amount of electrical power, each and every time. This is critical to maintaining a constant color temp. A speedlight puts out "more power" by simply extending the DURATION of the bulb burn.


What does this mean when comparing camera's with 1/200th X-Sync to those with 1/500th+ X-Sync? And moreover, should you choose your max X-sync speed? Or go higher?

A higher X-sync means the camera will be able to make better use of flash at high shutter speeds. Basically, the shutter mechanism is more efficient, working faster, so the sensor still has a moment where it's fully exposed even at very high shutter speeds.

The aspects of exposure that we need to think about are:

A. The balance between flash and ambient light.

B. The speed of the action you need to freeze

If there is no ambient light (it's perfectly dark) and the only light being used is the flash, then you could have a 10 minute exposure, and still freeze even hummingbird-speed action with that one, milli-or-even-micro-second flash burst. Of course, this would be a bit silly, since you'd need the hummingbird to be very cooperative for the 10 minutes, at least with second-curtain sync enabled. In this situation (ZERO ambient light), any shutter speed below X-Sync is probably an inconvience, and any speed above is a waste of energy for the flash.

FP mode on a speedlight does not waste energy. The reason you usually loose about 1/3 of your apparent "power" is only because the extremely short burst to accommodate the faster moving shutter curtain now limits the "duration" of the flash bulb burn time.

Remember, this burn time (at full electrical power - again, which is the ONLY way the bulb gets power), is what controls the flash's power output. The flash isn't working harder, it's just not able to give a very long burn to the bulb, which of course outputs less light.

If it's bright out, then a 10 minute exposure is going to leave you with a purely blown out image, flash or no flash. So, of course, we use shorter shutter speeds to cut down on that. The less ambient light, and the less sensitive the sensor, the less important the shutter speed to freezing action. The biggest difference between 1/200 X-sync and 1/500 X-sync is that with 1/500 you have the choice of having 1.5 stops less ambient light, if you want to. This way you have to be more reliant on the flash, it will be working 1.5 times as hard, but the action will be "more frozen".

1/200 to 1/500 the flash could care less as far as ETTL and your "subject" is concerned. (talking native X-sync per camera here) The flash duration of most speedlights will be somewhere in the range of 1/2000 ~ 1/15000. So, you can see that 1/200~1/500 matters little. It does affect how much ambient light hits the sensor.

A camera with native X-sync of 1/500 does give more freedom between shutter speed and aperture when using flash. IE: Using flash outside, you can use your higher sync to allow a wider aperture if desired for DOF.

Aperture is what directly affects the amount of flash light that hits the sensor, and of course the camera and flash talking ETTL to determine the flash duration for the correct exposure given all the other variables.

Here's an extreme example, a chickadee at a feeder near dusk, taken with flash as the primary (but not only!) source of illumination:


Have a look at the tips of the wing, the streaking you see is the ambient light illuminating the wing, then the flash freezes the image of the wing. (Ghosting) Because of the ambient light, the image is not terribly sharp, but just think! Here's the frantic movement of a small songbird, mostly frozen, by a shutter speed of 1/200th of a second. If I'd dropped the ISO and stopped down the aperture to reduce the exposure by a couple of stops, forcing E-TTL to compensate with more light from the flash, the wing might have been perfectly frozen, and at a shutter speed of 1/200 (the maximum X-sync of my rebel XT)

The reeaaaally strange part is, if I'd switched to FP flash and upped my shutter speed to 1/250 of a second (the natural reflex when one wants to stop action) it would have blurred the image more, because the actual flash burst would have been longer, which is the critical variable in this image since it relies on the flash almost entirely (the flash would also have to work much harder)

You don't mention critical info in this scenerio: Mainly, how many stops below correct ambient exposure are you on your camera settings?

Other things to consider; how close to your subject is your camera/flash?
How fast is your subject moving?
What direction is the movement from your camera? Parallel or straight away?
Each of these, along with other considerations, is important in knowing how far above ambient your flash output needs to be in order to "stop" motion. (2 stops is a good rule-of-thumb starting point)

If the ONLY things you would have changed would be to up your shutter speed to 1/250 from 1/200 and put the flash in HHS mode, not much would be different in your image if you were already 2 stops under on ambient. That's less than 1/4 stop on the ambient, it probably wouldn't even be noticable.

