A few questions concerning the flash

Discussion in 'Beyond the Basics' started by SlimPaul, Aug 10, 2009.

  1. SlimPaul

    SlimPaul TPF Noob!

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    Hi,

    I've been wondering a few things concerned with the flash. I hope you can help me. I shoot a Nikon D90 and an SB400.

    1. Can I increase (and how) the flash speed from 1/60 to let's say 1/200?
    2. Can I set the flash to automatically meter and fire with an appropriate amount of light (in A,S and M modes. When I choose the flash brightness manually, the shot often comes out too dark or overexposed)
    3. Is there a way to take photos of moving people in low light so that I can have both the person and the background properly exposed? When I shoot in a normal flash mode the background is dark, and when I shoot using slow/rear, the people come out blurry and have a ghosted look-alike coming out of them.

    I appreciate your help, thanks
     
  2. Big Mike

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

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    You can increase the shutter speed, up to the camera's flash sync speed, which might be 1/200 or 1/250. Any faster than that and you might get a black band in the photo, where the shutter curtain wasn't open all the way when the flash fires.
    The camera probably defaults to 1/60 for flash...but only in auto or P mode. Switch to manual or one of the priority modes and it probably won't do that.

    Does your flash have an TTL mode? If so, then yes.

    Flash photography is basically taking two photos at once. The first photo is the ambient exposure, it is controlled by the shutter speed and the aperture of the lens (ISO as well). So you are getting a dark background because your ambient exposure is under-exposed. Use a large aperture, a slower shutter speed and a higher ISO to increase the ambient exposure.
    The 2nd photo is the flash exposed part. It's exposure is controlled by the lens aperture and the power of the flash (not the shutter speed).
    Therefore, you can control the ambient exposure, independent of the flash exposure, by adjusting the shutter speed.

    When I shoot with flash, I put the camera into manual mode. This takes the camera's silly metering and defaults out of the equation. I do often leave the flash in E-TTL mode (Canon) so the flash exposure is automatic.

    The idea is usually to balance the flash and ambient exposure, so that you can avoid that 'dark background' look...but if you go too slow with your shutter speed, and if there is enough light, you will get blurry 'ghosted' images. It's always a trade off and learning to get it right will take some practice.
     
  3. KmH

    KmH Helping photographers learn to fish Supporting Member

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    This sounds backwards.

    Normal mode is front curtain sync. The flash fires as soon a the front curtain is open. If you are dragging the shutter (shutter speed less than the camera's flash sync speed) ambient light can be recorded as your subjects move and it looks like ghosting in front of them.

    With rear curtain sync the flash doesn't fire until just before the rear curtain starts to close. Any ambient light recorded would them appear behind the moving subject.
     
  4. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    You're misunderstanding. He's talking about exposing for the moving person or for the background. One he has just the person lit up and not the background, the other he shoots to the rear (background) with a slow shutter speed, the background is exposed properly, but the person is frozen with ghosting coming after the flash fires.
     
  5. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Ok, if the ghosting precedes the person's direction of motion, the camera is set for front/first curtain sync. If the ghosting trails the direction of motion, the camera is set to rear/second curtain sync.
     
  6. Dwig

    Dwig TPF Noob!

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    Yes, by setting the shutter speed yourself. You must use a mode that allows you to specify the shutter speed, either Manual or Shutter Priority. In Program and Aperture Priority the camera sets what it wants to use and you have no say in the matter.

    "Manual" means manual; you have to make the right choice. To get the camera and flash to negotiate and achieve a good exposure automatically, you have to set both to an automatic mode.

    No, not really. In low light there is not way to get a proper exposure for the ambient component except by using low shutter speeds, hence motion blur. Flash can't give the correct exposure for two different parts of the picture that are different distances from the camera (derived from Newton's Inverse Square Law). The only thing you can do to reduce, but not eliminate, the problem is to continue to use the slow/rear sync mode and also use very high ISO and manually (via Aperture Priority) select a very wide aperture, perhaps the lens' maximum aperture.
     
  7. JerryPH

    JerryPH No longer a newbie, moving up!

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    Nope, you cannot change flash speed. However, you can change SHUTTER speed. This can be done a couple of ways, by either going into a manual or semi-manual mode (like shutter or aperture priority on your camera)

    Your flash doesn't meter light... the camera does. Yes, it can automatically look at a scene, fire a test flash (also known as a pre-flash), measure it, set the power of the flash and then take the picture properly exposed even when in A and S modes. In manual mode, YOU are in control and make the decisions. The camera tries to help, but if you are too far off, there is not much that can be done (other than know what settings to change).

    Easy and comlex question at the same time.
    Flash, especially low powered flashes like the SB-400 are very limited in range. You cannot, for example, light a 20 X 20 foot room perfectly with your flash... it just is impossible.

    If you are trying to get a well exposed shot of the picture on the wall and your friend beside it, it is doable. If you are trying to get a picture of 25 poeple running by and you want all the people in the shot blur free and perfectly exposed... it is not going to happen.

    In low light scenarios, I can "freeze" moving people quite easily using flash (even while using 1/60th shutter speeds)... but then again, I get into more complex ways to control my light... such as by using off camera flashes and stronger ones as well.
     
  8. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    You're still not comprehending what he's trying to say.

    Obviously talking about the flash metering on the subject and lighting them and not the background. That would be with a faster shutter speed. Front/Rear curtain sync have nothing to do with this.

    Now he's slowing the shutter speed and focusing on the "rear". That means that he's exposing for ambient with the slower shutter speed and the flash is creating the ghosting.
     
  9. SlimPaul

    SlimPaul TPF Noob!

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    Thanks a lot for your responses guys. I'm so used to A mode that I totally forgot that I can set the speed up to 1/200 in S or M modes. Thanks for reminding me ;) I understand that I have to use pre-flash to have the camera select an appropriate flash power and freezing moving people at night just isn't gonna happen ;)

    Thanks again,
     
  10. JerryPH

    JerryPH No longer a newbie, moving up!

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    It is easy to make happen... its just knowing how and the limitations.
     
  11. iflynething

    iflynething TPF Noob!

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    I wish there was a lightbulb smiley! I would use it.

    I always tried to figure out how to get that ambient exposure right!

    Thanks Mike!

    ~Michael~
     

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