Flash Exposure Compensation in Canon A590IS

Discussion in 'Canon' started by Peabody, Feb 21, 2009.

  1. Peabody

    Peabody Guest

    I'm new to digital photography, and I'm trying to get familiar with
    my new A590. The camera has an overall Exposure Compensation
    setting as well as a separate Flash Exposure Compensation setting.
    The manual says this:

    "You can combine the flash exposure compensation with the camera's
    exposure compensation function for balanced exposure of the
    background when shooting with flash."

    I would like to understand exactly how these settings work when
    flash is used. The problem is that shooting indoors with flash in a
    dimly lit room, the main EC setting has no effect. The flash EC
    setting does have an effect if the ISO is 200, but not at 80. And,
    no matter what these settings are, it's always 1/60, F2.6. So I'm
    confused.

    Does the camera adjust the shutter speed during the exposure, or is
    it fixed before I push the button all the way down?

    Is it possible I have a setting wrong? I'm in Program mode,
    evaluative metering.

    But in general, how are these two settings supposed to function when
    flash in used? What situation could I photograph that would clearly
    illustrate how they work?

    By the way, I have to say I'm impressed by this camera - the things
    it can do for my $100 investment. And that's even before CHDK.
     
    Peabody, Feb 21, 2009
    #1
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  2. Peabody

    Robert Coe Guest

    : I'm new to digital photography, and I'm trying to get familiar with
    : my new A590. The camera has an overall Exposure Compensation
    : setting as well as a separate Flash Exposure Compensation setting.
    : The manual says this:
    :
    : "You can combine the flash exposure compensation with the camera's
    : exposure compensation function for balanced exposure of the
    : background when shooting with flash."
    :
    : I would like to understand exactly how these settings work when
    : flash is used. The problem is that shooting indoors with flash in a
    : dimly lit room, the main EC setting has no effect. The flash EC
    : setting does have an effect if the ISO is 200, but not at 80. And,
    : no matter what these settings are, it's always 1/60, F2.6. So I'm
    : confused.
    :
    : Does the camera adjust the shutter speed during the exposure, or is
    : it fixed before I push the button all the way down?
    :
    : Is it possible I have a setting wrong? I'm in Program mode,
    : evaluative metering.
    :
    : But in general, how are these two settings supposed to function when
    : flash in used? What situation could I photograph that would clearly
    : illustrate how they work?
    :
    : By the way, I have to say I'm impressed by this camera - the things
    : it can do for my $100 investment. And that's even before CHDK.

    Try using a higher ISO setting indoors. I don't know how far up you can go on
    that model before noise becomes a problem, but 400 ought to be usable.

    All(?) Canons seem to prefer you to expose for the ambient light and treat
    flash as fill. It's their reaction to the common complaint that flash pictures
    tend to show a brightly lit foreground, often with blown highlights, and a
    dark, muddy background. If you expose for the background, the camera/flash
    will try to compensate to avoid making the foreground too bright. Expensive
    SLRs do this with through-the-lens metering; cheaper cameras have to rely on
    EC. It appears in your case that the available EC is insufficient to
    compensate for the low ISO settings.

    The camera has two constraints when setting the shutter speed in P mode: keep
    it high enough to avoid a blurry image and low enough to keep the background
    from being too dark. (Cameras with focal-plane shutters have a third
    constraint, but that shouldn't be relevant here.) The settings it chooses for
    you (wide open with a slow shutter speed) suggest that the operative
    constraint is avoidance of a dark background, so a higher ISO setting should
    help.

    A shutter speed of 1/60 could present challenges. For example, it's usually
    too slow to use on young children, who can rarely be induced to hold still
    that long (an almost insurmountable problem if there are more than one of them
    in the picture). In such cases you may wish to switch from P mode to
    shutter-priority mode ("Time Value" mode in Canonese). But if you do, you'll
    have to pay even more attention to your EC and ISO settings.

    If you want more technical detail, Google for Gisle Hannemyr's Web site. He
    wrote a paper a few years ago that provides a detailed exposition of Canon's
    thinking regarding flash.

    Bob
     
    Robert Coe, Feb 21, 2009
    #2
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  3. Peabody

    Peabody Guest

    Robert Coe says...
    What's not clear with respect to this model camera is what
    kind of EC is involved in flash pictures. In Program mode
    with ISO set to Auto, and flash set to ON, when I push the
    button half way down, it almost always selects ISO 200, 1/60
    sec, and F2.6, which is wide open. The only exceptions
    are if the scene is brighter than that setting, in which
    case it will select some other appropriate combination.