Near the start of this you wrote how FP (High Speed X-sync) uses very short duration burst to work with the faster moving "slit", which is correct. Here, you've gone 180 degrees on that and state that the flash duration "burst" would be longer in FP mode, which it's not.

Again, the flash is not working harder in FP mode, other than turning the bulb on and off many times quickly.


Alrighty, well, that about sums it up for me, now I'd like to know if I nailed it!

Hope that helps...

pathfinder

I agree - FP synch, or High SPeed Synch as Canon calls it for its EOS flash system, is best used for control of ambient light exposure. To decrease the brightness of the background, for instance. I find it very useful for that. Just like in a stage production, put the brighter light on the star, and dim the background.

FP or High Speed Synch ( despite the name ) is the wrong choice if your desire is to use very short duration flash to stop action of very fast moving objects. A single pulse of very short duration light with x-synch at 1/250th will be more effective. Much of Edgertons's photos of bullets breaking light bulbs and ballons were done in a dark room with an open shutter set to bulb, and then the flash triggered by the passage of the bullet or dart or whatever.

ziggy53

Geeks might find this interesting.

Flash duration measurements of the Canon 580 EX in manual mode:

http://www.waynesthisandthat.com/flashdurations.html

Robinivich

rwells wrote:

But remember: The bulb ALWAYS receives the same amount of electrical power, each and every time. This is critical to maintaining a constant color temp. A speedlight puts out "more power" by simply extending the DURATION of the bulb burn.

This is news to me, and in hindsight, it makes a lot of sense, it's not an incandescent type bulb, but more fluorescent one, therefore the electricity involved is pretty constant. Thanks!

rwells wrote:

FP mode on a speedlight does not waste energy. The reason you usually loose about 1/3 of your apparent "power" is only because the extremely short burst to accommodate the faster moving shutter curtain now limits the "duration" of the flash bulb burn time.

Remember, this burn time (at full electrical power - again, which is the ONLY way the bulb gets power), is what controls the flash's power output. The flash isn't working harder, it's just not able to give a very long burn to the bulb, which of course outputs less light.

I'm not so certain I follow here, in terms of light output at any one instant, the bulb will be working as hard as it always does, but in terms of energy, if as an example, the "slit" moving across the sensor exposes only half of the sensor to light at any given instant, the flash would have to essentially create the same amount of light as for a single exposure, twice. Correct?

rwells wrote:

1/200 to 1/500 the flash could care less as far as ETTL and your "subject" is concerned. (talking native X-sync per camera here) The flash duration of most speedlights will be somewhere in the range of 1/2000 ~ 1/15000. So, you can see that 1/200~1/500 matters little. It does affect how much ambient light hits the sensor.

Totally agree, this was the key thing I wanted to point out.

Breaks over, guess I'll have to wrap up the rest in a bit!

:Edit:

Back again, and thanks ziggy for the info, quite enlightening actually

rwells wrote:

Near the start of this you wrote how FP (High Speed X-sync) uses very short duration burst to work with the faster moving "slit", which is correct. Here, you've gone 180 degrees on that and state that the flash duration "burst" would be longer in FP mode, which it's not.

Again, the flash is not working harder in FP mode, other than turning the bulb on and off many times quickly.

What I meant was, the duration of the flash will be longer in FP mode by comparison with X-sync. Assuming all else is equal (not going to be strictly true, but close enough for our purposes), because the flash needs to create the same amount of light multiple times to correct for the shutter curtain taking time to travel across the sensor, the interval between the first emission of light from the flash to the last, is longer in so-called High speed sync mode.

