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Thread: Help needed on using flash

  1. #21
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    Originally posted by Flare


    In this case, since its not TTL... the flash needs to know how much light to output. The flash need to know the sensitivity of the film and the amount of light that is entering the camera. Then it fires a preflash (I guess) and judge the reflection of the subject and then emitts a flash that should correctly expose the pic taking the sensitivity of the film and the aperture into consideration.

    i.e If the aperture is larger, the flash out put should be lower compared to a case where the aperture is larger. And a High ISO film will require less light, so the flash has to emit a lower output.
    Almost there, but not quite. Most non-TTL flashes nowadays are thyristor controlled. They have a sensor on the flash body itself (somewhere near the IR thingie for AF flashes) to detect when there's enough exposure and shut off the flash. No preflash is required. Of coz, this is also calculated based on ISO and aperture selected.

    Regards
    CK

  2. #22
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    Opps... Hee Hee...

  3. #23
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    Originally posted by ckiang


    Almost there, but not quite. Most non-TTL flashes nowadays are thyristor controlled. They have a sensor on the flash body itself (somewhere near the IR thingie for AF flashes) to
    are there non-thyristor controlled flash with sensor?
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  4. #24
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    Originally posted by mpenza


    are there non-thyristor controlled flash with sensor?
    Don't think so. Those flashes with a sensor are all thyristor controlled, so why would you want one without?

    Regards
    CK

  5. #25
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    Originally posted by ckiang


    Don't think so. Those flashes with a sensor are all thyristor controlled, so why would you want one without?

    Regards
    CK
    no lah, I'm not thinking of getting another flash. what I have is good enough for the moment and for the forseeable future

    I was just wondering about your statement cos I have a flash with a sensor that does not have thryistor. What I understand about the thyristor is that it is just a store of charges, so that the excess charges could be used for later flashes. This allowed for much faster recycling time.

    Without a thryistor, any excess charges are drained and the flash have to recharge to full before it can fire off again. This however does not mean that sensors cannot be used to control the amount of light. It's just that excess charges are wasted, rather than stored.

    I might be wrong though
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  6. #26
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    Originally posted by mpenza


    no lah, I'm not thinking of getting another flash. what I have is good enough for the moment and for the forseeable future

    I was just wondering about your statement cos I have a flash with a sensor that does not have thryistor. What I understand about the thyristor is that it is just a store of charges, so that the excess charges could be used for later flashes. This allowed for much faster recycling time.

    Without a thryistor, any excess charges are drained and the flash have to recharge to full before it can fire off again. This however does not mean that sensors cannot be used to control the amount of light. It's just that excess charges are wasted, rather than stored.

    I might be wrong though
    Thyristor is basically a switch, like a transistor. Capacitor stores charges. What happens is this.

    1. Camera triggers flash.
    2. Flash capacitor output is dumped via a invertor circuit into the flash tube.
    3. Flash fires.
    4. Sensor reads the reflected light. Once the intensity is sufficient, sends a signal to the thyristor.
    5. Thyristor cuts off the capacitor from the invertor circuit.
    6. Capacitor still has charge left, so takes shorter time to recharge.

    The flash might not say "Thyristor", but I think if it has a sensor, it should have some form of thyristor or other similar circuitry inside.

    Regards
    CK

  7. #27
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    thanks for the clarifications
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  8. #28
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    Originally posted by mpenza
    thanks for the clarifications
    You're most welcome.

    Regards
    CK

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