flash meter, light setup, and backgrounds

Discussion in 'Photography Beginners' Forum' started by texassand, Feb 23, 2007.

  1. texassand

    texassand TPF Noob!

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    Say I'm shooting indoors, with my alien bees lights, and I want a larger DOF, say F16 - would i just keep adjusting the strength of my light or moving it closer or farther from the subjects until my flash meter reads F16? Because I can't change from my flash sync speed right? I'm confused on if I can't change the shutterspeed, how I get the Fstop that I want. I just have a cheapy flash meter, nothing fancy.

    Basically, I am taking some shots of my pregnant best friend this weekend. Please take a look at some examples of what I want to do. These are for background questions only, I'm not composing these exact shots. The first - i love the dramatic lighting on this picture. How do I do that and how do i get the background so black? If I remember correctly, I need to put a lot of space between the subject and the background so there is no light on the background? Is that right?
    http://i57.photobucket.com/albums/g206/texassand/maternity8.jpg
    On the second one, how do I get the background so white? I know this one is tougher but anytime I use a white background, it always comes out grey and you can see the material. How do I blow it out like that?
    http://i57.photobucket.com/albums/g206/texassand/maternity13.jpg

    I have two alienbees lights, two silver umbrellas, on background light, and a rebel xt.

    Any guidance would be greatly appreciated!!

    Thanks!!

    Sarah
     
  2. Christie Photo

    Christie Photo No longer a newbie, moving up!

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    Yeah... forget the shutter speed. The flash is faster than your shutter. All you will do with the shutter is record other light in the room.

    I would think along the lines of varying the output of the lights. Other things happen as you move the lights in and out.

    For the high-key shots, the background should meter 2 and 1/2 times more than the subject. Remember... with that much light coming off the background, it becomes a light source in itself. You may need to move you subject away from the background if it provides too much bounce lighting.

    Pete
     
  3. Big Mike

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

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    Yes, if you want to shoot at F16, the you should adjust the flash output and/or the placement of the lights until the flash meter reads F16
    Shutter speed had no affect on the exposure from the lights. (as long as it's under the camera's max sync speed). The burst from the flash is probably 1/10000 of a second, much faster than the shutter...so changing the shutter will only change the ambient exposure...if there is any ambient light.

    To get the background to appear black or white...it has to be at least 2 1/2 stops different from your subject. So if you want black, then you need less light on the background and it helps to have a less reflective background (black velvet etc). For white, you need to have more light on the background...so add a strong background light or two.
     
  4. texassand

    texassand TPF Noob!

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    What do you mean by 2 1/2 stops? On the lights themselves? I need to look again at my alienbees, but I don't recall the power the of the light being referred to as "stops". I thought it was just 1/2 power, 3/4 power, etc.

    Also, placing background lights, if it's the white background for example,

    Do I put the light behind the background OR between the background and the subject pointing towards the background OR to the side of the subject but pointing at the background.

    For the black background - instead of changing the light output, why not just do completely without a light and just light the subject. Wouldn't this make the black background completely black and dark? Maybe I am not understanding.

    Thank you!!!
     
  5. Big Mike

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

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    Stops are the F stops on the aperture setting of the camera/lens. So if your subject is metered at F8...then one stop darker would be F5.6 and one more stop darker would be F4. So take a meter reading (flashes firing) at the subject...and then take another one of the background. The meter should also display F numbers...so it should be easy to figure out the difference.

    Put the lights in front of the background, pointing at the background. They can be behind the subject or off to the side or above etc.

    For a black background, you don't need a background light, that's right. But the problem is trying to keep the main & fill lights (to light the subject) from lighting up the background as well. Umbrellas can be bad for this because they spill light all over the place. If your walls, ceiling, floor etc. are reflective...then some of your main light will find it's way to the background. If you have the room...then moving your subject far away from the background is certainly a good thing to do.

    Also, a lot of the shots that you see...have had some post processing. If I can't get a background to look how I want it to, when shooting...then I will fix it with Photoshop :)
     
  6. Christie Photo

    Christie Photo No longer a newbie, moving up!

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    Mike fielded this one well. (Thanks, Mike!) I thought I'd expound on it a bit.

    With your back ground light OFF, determine the exposure for your subject. Let's say it's f11.

    Now, turn the BG light on, and adjust the power setting until your meter reads about f22 1/2. You should now have a white background.

    Think of it this way: the meter is telling you how to achieve GRAY... about 20% of the light falling onto the subject. So... if you want to achieve white, you need 5x the exposure (100%), 2 1/2 f stops.

    I hope I'm not confusing the issue. It's can be hard to picture all this in your mind when first doing it.

    Good luck!

    Pete
     
  7. texassand

    texassand TPF Noob!

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    ok, i undersand what you guys are saying.......as far as changing the stops, but here is where I am confused. I can only set my camera's aperature to ONE F-stop. So how in the heck would I have one light reading one F-stop - say F11 and the background light reading at F22? What would I set the f-stop to my camera on???
     
  8. shingfan

    shingfan TPF Noob!

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    you adjust the output level of the light...not the camera
     
  9. Big Mike

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

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    You set the F stop on the camera to expose the subject correctly...F11 in Pete's example.

    The F22 is the meter reading (well, so is the F11)...not the shooting aperture. We just use F numbers for metering because it's convenient.
     
  10. texassand

    texassand TPF Noob!

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    Ok so tell me if this is right:
    CAMERA:
    I set my shutter speed to flash sync speed which is 1/200 with Rebel XT's. I want a larger DOF so I would like F11 to accomodate my three subjects. I set my camera to F11.
    LIGHTS:
    Then with my strobes, I turn on ONLY my key/main light and adjust settings on that light until my flash meter reads F11. Then I turn on the background light and with BOTH the key and the backlight flashing, I adjust my background light until the flash meter reads F22 or so. Is this right?

    But won't that overexpose my subjects since the background is overexposed?
     
  11. Digital Matt

    Digital Matt alter ego: Analog Matt

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    The background is behind your subjects, and the exposure on it has nothing to do with your subjects. The only thing you need to worry about is too much light reflecting off of the background and onto your subjects. That's why they should be a few feet minimum from the background. The further, the better.
     
  12. texassand

    texassand TPF Noob!

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    Ok Big Mike,
    Now I'm really confused. I thought the meter readings tell me what to set as my shooting aperture - is that not right? What the heck do I set my camera at then?

    And Digital Matt - so do i not need to meter both the bg light and the key light at the same time?
     

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