Flash exposure

Discussion in 'Photography Beginners' Forum' started by Blind Bruce, Mar 6, 2016.

  1. Blind Bruce

    Blind Bruce TPF Noob!

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    Camera is set to Manual, flash to TTL, and auto ISO maxed at 6400. Test shot without flash is good but with the on board flash popped up,and the same setings, the shot is too bright. Why is this?


     
  2. wyogirl

    wyogirl Oh crop!

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    Your ISO for flash should be set at 100 usually.
     
  3. SCraig

    SCraig Been spending a lot of time on here!

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    Because you are taking a proper exposure and adding more light to it. You said that the exposure was correct without the flash, so naturally when you add more light on the subject with the same settings it is going to be overexposed.
     
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  4. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    Because your camera is set to manual, the exposure settings are fixed, so when you add additional light (the flash), you create over-exposure.

    Mmmm... that's a bit of a generalization. You can usually use a lower ISO with flash, but quite often it helps to increase ISO above base in order to get more benefit from the flash, save battery, or balance with ambient.
     
  5. Bebulamar

    Bebulamar No longer a newbie, moving up!

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    I wonder how flash and auto ISO work? Well for example the OP set the auto ISO max to 6400 and if he is in a dark environment the auto ISO for ambient light is almost certainly automatically set at 6400. Now when he turned the flash on, should the auto ISO automatically lower the ISO as it anticipates more light coming from the flash or it would stay at 6400?
     
  6. Alexr25

    Alexr25 No longer a newbie, moving up!

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    What were your manually selected shutter speed and aperture settings? There could be several reasons for the wrong exposure.
    If your shutter speed was set above the max flash sync speed (1/200 or 1/250 depending on camera model) then popping up the flash forces shutter speed to drop down to max sync speed even though you are in manual mode.
    Another possibility, and to my mind the most likely one is that the camera could not drop the flash output enough to give proper exposure. That is the problem you can run into when you are using auto ISO with flash and the reason I don't like auto ISO, the camera cranks up the ISO to give the correct ambient light exposure but then it can't turn down the flash output enough to prevent overexposure. Auto ISO in its present form does not seem smart enough to correctly handle flash.
     
  7. Blind Bruce

    Blind Bruce TPF Noob!

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    So, if I turn off the auto ISO and set it to 100, the flash output should increase to give a proper exposure, right?

    Life was silpler with my 4X5 speed Graphic and guide numbers.....not....
     
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  8. astroNikon

    astroNikon 'ya all Bananas I tell 'ya Supporting Member

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    In a "Studio" environment
    you would set a fixed ISO, such as 100 or 200.
    Then you would set the aperture and shutter for the specific value for the photo.
    Then you would set the one/multiple lights to obtain the type of lighting/shadows/high or low key / etc for the image that you want.
    Essentially you set everything yourself to obtain the type of image that you want.

    With a flash on top in TTL mode. If you set a fixed ISO the camera will compensate using the flash for lighting for proper exposure based on the cameras setting of ISO, shutter, aperture. Or if in one of the auto modes it will select it for you.

    If you are outside (or inside) shooting things that move fast (birds, sports, etc) and lighting changes quickly (and no flash because of distance) then AUTO ISO with MAX is the way to go.
     
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  9. beachrat

    beachrat No longer a newbie, moving up!

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    Guide numbers will always work my friend.
    You simply overshot with too much light.
    I'm positive of that.
     
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  10. weepete

    weepete TPF Supporters Supporting Member

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    Yeah, turn off ISO and manually meter and set exposure for the background. For some reason I get better results when I underexpose the background by around 2 stops on my light meter. I've yet to figure out why but I seldom do flash photography.

    If I remember correctly there's a custom function in my camera to set the time it takes to meter on a subject and set and exposure. I think that the lowest (on my camera) is 1/4 of a second. In this case your flash could be fireing before the metering sets the auto ISO for a correct exposure thus leading to an incorrect exposure. possibly....
     
  11. Braineack

    Braineack Been spending a lot of time on here!

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    you added light.
     
  12. Didereaux

    Didereaux Been spending a lot of time on here!

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    You didn't give the shutter speed you had set in the OP. I am betting it is above the synch speed of your flash. If so then the camera will automatically drop that speed to below the synch speed....thus you get over exposure plus the addition of the flash. If you wish to shoot manual then , as someone pointed out meter for the ambient using a speed slower than your synch speed (usually /200 or less) set your ISO to a reasonable one say 400 or less and take your readings. If the ambient is way low you may get a very slow shutter speed, if so you can use a tripod and shoot at those settings. The flash is very fast and so will 'stop' motion on your subject. This is an over simplification, but is basically the case. The thing to do is to set to Aperture priority, or shutter priority set a reasonable ISO, enable your flash and take a test shot. Look at the settings the camera made and go from there.

    Also see if your cameras built in flash supports hi speed synch, if so enable it. that will alllow for higher shutter speeds. But if your ambient light is low to begin with that is not the choice as high speed synch loses you about 2-3 stops of flash power.
     

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