Flash Diffuser and Exposure Compensation Question

Discussion in 'Digital Photography' started by sanPietro98, Jul 10, 2011.

  1. sanPietro98 macrumors 6502a

    sanPietro98

    Joined:
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    #1
    I have a Canon T2i and I have the Canon EF 70-300mm IS f/4-5.6 attached with a Canon Speedlite 270 EX external flash. My question is that when I attach a $10 plastic diffuser to the Speedlite, the shutter speed shifts from 1/100 down to 1/2 second in Aperture Priority mode @ f/5.6. In order to compensate, I need to set the Exposure Compensation to -5.0. When I do this, the exposure and color are great and natural, but this doesn't make sense to me as to why just adding a diffuser affects the shutter speed so much.

    I realize that the camera has a flash compensation setting, but I could not find a good setting for this that kept the exposure and colors to look good. I'll admit that I don't understand what the flash compensation actually does differently.

    could someone explain what is going on inside the camera to so wildly over expose the photo with the diffuser? or what I could be doing wrong?

    Thanks in advance.
     
  2. Eaton Photos macrumors regular

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    Jun 23, 2010
    Location:
    KY
    #2
    In all seriousness, your problem is using Aperture Priority. When it comes to using Flash, you need to learn to use Manual. Using Manual, lets you control your Shutter & Aperture, and thus control your Exposure Compensation. You can also use your ISO to help with Exposure Compensation too. Whenever I am using Flash, whether it be Indoors or Outdoors, I use Manual.

    Ex; For an indoor photo, say you are looking for ambient lights behind your subject, then you could start with an S/S of 1/60th @ 5.6, and then check your histogram. If its too dark, then cut your S/S in half, down to 1/30th @ 5.6. Check your histogram again. If your image is beginning to resemble what you are looking for, but not light enough, then cut your S/S to 1/15th. Once you get below 1/15th you will start to do what is referred to as, "Dragging the Shutter". What this is doing is letting more ambient light in, while still utilizing the flash to freeze your subjects. It is a technique, that is commonly used in both Wedding & Night Club photography.
     
  3. OreoCookie macrumors 68030

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    Location:
    Sendai, Japan
    #3
    I second that: when you want to learn how to use the flash, you should choose aperture and shutter speed manually. If necessary, you should also choose the flash power manually, but at least on my camera, using flash exposure compensation is sufficient (exposure compensation is not necessary since I'm in full manual mode anyway).
     
  4. sanPietro98 thread starter macrumors 6502a

    sanPietro98

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    #4
    Thanks for the advice. I'll try that. But I'm also interested in trying to understand the optics/mechanics of what is happening so I can learn to predict the camera's exposure. without the diffuser, the camera's exposure is reasonable. Is the diffuser interfering with the AF assist beam? Or is the nondirectional light confusing the light meter?

    But thanks for the advice. I will definitely experiment as you both suggest.
     
  5. OreoCookie macrumors 68030

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    Location:
    Sendai, Japan
    #5
    This is a very rough explanation: If you take a photo with flash, you have to think about two exposure settings in a sense: (1) the ISO value, shutter speed and aperture determine the exposure of the background and (2) the flash power determines the exposure of the subject in the foreground. A lot is determined by the balance between `foreground' and `background exposure.'

    Hence, you have to shoot manual to make that balance. With dslrs, it's actually quite easy to learn how to balance things: switch to M, set exposure settings that are close to what the camera suggests and take a photo. If the exposure of the photo is close to the exposure you want for the background, you're good to go. (Never mind if the photo is blurry and don't hesitate the crank up the ISO or use a large aperture.) Then switch on the flash. If foreground and background are not very far away from each other, you also need to worry about the extra light coming from the flash.
    This has nothing to do with the `AF light beam' (I'm assuming you mean the IR light that is used to improve focussing in low light.) Modern flash systems communicate electronically all sorts of settings to the camera which then takes those into account when determining the exposure. My Nikon flash knows about the distance to the subject, the focal length I use, the aperture, etc. The built-in zoom and flash power are then chosen appropriately. Larger Nikon flashes also come with a diffusor and if you put that on, the camera takes it into account.* With third-party diffusors, this doesn't really work. Diffusors scatter the light over a larger area and this reduces the intensity of the light (the energy the flash dissipates is the same, but it is spread over a larger area). Hence you get an under exposure. You can counter that in two ways: adjust the flash exposure correction (not the normal exposure correction!) or set the flash power manually. If you shoot indoors and the light situation isn't changing a lot, you get more consistent results if you choose the flash power manually.

    * I'm fairly certain Canon flashes can do the same things, but I have no first-hand experience with them.
     
  6. gnd macrumors 6502a

    gnd

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    #6
    While (1) is correct, (2) is not quite, the flash part of the exposure is determined by the flash power and aperture. If you keep the flash power constant, you can change the flash part of the exposure by changing the aperture.
     
  7. Edge100 macrumors 68000

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    #7
    ...or the ISO. ISO and aperture have exactly the same effect on the ambient/flash balance.
     
  8. OreoCookie macrumors 68030

    Joined:
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    Location:
    Sendai, Japan
    #8
    You're right, I forgot to mention that.
    I guess the reason is that I typically set the exposure for the background which means I'm not changing the aperture anymore when I manipulate the flash exposure.
     

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