If one was trying to capture a bullet in flight, one would not use 1/4000 sec with flash set to FP mode (which might sound like the best way to freeze action), but rather, sit in a darkroom with any shutter speed at or below X-sync so that the flash duration, which is the critical component to stopping motion, lasts longer in FP mode

The difference is probably immaterial, but the point is, FP sync is the flash firing bursts at something like 50Khz for the duration of the time the curtain takes to cross, so 1/1000th of a second for example, compared to say, 1/20000th of a second for an X-sync firing

Is that more clear?

rwells

Robinivich wrote:

I'm not so certain I follow here, in terms of light output at any one instant, the bulb will be working as hard as it always does, but in terms of energy, if as an example, the "slit" moving across the sensor exposes only half of the sensor to light at any given instant, the flash would have to essentially create the same amount of light as for a single exposure, twice. Correct?




What I meant was, the duration of the flash will be longer in FP mode by comparison with X-sync. Assuming all else is equal (not going to be strictly true, but close enough for our purposes), because the flash needs to create the same amount of light multiple times to correct for the shutter curtain taking time to travel across the sensor, the interval between the first emission of light from the flash to the last, is longer in so-called High speed sync mode.


If we realize that ETTL is your speedlight and camera talking to each other, the camera processor takes all this info into account and tries to come up with a "correct" exposure. Let's say for the ambient light available, aperture selected, subject distance, reflectance, etc, the flash needs to fire for 1/2000 sec. --- Now, you've engaged FP on the flash due to using a higher shutter speed above X-sync.

The flash has to "pulse" to work with the moving "slit" going across the sensor. In FP mode, the speed that the "slit" is moving across the sensor now helps to govern how much light reaches the sensor, given the aperture, not necessarily what ETTL would have liked for a correct exposure since it can no longer be in complete command of how long the flash duration is. That's why you "loose" approximately 1/3 of your flash output. There are infinite variables here.

All this is very complicated, and Canon's speedlights are wicked smart and fast at figuring this out. So, as you can see, there are many, many variables that go into how your flash will react in FP mode. But, usually, if the subject is close, it does a very good job. You just have to learn how it reacts under the given circumstance.



The difference is probably immaterial, but the point is, FP sync is the flash firing bursts at something like 50Khz for the duration of the time the curtain takes to cross, so 1/1000th of a second for example, compared to say, 1/20000th of a second for an X-sync firing

Is that more clear?


Yes, that's what I'm saying. So as you again stated, FP flash mode produces a SHORTER DURATION flash burst per "slit" opening. Not a longer one.

I think what's messing you up, is that your taking the "total" time the flash might be firing. That doesn't matter in FP mode. It's "how long did it fire for the "slit" area to be exposed on the sensor".

Hope that helps...

nstam

very intrigued

rwells wrote:

Hope that helps...

im going to blabber about random things... to try and understand the flash better

1)
when stated above 'working much harder'
i thought that a bright flash actually put more light out during the same duration of time of a dimmer flash, but now i have come to understand is that the bulb is just on for a longer amount of time for a "brighter" flash. which makes sense!
knowing the flash is on longer; is it correct to state that the energy is more, where the power remains the same??
[energy = power (mulitplied by) time]

2)
FP using faster shutter speeds: is it correct to state that each flash fired during each segment of the curtain travelling accross the sensor is just exposing the tiny "slit" of the sensor each time "properly" and there for having 'multiple' images within a shorter amount of time? and therefor having a 'more blurry' image because of the segment travel?



3)
why isnt it a voltage regulator that determines how much light is emitted instead of it being a time thing? i would think a variable voltage would bring up much faster x-sync speeds?



im very intriqued with this topic, because i have the limitations of the x-sync of canon's line up, where as my oly-720 could fire flash at 1/1000s and being stopped all the way down, and it still work! of course waaaaaay back then, i wasnt pixel peeping or taking pictures of fast moving subjects, so maybe it was a case of FP flash....

i love photography! gets so much more interesting with each day!
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rwells

nstam wrote:

im going to blabber about random things... to try and understand the flash better

1)
when stated above 'working much harder'
i thought that a bright flash actually put more light out during the same duration of time of a dimmer flash, but now i have come to understand is that the bulb is just on for a longer amount of time for a "brighter" flash. which makes sense!
knowing the flash is on longer; is it correct to state that the energy is more, where the power remains the same??
[energy = power (mulitplied by) time]

Yes

2)
FP using faster shutter speeds: is it correct to state that each flash fired during each segment of the curtain traveling across the sensor is just exposing the tiny "slit" of the sensor each time "properly" and there for having 'multiple' images within a shorter amount of time? and therefor having a 'more blurry' image because of the segment travel?

