Flash Question

Discussion in 'Photography Beginners' Forum' started by JimmyO, Feb 21, 2008.

  1. JimmyO

    JimmyO TPF Noob!

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    Hey!

    Ive been playin around with my sb-28dx on my d40x. I can only use it on auto becuase apparently the ttl mode is not combatible with my camera

    Anyways...When i take pictures i can only take them up to 1/250 shutter speed becuase if i go any faster only part of the picture will get the flash. So say i use 1/1000 only the top eigth of the picture is bright

    Why is this?
     
  2. Big Mike

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

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    These camera use a curtain (focal plane) type shutter. It has two curtains, one that travels up and one that travels down. So when you press the button, the two halves open to expose the sensor/film. They may not open at exactly the same moment but they will be open the same length of time, so that the exposure is even.

    Now, the flash is a burst of light that is much faster than the shutter....very short. So, for the flash to be effective, it has to fire when both shutter curtains are fully open.

    Because of the way the shutter is designed and how the flash triggers...it can only go to a maximum shutter speed, other wise, the flash is fired when both curtains aren't open...and that's where you get that band of unexposed photo.

    The fastest shutter speed is called the Max Sync speed. On older SLR cameras, it was only 1/60, so you should be happy with your 1/250. On cameras with a leaf type shutter, there isn't a max sync speed and you can use any shutter speed.

    If you put on a flash that was compatible with your camera (SB-600, for example) the camera would probably limit the shutter speed when the flash was attached.
     
  3. JimmyO

    JimmyO TPF Noob!

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    I just figured it would be combatable, bad idea :(

    Ohh well. Im planning on mainly using this off camera so i wouldnt have the i-ttl anyways....

    Thanks for getting back to me:hug::
     
  4. Socrates

    Socrates TPF Noob!

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    Incorrect. Both curtains travel in the same direction. Prior to pressing the shutter button they're both at the top. One moves down to open the shutter to the film/sensor. Then, after the correct delay, the other moves down to close the shutter.

    When you cock the camera (manually or automatically) for the next shot, both curtains are brought back to their starting positions.

    While reading further, bear in mind that an electronic flash is VERY brief.

    At high shutter speeds (brief opening times), the miles per hour speed of the curtains becomes relevant. The second curtain begins to close before the first curtain is fully open. Thus, you never have an instant when the shutter is 100% open. Instead, you have a traveling slit. If the flash fires, you illuminate only that slit, which begins when the flash is triggered (when the first curtain has reached its full open position).

    The limitation was worse with older cameras with curtains that moved sideways (because they must travel further).

    Leaf shutters do not have this limitation because there is always an instant when the shutter is 100% open.

    The primary benefit to a FP shutter is with NON-flash photos. Even with the traveling slit, the entire frame is exposed uniformly. A leaf shutter opens from the center and then closes back toward the center. Consequently, the portion of the frame closest to the center is always exposed more than the portion near the edges.

    Years ago, we had "FP flashbulbs," which were long-burning bulbs. The FP sync fired the flash and then opened the shutter and closed the shutter while the bulb continued to burn. SOME electronic flashes (e.g., Nikon SB-600 and SB-800 with the correct camera body) simulate long-burning bulbs with a series of a large quantity of low-power flashes.
     
  5. Big Mike

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

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    Thanks for the correction...I always though there was a quirk in how I understood those shutters to work.
     
  6. Socrates

    Socrates TPF Noob!

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    NP.

    I feel that maximum benefit can be obtained from ANY equipment, be it a camera or a car, if the user understands the principles of operation of the equipment.
     
  7. Big Mike

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

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    I guess that's why you are one of the world's best know philosophers. :D
     
  8. Helen B

    Helen B TPF Noob!

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