    What I can't tell is whether the camera makes any further
    adjustments while actually making the exposure. So far as I
    can tell, the A590 has no pre-flash, so it seems the only
    other possibility is if it shuts off the flash in real time
    when the aggregate light reaches a certain level. I can't
    tell if that's happening or not. If not, then the flash EC
    may be nothing more than a flash power level control. I
    just can't tell.
    I've found this site of his:

    http://dpanswers.com/canon_flash.html

    and I think I understand Canon's way of handling flash. I
    just wish I knew more about what the A590 actually does.

    Upon reflection, it seems that my previous tests didn't have
    the background natural illumination at a high enough level
    to make any difference. So I think I need to try everything
    again with more light.

    Everything is further complicated by the Scene settings
    Night Snapshot and Night Scene, and of course Canon says
    nothing specific about what the Scene settings actually do,
    in technical terms I mean.

    Thanks very much for your reply.
     
    Peabody, Feb 21, 2009
    #3
  4. Peabody

    Robert Coe Guest

    : Robert Coe says...
    :
    : > All(?) Canons seem to prefer you to expose for the
    : > ambient light and treat flash as fill. It's their
    : > reaction to the common complaint that flash pictures
    : > tend to show a brightly lit foreground, often with blown
    : > highlights, and a dark, muddy background. If you expose
    : > for the background, the camera/flash will try to
    : > compensate to avoid making the foreground too bright.
    : > Expensive SLRs do this with through-the-lens metering;
    : > cheaper cameras have to rely on EC.
    :
    : What's not clear with respect to this model camera is what
    : kind of EC is involved in flash pictures. In Program mode
    : with ISO set to Auto, and flash set to ON, when I push the
    : button half way down, it almost always selects ISO 200, 1/60
    : sec, and F2.6, which is wide open. The only exceptions
    : are if the scene is brighter than that setting, in which
    : case it will select some other appropriate combination.

    I'm not a fan of auto-ISO, and I certainly wouldn't use it indoors with flash.
    To minimize noise, the camera probably picks the lowest setting it thinks you
    can get away with. And if the background is fairly bright, it may be
    encouraging the low setting. You might get some relief by switching to
    centerweighted metering, but the simplest solution os to turn anto-ISO off and
    select a more realistic value, e.g. 400. It may even be that auto-ISO is
    taking your EC settings into account. That would seem to defeat the purpose of
    EC, but manufacturers sometimes make strange design decisions. (My 50D manual
    warns of an obscure feature that does defeat EC and suggests turning it off
    under certain conditions, but I doubt that your camera has that particular
    feature.)

    : What I can't tell is whether the camera makes any further
    : adjustments while actually making the exposure. So far as I
    : can tell, the A590 has no pre-flash, so it seems the only
    : other possibility is if it shuts off the flash in real time
    : when the aggregate light reaches a certain level. I can't
    : tell if that's happening or not. If not, then the flash EC
    : may be nothing more than a flash power level control. I
    : just can't tell.

    Cutting the flash off when it reaches a certain level (whatever the means by
    which that decision is made) is pretty much the only option available. At the
    time intervals at which the flash will fire, there probably wouldn't be time
    to do anything mechanical.

    : > If you want more technical detail, Google for Gisle
    : > Hannemyr's Web site. He wrote a paper a few years ago
    : > that provides a detailed exposition of Canon's thinking
    : > regarding flash.
    :
    : I've found this site of his:
    :
    : http://dpanswers.com/canon_flash.html
    :
    : and I think I understand Canon's way of handling flash. I
    : just wish I knew more about what the A590 actually does.
    :
    : Upon reflection, it seems that my previous tests didn't have
    : the background natural illumination at a high enough level
    : to make any difference. So I think I need to try everything
    : again with more light.

    Or at a higher ISO. The effect should be about the same.

    : Everything is further complicated by the Scene settings
    : Night Snapshot and Night Scene, and of course Canon says
    : nothing specific about what the Scene settings actually do,
    : in technical terms I mean.

    One thing they do, at least on my 50D, is force auto-ISO on. So if auto-ISO is
    part of your problem, you probably don't want to use the scene settings with
    flash.

    One thing you might do is probe Canon's Web site for more technical info. At a
    minimum you could download the user's manuals for one or more of their more
    expensive cameras. Many features tend to behave similarly across different
    models, and the manuals for the more elaborate cameras may describe them
    better.

    Bob
     
    Robert Coe, Feb 21, 2009
    #4
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