Canon t3i - Flash Sync - Why only 1/200 of a sec?

Started Jun 18, 2012 | Discussions
btdoyle
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Canon t3i - Flash Sync - Why only 1/200 of a sec?
Jun 18, 2012

I was recently on a sailing trip and was trying to use the built in flash on my t3i to prevent dark faces under hats and to deal with a backlit very sunny scene. I ran into a issue where the maximum flash sync shutter speed allowed was only 1/200 a sec.

In Aperture Priority mode, this was causing a lot of blown on scenes.

Does anyone know why this is or have a suggestion for how I can get around this?

I really wanted to keep control over the aperture so I could effect the bokeh. Is there another setting that I am missing someplace?

Thank you in advance,

Brandon

Canon EOS 600D (EOS Rebel T3i / EOS Kiss X5)
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Shorthand
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Re: Canon t3i - Flash Sync - Why only 1/200 of a sec?
In reply to btdoyle, Jun 18, 2012

The explaniation is better seen than described - the short version is that 1/200s is the fastest shutter speed where there is a moment where the shutter is completely open. Faster shutter speeds actually swipe a narrow band across the sensor so that each part is only exposed for 1/2000s but the swipe still takes about 1/200s.

See:

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Doug Pardee
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Re: Canon t3i - Flash Sync - Why only 1/200 of a sec?
In reply to btdoyle, Jun 18, 2012

You need an external flash unit that has High Speed Sync (HSS).

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Shorthand
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Re: Canon t3i - Flash Sync - Why only 1/200 of a sec?
In reply to btdoyle, Jun 18, 2012

btdoyle wrote:

Does anyone know why this is or have a suggestion for how I can get around this?

I really wanted to keep control over the aperture so I could effect the bokeh. Is there another setting that I am missing someplace?

OK, I wanted to make this a separate post.

For this, see:

There are two ways to handle this:

  • Use a neutral density filter

  • Use a flash with high speed sync

  • Often one actually wants to do both because high-speed sync is an ineffeicient use of your flash. Therefore, if you're fighting sunlight, you generally want to keep the flash in single-pop mode.

Here is an example - I was using a single off-camera bare flash and an 3-stop ND filter on my 50mm f/1.8.

If you have any more questions after reading through all three of the links above, please check back and we'll be happy to clarify.

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Narcosynthesis
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Re: Canon t3i - Flash Sync - Why only 1/200 of a sec?
In reply to Shorthand, Jun 20, 2012

The shutter consists of two curtains, one initially covering the aperture, and one 'open'. When you hit the shutter button the first curtain moves, exposing the sensor to light, and then after the required length of time the second curtain moves to cover over the sensor again.

With long exposures the time it takes to move the curtains is somewhat irrelevant, since it is a tiny fraction of the exposure time. With very quick exposures though, because the shutters take a set amount of time to travel across the sensor, the second curtain starts moving before the first curtain has fully opened - so rather than having a pause in the middle between the movements of the first and second curtains, the exposure essentially becomes a letterbox, moving down across the image.

Flash on the other hand is always a instantaneous burst of light in the order of thousandths of a second, whatever the exposure time. With slower shutter speeds it fires while the sensor is completely uncovered, and lights up the whole image in that time. The problem comes once you get to the point that the second curtain is moving before the first finishes, there is no point during that exposure where the entire frame is uncovered that the flash can fire - if you could get the flash to fire, you would end up with part of the frame exposed as expected, but also part of it dark as the shutter was obscuring it as the flash fired. This shutter speed is what is called the sync speed - ie the maximum shutter speed where the flash can fire and evenly expose the whole frame.

As for how to deal with sync issues, there are a few solutions:

Darken your ambient exposure by other means: close down the aperture, reduce ISO or add an ND filter and you will reduce the shutter speed - reduce it to below the sync and you will be able to balance the flash and ambient. The downside is that darkening the exposure will mean you will need more flash power to overcome this.

Another solution is to use a flash with high speed sync (HSS in Canon terms) - basically instead of firing once to expose the whole image, the flash strobes multiple times over the exposure length to light each area as the opening in the shutter curtain passes is. Again the downside is that you lose flash power - as soon as the flash has to strobe it has to split the available power between each pop, rather than dumping it all at once, so your maximum power will dl be able to shoot at your chosen shutter speed.
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PhilPreston3072
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Re: Canon t3i - Flash Sync - Why only 1/200 of a sec?
In reply to btdoyle, Jun 20, 2012

btdoyle wrote:

I was recently on a sailing trip and was trying to use the built in flash on my t3i to prevent dark faces under hats and to deal with a backlit very sunny scene. I ran into a issue where the maximum flash sync shutter speed allowed was only 1/200 a sec.

In Aperture Priority mode, this was causing a lot of blown on scenes.

Does anyone know why this is or have a suggestion for how I can get around this?

I really wanted to keep control over the aperture so I could effect the bokeh. Is there another setting that I am missing someplace?

Thank you in advance,

Brandon

As the previous poster mentioned, if you want to use Av mode you'll need an ND filter to reduce the light intensity but this will also reduce flash intensity too, which the built-in flash is not great at. The same thing will happen if you use P or Tv mode (set to 1/200) because the closing of the aperture reduces the flash intensity.

The ideal work around is to use an ND filter and the strongest external flash you can afford and don't shoot above the maximum flash sync (for T3i is 1/200). Whilst you can shoot above 1/200 in HSS mode, the flash intensity can drop significantly.

Here's a great demo of the effect HSS does to the flash output:
http://neilvn.com/tangents/2010/08/02/high-speed-flash-sync/

Neil also recommends using an ND filter with the camera's maximum flash sync for best intensity:

http://neilvn.com/tangents/2010/07/16/using-a-neutral-density-nd-filter-with-flash/

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