Bad exposure with 1/60 sync and flash

Discussion in 'Beyond the Basics' started by cumi, Feb 6, 2006.

  1. cumi

    cumi TPF Noob!

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    I have a Canon PowerShot A520 and shoot mostly in aperture-priority mode. When I use the (build-in) flash and aperture-priority mode, the photos are mostly overexposed. I think it's because, the shutter-speed is not changing when I change the aperture. It stays allways 1/60 sec, regardless how open is the aperture.

    What am I doing wrong? Is this normal? How should I use the flash and how can I live with constant 1/60 sync time? Is maybe the P-mode better?

    I recognised, that when I move a bit back and zoom to subject (also bigger F-Number), the photos are not so overexposed anymore.

    Any experiences?

    Thanks,
     
  2. Azuth

    Azuth TPF Noob!

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    If you can't adjust the power of the flash (perhaps a fill-in setting?) you're probably a little stuck. You need to set the aperture correctly to expose for the flash power. If you can't manually adjust the aperture then it's an issue.
    Can you adjust the ISO setting?
    Or perhaps some sort of exposure compensation?
    Or try using something to difuse the flash?
     
  3. cumi

    cumi TPF Noob!

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    I can adjust the flash power only in manual mode (3 stops).

    Why doesn't do this the camera for me? Without flash it does it!

    Maybe, I should take the photos in shutter-priority mode and set 1/60 sec to have the correct aperture set automatically...?

    I can do it, but the problem is that the shutter speed stays the same. Normally, without flash, when I move 1 step lower with aperture, the shutter moves 1 step higher automatically. That's how the aperture-priority mode works. The problem comes with flash, when the shutter speed stays always 1/60, regardless if I move 1 or 10 steps lower or higher with aperture. The reciprocity is not working...
     
  4. Rob

    Rob TPF Noob!

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    Generally speaking cameras have a constant fixed synch speed usually around the 1/60th to 1/500th range. The aperture affects the exposure, but with a built-in flash, the flash power should be stepped accordingly. It sounds odd that it doesn't work as-is.

    Perhaps your camera does not permit flash synch in AP mode, but I would be surprised if this were the case.

    Do you have another adjustment for the flash output? If you can alter flash power using EV or stops, then try stopping the flash back at wide apertures to avoid overexposure. Or, stick a rizla or something over it!

    Rob
     
  5. cumi

    cumi TPF Noob!

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    An example:

    1.
    [​IMG]

    Face overexposed.

    File: IMG_0800.JPG
    File size: 941KB
    Image Serial Number: 107-0800
    Camera Model: Canon PowerShot A520
    Firmware: Firmware Version 1.00
    Date/Time: 2005:10:21 18:57:46
    Shutter speed: 1/60 sec
    Aperture: 2,6
    Exposure mode: Program
    Flash: Built-in + Red eye reduction
    Flash EF guide number: 3,28
    Metering mode: Center-weighted average
    Drive mode: Single frame shooting
    ISO: Auto
    Lens: 5,8 to 23,2mm
    Focal length: 5,8mm
    Subject distance: 0,41 m
    AF mode: Single AF
    Image size: 1704 x 2272
    Rotation: none
    Image quality: Fine
    White balance: Auto
    Picture Effect: Normal



    2.
    [​IMG]

    Moving a bit back and zooming using a F5 instead of F2.6 and it looks a bit better.

    File: IMG_0801.JPG
    File size: 895KB
    Image Serial Number: 108-0801
    Camera Model: Canon PowerShot A520
    Firmware: Firmware Version 1.00
    Date/Time: 2005:10:21 18:58:00
    Shutter speed: 1/60 sec
    Aperture: 5
    Exposure mode: Program
    Flash: Built-in + Red eye reduction
    Flash EF guide number: 6,31
    Metering mode: Center-weighted average
    Drive mode: Single frame shooting
    ISO: Auto
    Lens: 5,8 to 23,2mm
    Focal length: 18,7mm
    Subject distance: 0,97 m
    AF mode: Single AF
    Image size: 1704 x 2272
    Rotation: none
    Image quality: Fine
    White balance: Auto
    Picture Effect: Normal


    One more thing: what does the Flash EF guide number means? Why is it double on the second photo?
     
  6. Rob

    Rob TPF Noob!

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    The flash guide number is the camera altering the output of the flash. Essentially, the number is the f stop multiplied by the flash to subject distance at 100 ISO. In this case the distance is measured in meters.

    First Photo: GN 3.28 = f 2.6 * 0.41 m (ISO unknown) This is obviously a camera cock-up as a GN of 1 would be appropriate at this aperture and distance. Hence the picture is over exposed.

    Second Photo: GN 6.37 = f 5 * 0.97m (ISO unknown) This is about right as the GN works out at about 4.85.

    Remember that the ISO needs to be known to calculate exactly. It looks like your camera isn't very good at close range flash shots at a wide aperture. No real surprise there, just move back a bit and zoom to narrow the field of view and the aperture.

    Rob
     
  7. Rob

    Rob TPF Noob!

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    If you set the ISO to exactly 100 (if possible) then you should be able to confirm the maths easily. You can compensate if you know the ISO is say 120, but the formula gets a little more complicated.

    Rob
     
  8. cumi

    cumi TPF Noob!

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    So I didn't do anything wrong.

    The manual says the flash can be used from 0,45m. Maybe I was 4cms to close... :lol:

    And actually the manual also says, that the flash sync is between 1/60 and 1/500, but I've never seen something different than 1/60... :(
     
  9. markc

    markc TPF Noob!

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    Cumi, I hope you don't mind. I'm not posting this as an edit. I just wanted to show you the difference between wide angle and telephoto. I got the size and exposure close to make an easier comparison.

    I'm guessing the one on the right looks more like your son really does. Wide angles exagerate distances from the lens. Notice how his face looks like it's been inflated like a baloon? His nose looks bigger on his face and his ears smaller. When shooting portraits, it's usually best to use a slight tele.

    [​IMG]
     
  10. cumi

    cumi TPF Noob!

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    Thanks markc! This was also a reason, why I took the second photo. You illustrated the difference perfectly, could be a nice school-example! :thumbup:

    Any ideas for the reason of the overexposed face on the first one?
     
  11. markc

    markc TPF Noob!

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    If you were closer than what is recommended for the flash, the flash output may not be able to drop low enough. You can get pretty harsh light by being too close. And because of the odd way light works (inverse square law), you notice light drop-off much quicker. See how the bed goes so dark compared to him? If you are .41m from him, the light will be 1/4 strength at .82m. At .97m, the light doesn't dropt to 1/4 strength until 1.94m. The graph looks like this.

    A flash's output drops remarkably just out of the flash. Sunlight has already traveled an amazing distance, so several meters will have no visible effect on it's dropoff.

    .41m is only .04m from the minimum recommended .45m, but .04 is 10% of .45m. If my math is right, that's something like 22% brighter.
     

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