# Canon Sync Speed



## lennon33x (Nov 8, 2013)

So, I am looking to purchase a new camera here around Christmas, and I thought I was set on the EOS 1D. I like its large pixel size, sync speed and high shutter speed. I do a lot of flash photography, so I had the following question.

Is the high sync speed limited to using only Canon speedlites? Or can you use ETTL flashes like Yongnuo, etc. to fire the flashes at a high shutter speed? 

I use a Pixel Opas transceiver for my off camera flash and strobes, which can be synced to as high as 1/300 (I'm sure higher, depending on the camera). I have a Rebel T3 now, which only indicates use for 1/200, but I can get up to 1/250 without a problem most of the time. 

So my general question is does the sync speed on the camera limit the brand of flash that you can use? How does this pertain to strobes?


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## Luigi_xp (Nov 12, 2013)

The problem of Yonguno flashes is the poor documentation, i'm not sure if them will work at these fast sync speeds, but theorically there's no why they shouldn't work.
Getting an EOS 1D? man, i envy you. (in fact it's a little too overkill for me and heavy, but hearing the burst of an 1D... oh my...) Have you considered an 5DMkIII or maybe another camera, since upgrading from a T3 direct to a 1DX is a VERY big upgrade... anyway if you have lenses to use with an 1D why not?


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## TCampbell (Nov 12, 2013)

reavesce said:


> So, I am looking to purchase a new camera here around Christmas, and I thought I was set on the EOS 1D. I like its large pixel size, sync speed and high shutter speed. I do a lot of flash photography, so I had the following question.
> 
> Is the high sync speed limited to using only Canon speedlites? Or can you use ETTL flashes like Yongnuo, etc. to fire the flashes at a high shutter speed?
> 
> ...



The sync speed is based on the physical traversal time of the camera shutter curtains.

DSLR cameras have focal plane 'curtain' shutters - and there are two 'curtains' on the shutter.  To take an exposure, the first curtain begins to slide open and, at some point, the second curtain will follow to close.  The reason for this is because the if the camera just had a single curtain then one side of the image would get more exposure than the other AND it allows for very fast exposures.  

It's easy to imagine in a thought experiment:  Suppose the shutter is so slow that it actually takes a full 1 second for the shutter to completely slide "open" and another second for the shutter to completely slide "shut" again.  If you start the clock the moment the shutter begins to open, the pixels closest to the edge where the shutter curtain opens are immediately exposed to light... but the pixels on the opposite edge won't get any light at all for a full 1 second (the time it takes the curtain to finish sliding open.)  Once the shutter is completely open, the shutter could then start to close again -- but if you only had a single shutter curtain then it has to reverse... immediately covering the pixels that were only just exposed to light a fraction of a second ago -- and it will be another second before it completes the closure process.  This means the pixels nearest to the opening edge will actually get a full TWO SECONDS of exposure and the pixels at the opposite edge will only get a fraction of a second.  You'd have a very wonky image.

By using TWO curtains, after the first curtain opens, a second curtain can follow to "close".  Now all pixels will be exposed to light for the same total amount of time.

If my very slow curtain shutter needs to take a 1/4 second exposure, what I need to do is start the first curtain opening, and then when the first curtain has been moving for only 1/4 of a second (1/4 of the way across the sensor), the second curtain will begin to close.  This means that a "slit" 1/4 of the sensor width wide will sweep across the shutter so that no pixel is exposed to light for more than 1/4 of a second.  However... the actual amount of time needed to completely the exposure will be 1.25 seconds.  (BTW, I write this as though the curtains travel horizontally and on old film SLRs they actually did -- but on most modern DSLR cameras the curtains usually move vertically rather than horizontally.)

But this creates a "problem" for flash photography.  The flash will only illuminate for a fraction of a second -- let's call it 1/1000ths  second (in reality many flashes are actually much faster).  If the flash fires the moment that the curtain starts to open then then only the first row of pixels will get the benefit of light because the rest of the sensor is still covered by our extremely slow moving shutter.  So we play the waiting game... we have to wait for the shutter to completely open, then burst the flash, THEN start the second curtain closing.  This allows all pixels to get the benefit of the flash.  But since it takes a full second to open, and another full second to close (plus the flash duration), the total time needed to take that exposure ends up being 2 seconds BUT it only counts as a 1 second exposure because no individual pixel will be exposed to light for more than 1 full second.

If one full second is too long, we can speed things along by using "high speed sync".  

Suppose we only want a 1/4 second exposure rather than the 1 full second exposure we had before.  We can wait for the first curtain to open 1/4 of the way (which will take 1/4 second) THEN pulse the flash (which happens very quickly), THEN start the second curtain closing meanwhile the first curtain is still continuing on it's way.  When it is 1/2 way across (another 1/4 second later) and pulse the flash again, and so on until we have pulsed the flash 4 times.  Now, no individual pixel was exposed for more than 4 seconds, we had a "slit" but since we kept pulsing the flash as the curtain shutter operates, all pixels actually do get an even amount of light.

In reality, this all happens much faster.

When your camera says it has a max sync speed of (I'll just toss out a typical value) 1/200th, what that REALLY means is it takes the curtain shutter 1/200th of a second to traverse the distance from one edge to the other (the flash can then fire, and then second curtain can close.  But while the closing process will take another 1/200th second (requiring 2/200ths (so really 1/100th) to take the exposure, no individual pixel will actually be exposed to light for more than 1/200th second so it still counts as a 1/200th sec shutter speed. (shutter speeds are expressed in the amount of time each pixel will be exposed... not the physical amount of time needed to take the shot.)

Since it's a limitation on the maximum speed that our mechanical shutter can slide across the sensor, buying a different flash isn't going to help -- because it isn't the flash that's limiting the shutter exposure speed... it's the camera shutter.

Most external E-TTL flashes support "high speed sync" (Canon's do).  You do have to enable the mode -- it's not on by default.  Also, you take a hit in total flash power when you enable the mode.  The physical number of pulses required to get the image depends on the actual shutter speed.  If you want 1/400th on a camera with a max shutters peed of 1/200th then that means the flash MUST fire at least twice -- limiting you to no more than 1/2 of the total power of the flash in a single pulse (because it has to reserve enough power in the capacitors for all the required pulses.)  If you wanted say, 1/800th then it would need 4 pulses and no individual pulse could take more than 1/4 of the total power.

You can get around the power limitation by clustering speedlites.  Create a gang of 4 lights and you can fire effectively at "full power" even when shooting at 1/800th BECAUSE each unit only fires 1/4 of it's power but you have four units firing.

Here's a video (by Canon) showing how they cluster their speedlites for high speed action photography -- and they are clustering the flashes.






Note that this is using Canon's E-TTL flash system via the new radio control.  Some third party external triggers cause a slight degradation in performance and this can mean you have to dial the fastest shutter speed (for a single flash -- not high-speed sync) by about 1 notch (1/3rd stop of shutter speed).  So if (for example) your max sync speed is really 1/200th native on the camera, you might actually have to limit yourself to 1/160th to avoid problems caused by a slight delay in the off-camera trigger.


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