Flash and Half Dark Pictures

Discussion in 'Photography Beginners' Forum' started by ricepudding, Nov 23, 2007.

  1. ricepudding

    ricepudding TPF Noob!

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    Today I started using the old Vivitar 3500 flash off my Mom's old slr camera on my D80. I got some pics with a half dark picture/half light.

    At first I thought the darn thing broke my camera with a high voltage or something. But then I adjusted my shutter speed and realized it needed to be slower to capture the whole picture in light.

    Can you help me understand this in general.....maybe give me a couple of crash course paragraphs on external flashes and good pictures??

    I've read on the net that this is called flash-sync speed??? the fastest speed at which the camera can get the picture and have it all lit from the flash?! Is that right? And then I read the D80 has a max speed of 1/200 sec. Does this mean when I use my external flash I can never use a shutter speed of anything higher than 1/200?

    Thanks for your help!
     
  2. Double H

    Double H TPF Noob!

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    Yes. You will have to depend more on adjusting the aperture/light positioning.
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    The shutter in these cameras is a focal plane curtain shutter. It's made up of curtains which open in two directions. They don't necessarily open at exactly the same time, there is a period of time where one curtain will be open while the other is still closed.

    The burst of flash is very fast/short...much shorter than the time that the shutter will be open. So the flash has to be timed to go off when both curtains are open...otherwise, you get what you have seen...a photo where only half of the scene is illuminated by the flash.

    The max sync speed is the fastest shutter speed at which the camera is able to have both shutter curtains open long enough to fire the flash.

    This isn't really a problem when the light level is low...but it may become a problem when shooting outdoors and it's bright. You can use a smaller aperture to keep the ambient light from over exposing your photo...but the smaller the aperture, the more flash power will be required.

    Some higher end flash units have a high sync feature. What this does is pulse the light like a strobe...allowing you to use a faster/shorter shutter speed. The trade off is a reduced range and increased battery consumption.
     
  4. the_beginner

    the_beginner TPF Noob!

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    Would a setting of Rear Curtain (or similar) help alleviate this problem?

    I'm still unclear on that setting, but I think it's supposed to fire the flash when the rear (2nd) curtain is opened. Is that accurate?
     
  5. Helen B

    Helen B TPF Noob!

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    Second curtain sync fires the flash just before the second curtain closes. The normal first-curtain sync sequence is for the first curtain to open, then the flash is fired when the first curtain is fully open (the second curtain starts out open). The second curtain then begins to close (if you are shooting at sync speed or slower). How soon it begins to close depends on the shutter speed. At the sync speed it will start to close just after the first curtain has fully opened. At longer speeds there will be a delay between the first curtain being fully open and the second curtain starting to close.

    Just for simplicity, suppose that the sync speed is 1/100 second. That means that in 1/100 second the first curtain can open, the flash can fire, and the second curtain can start to close. So it takes about 1/100 second (approx, slightly less) for a curtain to travel across the full distance. (notice that it actually takes 1/50 second (approx) total time to make a 1/100 second exposure in this case, though no one single point on the sensor/film is exposed for more than 1/100 second)

    If you have a shutter speed of 1/12.5 second with curtains that take 1/100 second to travel then the following would be the sort of thing that happens (timings approximate, for illustrating the point):

    Start condition: first curtain closed, second curtain open.
    0-1/100 second: the first curtain opens;
    1/100 second: first-curtain flash sync point;
    1/100 - 8/100 seconds: delay while most of exposure is made, curtains are both open;
    8/100 seconds: second curtain flash sync point;
    8/100-9/100 seconds: second curtain closes.
    End condition: first curtain open, second curtain closed.

    If the sync speed is 1/200, then the curtains will be able to open or close in about 1/200 second.

    The above applies to a focal plane shutter.

    Does that help?

    Best,
    Helen
     
  6. Helen B

    Helen B TPF Noob!

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    The answer to that was probably not very clear in my explanation above.

    If the shutter speed is sorter (faster) than the sync speed then the entire sensor/film are will not be exposed at any one instant - it will be exposed by a moving slit between the curtains. The second curtain will start to close before the first curtain is open. When the first curtain is fully open the second curtain will be partly closed. The width of the gap between the curtains determines the shutter speed.

    In the good old days there was a simple way round this: use a focal plane flash bulb. This was a bulb that burned at an approximately equal brightness for long enough for the exposure to be made. Sounds odd: if you want a faster shutter speed, use a bulb that has a longer burn time.

    The modern equivalent is to fire an electronic flash multiple times in quick succession so that a series of exposures are made as the gap between the curtains travels across the sensor/film.

    Some DSLRs take a completely different approach - if you set a speed shorter than the sync speed the exposure is made electronically by reading the sensor over a short period, while the physical shutter is fully open - just like the way a point-and-shoot digital works.

    Best,
    Helen
     
  7. the_beginner

    the_beginner TPF Noob!

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    Thanks! That makes total sense now. Way more helpful than "do this if your shot is like this" like the manuals tend to describe things. :)
     
  8. DSLR noob

    DSLR noob TPF Noob!

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    If you were aware of this phenomenon, then why did you shoot regardless? Sorry, I just can't stand the idea of someone risking their DSLR for a few experimental pics with flash.

    Be safe, get a dedicated flash unit for your D80 or a safe sync.
     
  9. Helen B

    Helen B TPF Noob!

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    Here is a rough diagram of the way in which the curtains operate. It is for a shutter with a sync speed of 1/200 second, at 1/12.5 second, 1/200 second and 1/1000 second. The timings are in thousandths of a second ( = milliseconds). In each case the entire exposure lasts for the indicated time plus the time it takes for a curtain to travel across the shutter.

    [​IMG]
     

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