Exposure Question (Flash and Exposure Compensation)

Discussion in 'Beyond the Basics' started by iflynething, Sep 3, 2008.

  1. iflynething

    iflynething TPF Noob!

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    Is there a difference in underexposing the flash and just turning it down?

    I'm using an SB-800 and you can hit the ( - ) on it and that give you a -2 or -1 or whatever with the EV. When I do that, am I actually turning the flash output down?

    One other question is when I over compensate or under compensate for a picture. What is that exactly doing and how does it effect (affect) the exposure. Wouldn't it come out the same way. What exactly does exposure compensation do. Any pictures of some of this done?

    Thank you

    ~Michael~
     
  2. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    In both instances you are adjusting what the meter measures for. Without any compensation set the meter measures for middle gray. With exposure comp set to +1 the meter is measuring for 1 stop brighter than middle gray, -1 means 1 stop darker than middle gray.
     
  3. iflynething

    iflynething TPF Noob!

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    Oh alright. So the photo still may look correctly exposed?

    ~Michael~
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    Yes mostly the camera will expose normally except impose a 1/60th minimum shutter speed. The flash then works separately and meters separately.

    So you're outside, the scene may look correct normally. If you enable the flash nothing changes on the camera and the flash (since the scene is already bright) fires with very little power.

    But when you're inside the camera clamps the shutter speed to the camera would naturally under expose because 1/60th is simply too fast for the environment in this example. The flash would then fire much harder to expose the image correctly.

    Now when you change the flash exposures this is done regardless of the camera. The camera does nothing to its exposure. However if you change the scene exposure on the camera, (at least with Nikon iTTL) the flash also follows this to make sure that only the flash compensation affects the flash:environment ratio, and the normal compensation just affects overall image exposure.
     
  5. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    It depends on the subject, lighting, and the opinion of the photog as to what "correct exposure" is. Don't worry about exposure compensation or flash compensation until you understand how the meter works. It'll make a lot more sense then.

    http://www.cambridgeincolour.com/tutorials/camera-metering.htm
     
  6. Big Mike

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

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    Every flash photo is actually two exposures...one is the ambient and one is the flash exposure. Most times, they overlap perfectly or one is a lot stronger than the other one...so you don't see two exposures.

    The camera's regular meter measures the ambient exposure and you can control that with EC. It's a result of the aperture, shutter speed and ISO. The flash exposure is metered by the camera via a pre-flash and is controlled by the aperture, ISO and flash output. So when you are adjusting the FEC (flash exposure compensation) you are adjusting the flash output which has no effect on the ambient exposure.
    Similarly, you can adjust the ambient with no affect to the flash exposure, by changing the shutter speed.
     
  7. iflynething

    iflynething TPF Noob!

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    Got ya Big Mike. So bringing down the flash is actually decreasing the flash output.

    So, what good is exposure compensation on the camera, if the picture is still going to come out correctly exposed?

    ~Michael~
     
  8. Big Mike

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

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    That will affect the ambient part of the exposure.

    For example, if you are shooting people outside near dusk. The flash will light up the people in front of you but won't light up the landscape or the sunset behind them. You can adjust the FEC to tune the exposure of the people and you can adjust the EC to suite the exposure of the background that you want.

    This is why I always shoot in manual mode when using flash. Whatever aperture I set, the E-TTL metering of the flash will control the flash output for that aperture (then FEC to taste). Then I can change the shutter speed to independently control the background (ambient) exposure.
     
  9. iflynething

    iflynething TPF Noob!

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    Got you, so if I'm wanting the backgroud to be more exposed in a portrait shot (or any for that matter) just overexpose it to get the background a little brighter?

    Thanks for the help Big Mike.

    ~Michael~
     
  10. Big Mike

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

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    That's the idea. But remember that you would be increasing the exposure of anything lit by ambient light. So if your subject is also lit by ambient light, they will get brighter as well. My example works best when it's dark and there is little ambient light on the subjects.
    This is also an issue if your shutter speeds get too long. For example, when shooting in near dark, the shutter speed can be really long and the flash will still freeze the subjects...but if there is enough ambient light, the subjects will show up in the ambient exposure and probably be blurry from camera shake or their movement. The flash exposure will be separate and should be sharp...but the two will overlap and give you funny results. I do this on purpose sometimes, to convey movement (at a wedding reception for example)...but note that you should have the flash set to rear/2nd curtain sync when doing this.

    Sorry....when on a bit of a tangent there :er:
     
  11. Bifurcator

    Bifurcator TPF Noob!

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    If I wanted the BG brighter I would use a second light source between the subject and the BG if possible.
     
  12. Big Mike

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

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    That would work, and I do that for studio shots and even at wedding receptions. It doesn't work when your background is a landscape though. :er:
     

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