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Thread: flash and f stop

  1. #1

    Default flash and f stop

    hello,
    if i'm using external flash and since the shutter speed is fixed at 1/60. what is the difference if i adjust the aperture between f/2.8 and f/22?
    what kind of effect will i get?
    how do i get a very dark background and correctly exposed subject in the photo?

  2. #2

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    Quote Originally Posted by nature
    how do i get a very dark background and correctly exposed subject in the photo?
    use manual exposure mode

    shutter speed set 1/60 and faster

    aperture set around f2.8 or f4 can already

    direct or bounced flash up to u

  3. #3
    Moderator ortega's Avatar
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    actually it depends on the flash mode setting that you are using.
    In M mode the distance between the flash and the subject will affect the aperture to use.

    Shutter speed to use depends on what your camera can or cannot do.
    Some cameras can go up to 1/500 (D70). Useful for daytime shoots.

    But as a guide 1/60 or slower. The slower the shutter speed gives you more
    exposure for ambient light. (My preference)

    To get a very dark background, choose the fastest shutter speed
    that your camera can use. and a smaller aperture (less ambient light)
    Direct flash only, bounce flash will light up everything, including the background.

    Experiment using your own equipment, as we do not know what you are using and what is the shooting situation.

  4. #4

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    Quote Originally Posted by nature
    hello,
    if i'm using external flash and since the shutter speed is fixed at 1/60. what is the difference if i adjust the aperture between f/2.8 and f/22?
    what kind of effect will i get?
    how do i get a very dark background and correctly exposed subject in the photo?
    If ISO, focal length, shutter speed are fixed and aperture values varies, you should get constant exposure if you are using a good flash. Varies aperture will result in DOF issues. Example, a group shot taken in f2.8 will get soft quality.

    For dark background, position your flash head direct to the object and use faster shutter speed.
    See my Photo Gallery at the Clubsnap

  5. #5

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    Quote Originally Posted by ortega

    To get a very dark background, choose the fastest shutter speed
    that your camera can use. and a smaller aperture (less ambient light)
    Direct flash only, bounce flash will light up everything, including the background.

    Experiment using your own equipment, as we do not know what you are using and what is the shooting situation.
    so that means if i use manual, i set the fastest shutter, 1/125 and smallest aperture, f/22, and use direct flash, i will get the dark background and correctly exposed subject, right?

    but manual usually doesn't give you the right exposure if i'm not using flash, so does this means that the speedlight will calculate the correct amount of flash to fire to get the correct exposure for the subject?

  6. #6

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    Quote Originally Posted by nature
    so that means if i use manual, i set the fastest shutter, 1/125 and smallest aperture, f/22, and use direct flash, i will get the dark background and correctly exposed subject, right?
    Logically you are right if the flash is strong enuff to light up 1/125sec and f22 settings. Usually people will set faster the shutter speed instead of using smaller aperture values to darken the background due to quality issue. ( Depend on lens MTF, certain range of apertures like f8 will give best quality )

    Quote Originally Posted by nature
    but manual usually doesn't give you the right exposure if i'm not using flash, so does this means that the speedlight will calculate the correct amount of flash to fire to get the correct exposure for the subject?
    Yes, that is the best part of intelligent flash.
    Last edited by megaweb; 15th March 2005 at 08:52 AM.
    See my Photo Gallery at the Clubsnap

  7. #7
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    Nature, if I am not mistaken, you are refering to one of those pics that you take at the beach, trying to capture the lights from the ship with someone in front but end up only getting the person correctly exposed with a very big black piece of cloth.
    If this is it, then it is because your camera was set too fast to capture the scenery.
    With flash photography, especially manual external flash (not one of those dedicated ones) shutter speed plays very little by way of getting the subject that is within the flash range correctly exposed. Whether you set the shutter speed from 1/60 to holding the shutter open in bulb mode, your subject will still be correctly exposed (provided your aperture to subject distance is correct per the table on the flash). This is because the speed of the flash is too fast for the shutter speed to make any difference, usually about 1/20,000 - 1/30,000.
    But as you continue to extend your shutter exposure, you'll start to notice something. . .your background starts to get brighter and brighter. That's because long after the flash has extinguished, your shutter is still kept open to let the dim natural light in the distant to continue to expose your film or ccd if you are taking with digital.
    As a simple rule, in such situations, you usually set the aperture to suit the subject to flash distance and then set the shutter to correctly expose the background.
    But a word of caution, if you are not taking in complete darkness and your subject is not standing still, there will be some ghosting effect around him/her.

  8. #8
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    Oops didn't think I was writing that much :P
    Anyway, Nature, As I was saying above, if you still would like to take such pictures in that kind of situation, you can always make your subject duck out of sight after the flash has gone off. To minimise the ghosting effect, try using darker coloured outfit.
    I hope I have not given you too much information overload

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