Puzzling Flash Exposure Problem With Canon S100

Discussion in 'Digital Cameras' started by Mort, Jan 2, 2012.

  1. Mort

    Mort Guest

    Hi,

    While testing my new Canon S100, I came across the following exposure
    problem, which I cannot explain. (I am taking pictures for many
    decades,and am well aware of f stops, shutter speeds,and film/chip ISO
    speeds.)

    I was testing the built-in flash, using a bookcase in my study, and
    examining the focus and exposure under various conditions. On one
    picture, in P setting, I set the ISO at 200, used a medium focal length
    from its zoom range, and the picture was about 2 f stops overexposed. I
    then set the ISO to AUTO, and took the exact same picture under
    otherwise identical conditions, and its exposure was excellent, even
    though the ISO readout was 640, while the f stop and shutter speed were
    identical to the overexposed pic.

    How can this happen? I would expect ISO 640 to give almost 2 f stops
    extra exposure as compared with ISO 200, given the same f stop and
    shutter speed and distance. The battery was fully charged. My non-flash
    pix are all fine, as are most of my flash pix in non-testing circumstances.m

    Can anyone explain this to me? I would be most grateful for any helpful
    responses.

    Thanks.

    Mort Linder
     
    Mort, Jan 2, 2012
    #1
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  2. Mort

    wtrplnet Guest

    Don't know if this will help, but it's worth noting. I too am a new
    owner of the S100. If you haven't already done so download the manual
    from the Canon web site. It is far more complete than the one that comes
    with the camera. You may find help there.

    I'm in early testing of the camera and I haven't run into any problems
    with it. The results you relate are indeed curious.

    /)_
     
    wtrplnet, Jan 3, 2012
    #2
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  3. Mort

    Dudley Hanks Guest

    Just a thought:

    If the bookcase is darkly coloured, the meter might have averaged a dark
    area and caused the cam to over-expose the ISO 200 pic, which probably
    resulted in a heavy draw on the flash charging circuit.

    Depending on how much time you took to reset the cam to auto mode, the flash
    might not have been fully charged when you took the next, automatically
    exposed image, causing the camera to bump up the ISO and possibly fire the
    flash at less than full power.

    With an under-powered flash and high ISO, the end result ends up being a
    properly exposed image, more by a curious combination of factors than by
    accurate metering and calibrated firing.

    It's just a guess. But, you could retest by setting up the same scenario
    and reshooting, but leaving more time in between shots, and by shifting the
    metering points around for a few shots from different angles.

    Good Luck,
    Dudley
     
    Dudley Hanks, Jan 3, 2012
    #3
  4. Mort

    Mort Guest

    Hi,

    Thanks a lot for your reply. I did indeed download the full owners'
    manual from the Canon website. I make it a practice these days to
    download and print out manuals from electronic items, then have a
    Fedex/Kinko store bind them with tube binding, at a modest cost.

    Unfortunately, the manual offers no insight into my problem. It defies
    logic that with identical shutter speeds, f stops, focal length, and
    subject, that the lower ISO setting gives an overexposed pic while the
    higher ISO setting gives the proper exposure. This occurs both onscreen
    and in the prints, so it is not a PC screen problem.

    Inasmuch as the vast majority of my S 100 pix are excellent, I won't
    dwell upon this small problem.

    Thanks again.

    Mort
     
    Mort, Jan 4, 2012
    #4
  5. Mort

    Robert Coe Guest

    : wtrplnet wrote:
    : > On 1/2/2012 2:58 PM, Mort wrote:
    : >> Hi,
    : >>
    : >> While testing my new Canon S100, I came across the following exposure
    : >> problem, which I cannot explain. (I am taking pictures for many
    : >> decades,and am well aware of f stops, shutter speeds,and film/chip ISO
    : >> speeds.)
    : >>
    : >> I was testing the built-in flash, using a bookcase in my study, and
    : >> examining the focus and exposure under various conditions. On one
    : >> picture, in P setting, I set the ISO at 200, used a medium focal length
    : >> from its zoom range, and the picture was about 2 f stops overexposed. I
    : >> then set the ISO to AUTO, and took the exact same picture under
    : >> otherwise identical conditions, and its exposure was excellent, even
    : >> though the ISO readout was 640, while the f stop and shutter speed were
    : >> identical to the overexposed pic.
    : >>
    : >> How can this happen? I would expect ISO 640 to give almost 2 f stops
    : >> extra exposure as compared with ISO 200, given the same f stop and
    : >> shutter speed and distance. The battery was fully charged. My non-flash
    : >> pix are all fine, as are most of my flash pix in non-testing
    : >> circumstances.m
    : >>
    : >> Can anyone explain this to me? I would be most grateful for any helpful
    : >> responses.
    : >>
    : >> Thanks.
    : >>
    : >> Mort Linder
    : >
    : > Don't know if this will help, but it's worth noting. I too am a new
    : > owner of the S100. If you haven't already done so download the manual
    : > from the Canon web site. It is far more complete than the one that comes
    : > with the camera. You may find help there.
    : >
    : > I'm in early testing of the camera and I haven't run into any problems
    : > with it. The results you relate are indeed curious.
    : >
    : > /)_
    :
    : Hi,
    :
    : Thanks a lot for your reply. I did indeed download the full owners'
    : manual from the Canon website. I make it a practice these days to
    : download and print out manuals from electronic items, then have a
    : Fedex/Kinko store bind them with tube binding, at a modest cost.
    :
    : Unfortunately, the manual offers no insight into my problem. It defies
    : logic that with identical shutter speeds, f stops, focal length, and
    : subject, that the lower ISO setting gives an overexposed pic while the
    : higher ISO setting gives the proper exposure. This occurs both onscreen
    : and in the prints, so it is not a PC screen problem.
    :
    : Inasmuch as the vast majority of my S 100 pix are excellent, I won't
    : dwell upon this small problem.
    :
    : Thanks again.
    :
    : Mort

    Does the S100 have a focal plane shutter?

    Bob
     
    Robert Coe, Jan 5, 2012
    #5
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