why did my camera use such a large aperture when I used the flash?

Discussion in 'Photography Beginners' Forum' started by skywalkerbeth, Feb 14, 2008.

  1. skywalkerbeth

    skywalkerbeth TPF Noob!

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    I took a photo last weekend and I finally had to use the flash... I set the camera to automatic and the flash popped up. The aperture was set to 2.8 though! The resulting photo of the group of firefighters has the middle guys really in focus, with the end ones out of focus (I took it standing off to the side, since I have the 35mm prime and that was the only way to get them all in).

    Since this was automatic - I didn't set anything - why did the camera select 2.8 and how do I override that?
     
  2. JimmyO

    JimmyO TPF Noob!

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    Im not familiar at all with your camera, but there should be a shooting mode where you can set everything manually, and it should be marked with an "M" on the main dial. Refer to your manual for how to do this
     
  3. EOS_JD

    EOS_JD TPF Noob!

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    In Auto mode i think the camera tries to meter for the scene - without taking into account the flash. So if it was dark, it will open up the aperture to let enough light in for a correct exposure.

    In order to use a smaller aperture, use Av mode (aperture priority) and set the aperture yourself. Flip up the flash by pressing the adjacent button. In semi-manual and manual mode the flash will not pop up automatically.

    Alternatively use M and set the Aperture and shutter speed yourself - remember to pop up the flash yourself though. The pop up flash is not that powerful so if you find it underexposes you may need to increase your ISO.
     
  4. skywalkerbeth

    skywalkerbeth TPF Noob!

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    Thank you - I use manual all the time - I just have never used the flash before! I will experiment.
     
  5. Big Mike

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

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    Actually, it does that in Av or TV...but not in auto...otherwise the shutter speed would have been very slow.

    In auto, the camera will probably set a predetermined shutter speed (1/60 I would guess). Then it sets a large aperture (often the max, but maybe it's limit is F2.8). This probably won't be enough to get the exposure (when it's dark) so the flash does the rest.

    Flash exposure is controlled by two things, the power of the flash and the aperture of the lens. So the larger the aperture, the less power needed from the flash. So actually, if you had used an aperture of F8 (to get more DOF), the flash may not have been powerful enough.

    When in Av or Tv, the camera will give you settings for the exposure, as is you didn't have the flash on. So you will probably end up with a long shutter speed. The flash is more of a fill light for the subject, which can be good but beware of what long shutter speeds can do.

    I put the camera into Manual when using flash. This way, you set the aperture that you want for the DOF that you need. The flash will put out the proper amount of light for that aperture (as long as it's within it's capabilities). The shutter speed is then used to control the amount of ambient (background) light that is recorded. A longer shutter speed will give you more ambient exposure (so your subjects don't look like they are in a cave)...but you do have to be aware that too long of a shutter speed (with too much ambient light) will give you blur or ghosting.

    Also, you can use FEC (flash exposure compensation) to make the flash put out more or less light.
     
  6. JerryPH

    JerryPH No longer a newbie, moving up!

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    Besides all that is said here, no one addressed the fact that the on camera flash are not very effetive at lighting very wide. First they are of limited use at anything beyond 10 feet and maybe 6-8 feet wide before light quality drastically starts to fall off. Also since the camera was using the complete potential of the flash, it also metered that it could not do so until it opened up the lens to it's maximum, so whatever you were trying to shoot, for it to be able to light up what it saw, an F/2.8 was needed. The fact that you had a narrow DOF is just a result of this.

    You may have had better luck going to aperture priority mode and doing some post processing in the pic, but its hard to be more specific without knowing the details of the picture.
     
  7. EOS_JD

    EOS_JD TPF Noob!

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    Sorry Mike.... Yes you are right there. I knew P mode set the shutter speed to 1/60th so I should have known Auto would do the same!!
     

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