No - On Canon's speedlights, FP mode fires the flash at around 50kHz. Don't get me lying about how many flashes that would be for a given shutter speed, say 1/2000. But, what it does mean is that the faster the shutter speed, the less "time" the rapid flashes have to expose the sensor "at any given spot". Therefore less "energy" output of the flash, if you want to word it that way.

I've not seen that FP mode would blur an image more. Just that my working distance (for proper exposure) is reduced the faster I move the shutter speed up in FP mode.

I would think that there may be some issues created if you only "cross the line" and bump up your shutter to say 1/320 & go into FP mode, and your trying to shoot something fast at fairly close range. Just don't do that, go ahead and up the shutter to a faster speed.

Remember: It's all a balancing act...



3)
why isnt it a voltage regulator that determines how much light is emitted instead of it being a time thing? i would think a variable voltage would bring up much faster x-sync speeds?

Because light temperature is very important. If you don't put the same amount of energy to the bulb each time, you won't get the same kelvin output. Canon's 580XXX series even goes so far as to send kelvin info to the camera via ETTL-II so that they can both work together at keeping a constant WB. (ALL bulbs will change output temperature "Kelvin" over the life of the bulb.) These same flash units also have circuitry that maintains the exact same electrical input to the bulb, even as your batteries are getting depleted, trying once again to maintain the same kelvin output.



im very intriqued with this topic, because i have the limitations of the x-sync of canon's line up, where as my oly-720 could fire flash at 1/1000s and being stopped all the way down, and it still work! of course waaaaaay back then, i wasnt pixel peeping or taking pictures of fast moving subjects, so maybe it was a case of FP flash....

i love photography! gets so much more interesting with each day!
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Hope that helps...

SavedByZero

Just shoot with a leaf shutter and don't worry about sync speeds.

pathfinder

Not many DSLRs with a leaf shutter these days.....

Several film 2 1/4 square SLRs came with interchangeable lenses with leaf style shutters within each lens. The lenses were substantially more expensive as a result too. But were great for synching with studio strobes, weren't they?

Robinivich

Thanks for the informative replies everyone, particularly rwells

I think I'm now on more or less the same page here,

"Just crossing the line" say 1/320, might produce unexpected results, but after that the difference would likely be difficult to measure.

We obviously have to assume for these theoretical discussions that there is no ambient light, aperture and sensitivity remains constant, and that our target maintains the same reflection characteristics. (otherwise things get just goofy) Don't get me wrong, I'm a practical person, but I like understanding the fundamental operation of things, and this thread has already helped a TON.

What's been mentioned with colour temperature being influenced by the electrical current to the bulb makes perfect sense, thanks for the explanation.

I'm also starting to work out what you're saying about the flash algorithms, my point with the "flash working harder" is in the sense that (in our constant dark scenario) the flash at MAX power and 1/200 is going to be able to expose the sensor to significantly more light than the same flash at 1/2000 and FP sync, the flash has to use more energy because it can't expose the whole sensor with the same pulse of light, therefore the TOTAL light output (irrespective of duration of the indiviual bursts) will need to be higher (possibly several times more than what the flash is capable of at once).

This brings up another question, what is the real limiter of the flash power?

Is it the length of the pulse for max power? (in which case FP flash might even be capable of more power?!?)

Is it how much charge can be stored in the capacitors of the flash? Or maybe the amount of light the flash can fire without risk of overheating? (in which case the sheer quantity of light would be the deciding factor)

Is it something else that hadn't occured to me?

Thanks for all of this, it's been fascinating learning.