Difference in exposure between automatic and manual modes

Discussion in 'Photography Beginners' Forum' started by Bluepoole, Jun 24, 2009.

  1. Bluepoole

    Bluepoole TPF Noob!

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    Hi all

    I've bought a Canon 1000D, and was playing around with the settings. I was taking pictures of a person standing against a white wall.

    I then noticed that there was a difference in exposure of the wall between full automatic mode, and manual (M) mode. With full auto, the wall was exposed 100% on both sides of the person. With manual mode, the side which received some light from a window was correctly exposed but the wall on the other side was noticeably darker (not really underexposed, but still, there is a definite difference).

    It is strange to me because the settings for both shots were identical: lens used was the Canon 17-85mm IS (same focal length for both shots), ISO 400, 1/60 and f5.0. External flash was used, Canon 430EX MkII, 60 degree head tilt, flash bounced off the ceiling. Flash was on full auto mode for both shots.

    The person was correctly exposed in full auto mode, but also darker in manual mode.

    So why is there a difference in exposure on the one side of the wall? I'm very curious to learn about issues like these so any input would be appreciated!
     
    Last edited: Jun 24, 2009
  2. Cool G5

    Cool G5 TPF Noob!

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    AFAIK It is because of metering.
     
  3. benlonghair

    benlonghair TPF Noob!

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    It's because your meter is trying to make the entire photo 18% gray. It takes an overall reading of the area and takes an average.

    In manual, you meter off your subject and then expose and end up with a white wall and correctly exposed subject. (It might meter differently in manual also.) I would highly reccomend Understanding Exposure. It will explain everything. Well, almost everything. Personally I thought I knew what I was doing before reading it. Now I have no idea if you know what i mean.

    Can you post the pics with EXIF data?
     
  4. Steph

    Steph No longer a newbie, moving up!

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    If aperture and shutter speed were the same in both manual and auto and you don't get the same result, then the flash output was different in both cases. Read you flash manual. It probably works differently when the camera is in auto or manual modes.
     
  5. Bluepoole

    Bluepoole TPF Noob!

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    Thanks for the replies so far...

    I hear what you guys say about metering...but I've double checked and the metering was also the same (evaluative metering) for both shots.

    Steph, I also wondered about the flash output, I know for certain that the flash was on full auto mode for both shots - will try the setup again with the flash on manual mode and the same settings for both shots, and see what happens.

    One mistake I made in my original post (I've corrected the post now), was to say that the person was exposed the same in both shots (but actually, he was also underexposed in manual mode). Took a lot of shots, so I did not check thoroughly on that detail!

    I will try and post the photos later on, need to download it to my PC first. In the mean time, any other suggestions or input would also be welcomed.
     
  6. Steph

    Steph No longer a newbie, moving up!

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    What I am trying to say is that the output of the flash (when set to auto) might be different when your camera is set to auto or manual. I don't know for sure as I am not familiar with the Canon flash system. This should be explained in your flash manual or maybe here.
     
  7. musicaleCA

    musicaleCA TPF Noob!

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    I think you've got it right Steph. I've got a bunch of Canon flashes though and the manuals don't really go into explicit detail on the mechanics of ETTL.

    That said, the only variable setting here was the flash output, so my guess is the ETTL changed the flash output between the two shots. ETTL's great, and super-fast, just, not very good if you want consistent exposures. :-/
     
  8. johngpt

    johngpt No longer a newbie, moving up!

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    Bluepoole, you're experiencing something that had bugged me as well as I'm learning how to work with flash.

    I'm inserting a link to a site that was extremely helpful to me. Take your time and explore his site. There is a lot of quite useful information.

    http://www.planetneil.com/tangents/flash-photography-techniques/

    I now almost exclusively go to manual mode when utilising flash, whether built-in, or external.
     
  9. Big Mike

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

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    Yes, it seems that the variable aspect is the E-TTL mode on the flash. It does cat somewhat differently, depending on the mode that it's in.

    You can read just anything there is to know about Canon flash in The Canon Flash Bible.
    (it's also good reading if you are having trouble falling asleep ;)
     

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