flash sync speed for dummies...

Discussion in 'Wedding and Event' started by brian_walsh|7, Jun 14, 2007.

  1. can anyone define and explain flash sync speed in laymen's terms. it seems to
    me one of the more complicated topics. mainly: why can't my d80 use a faster
    shutter speed than 1/200 when my flash is active? the flash bursts lasts only
    like 1/10,000 of a sec. , so why can't my shutter fire faster than 1/200?
    especially outside in the sun, i'd like to use my 2.8 aperture for the shallow
    dof, but can't because the shutter can't go faster than 1/200 and everything
    would be blown.

    can anyone shed some light about the basics of the flash sync so a 'dummy' can
    grasp it???
  2. if you have a Nikon SB-800 and your D80 supports ityou can switch it to FP mode and sync at shorter shutter speeds but the maximum amount of light is reduced to 1/16th or 1/32nd of maximum power --and no TTL.

    As to why you can't use a shorter shutter speed the "dummies" answer is that it has to do with the mechanics of getting the sensor fully open during the 1/1,000th of a second or shorter flash duration. The FP mode works at shorter shutter speeds as the SB-800 is emitting a stream of low power flashes that are synced with the movement of the shutter.
  3. I wrote this for another forum a few weeks ago.

    To understand flash sync you have to understand how the curtain on a SLR works.

    I'll take a stab at explaining how the curtain works on a DSLR and maybe it will help.

    A modern SLR camera has a dual curtain that usually slides from top to bottom. By dual curtain, I mean, one curtain opens sliding from top to bottom and a second second curtain closes behind it also from top to bottom. It is important that the curtains open and close in the same direction so that the frame will be exposed evenly.

    Lets say you are taking a picture at 1/30 of a second. The first curtain would open all the way, the flash fires, light hits the sensor, and then 1/30 of a second later the second curtain closes. Keep in mind that the duration of the flash is only about 1/20000th of a second. As long as it fires within the time frame that the first curtain is completely open, and before the second curtain starts to close, life is good.

    That's actually a little bit of a simplification. You have to remember that it takes time for the curtain to travel from the top to the bottom of the frame. So the camera has to take that into account when deciding when to start closing the second curtain.

    On a Pentax, it takes 1/180th of a second for the curtain to move from one top the frame to the bottom. The only way that the camera can achieve a shutter speed faster than 1/180th is for the second shutter to start closing before the first shutter is even all the way open.

    I guess the best way to think of it is, at those higher shutter speeds, the curtain becomes a "slit" that travels across the frame. The higher the shutter speed, the narrower the slit.

    Now - How does this relate to sync speed. A cameras "normal maximum sync speed" is the maximum shutter speed in which the camera has the first curtain fully open before the second curtains starts to close. Or in other words, the maximum speed that the camera can operate without going into "slit mode". If you try to take a normal flash photo while the camera is operating in "slit mode". You will actually capture a photograph of the shutter itself. Probably not what you wanted.

    The only way that a camera can operate in the "high speed flash" mode. Is to be able to fire the flash multiple times as the slit travels across the frame. That takes quite bit of electronic magic and requires a much more expensive flash. Those flashes have to be in constant communication with the camera during the actual exposure. It's amazing to me that they can even do that!!

    Another way to achieve HIGHSPEED sync is to do it totally electronically without involving the mechanical shutter at all.

    So there you have it. How a camera works 101. Hope it helped.
  4. " --and no TTL" - that was the older Nikon FP technology.

    The newer FP provided with SB800 for D200, D80, provides TTL automation of exposure. This latest FP is great improvement over the older FP.
  5. Ken Rockwell has a very good write-up on what flash sync speed is and how it works:
    Sync speed is the fastest shutter speed you can use with flash, period. You cannot use a faster shutter speed than the sync speed with flash. [...] sync speed is critical for using fill in daylight, and fill is critical to getting good images in direct sunliught.That's what makes sync speed critical, in addition to stopping motion.​
    See: The Importance of Flash Sync Speed
  6. The short answer:
    When the shutter speed is above 1/200 of a second, the whole sensor/film is not illuminated at the same instant. Think of a Xerox machine, where the light bar moves from one side of the sheet to the other. If the light inside the moving part only flashed for 1/10,000 of a second instead of being constantly on, only part of the sheet would be duplicated.
  7. For a FP shutter, its the fastest shutter speed available where the first shutter curtain has opened completly, but the second curtain hasnt moved. This allows the light from the flash to be registered on the film. Usually its 1/60th of a second, but some have 1/30th 1/125 or 1/250th.

    For a leaf shutter, it syncs at any speed
  8. Mark, that's the best explanation I've ever heard. Very clear and easy to visualize.
  9. There are ways to hack the sync speed. Follow the link for an explanation.

  10. Mark - That's brilliant!
  11. Mary,

    I will vote to move Mark's writeup to the FAQ. It's the shortest explanation of sync limit that I've ever seen yet it explains the guts of the issue and leaves no questions behind. Unbelievable! Hats off!

  12. "For a FP shutter, its the fastest shutter speed available where the first shutter curtain has opened completly, but the second curtain hasnt moved." - Yes that is right when you think FP = Focal Plane shutter, and not the electronic FP flash mode.

    Just to make it clear.

    We are also talking here FP = electronic flash/camera mode that allows use on any faster shutter speed than the mechanical max sync. E.g. all the way to 1/8000 sec on Nikon D200, etc.
  13. This is an old topic so I hope someone's still reading this ... I have a Nikon D300 and I'd like to understand why I might change the flash sync speed from the default of 1/250 s to 1/320 s (Auto FP) in one direction, or to 1/60 s in the other. Can anyone explain why one changes the flash sync speed?

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