Minolta 7xi built-in flash...proper exposure ?

Discussion in 'Minolta' started by boby777, Jan 12, 2006.

  1. boby777

    boby777 Guest

    Lets say I go to aperature priority and set the aperature to f 27 and taka a flash pix, using the built in flash. Then go to a low f stop, like around 3.5 or 4, and I take a flash picture of the same subject, using the built in flash...
    How does the 7xi know how to properly expose the picture? In flash mode the speed stays at 1/200. And the aperature is set by me, so I can vary the depth of field.

    Seems that the built in flash would have to vary either its light output intensity or light output duration in order to provide the proper exposure (assuming I'm within the proper range of the built in flash, which is probably only up to 15 feet).

    Thanks for any info.

    Bo
     
    boby777, Jan 12, 2006
    #1
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  2. The camera and the flash collaborate on varying the duration of the
    flash. On a film camera of that generation, I imagine it actually
    reads the light reflected off the film during the exposure, and uses
    that information to decide when to quench the flash.

    You'll find the builtin flash has a *really* short range at "f 27"
    (which doesn't exist on much of any lens I've ever seen).
     
    David Dyer-Bennet, Jan 12, 2006
    #2
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  3. boby777

    Alan Browne Guest

    Not likely at any range. The GN of the 7xi flash is 12, IIRC.


    Then go to a low f stop,
    "Modern" flash for cameras is referred to as TTL (Through The Lens) when
    used automaticaly.

    When the shutter opens completely, the flash fires and tries to keep
    firing until the capacitor is depleted. The light bounced off of the
    scene returns through the lens, bounces off of the film and is received
    by a sensor** at the bottom of the camera between the lens and film.

    Those sensors are measured over time (oh so brief) and when an amount of
    light hits that corresponds with the ISO of the film and the meter
    selection (partial/matrix), then a signal is sent to the flash to cease
    output.

    The varistor opens and the flash stops. Very abruptly. This results in
    (we hope) correct exposure, and also saves the unused energy in the caps
    for the next shot, thereby sparing your batteries which is a secondary
    but very practical benefit to the method.

    This all occurs very quickly, usually in less than 2msec and often in
    much less than 1 msec.

    This does not need to consider shutter speed (obviously) or (less
    obviously) the aperture setting as that light hitting the sensor has
    already been affected by the aperture (TTL). So for each stop the
    aperture is closed, the sensor will need to see the lower intensity
    light for a longer period.

    The above does not apply to digital SLR's by the way. Light does not
    bounce off of the CCD/CMOS cover in a useful way for this function. So
    most digital cameras pre-flash (weakly) and then fires a computed amount
    (ISO, Aperture and return light) of flash power on the actual shot.
    Again, the thyristor is the beast that cuts the flow to the flash abruptly.

    Is TTL a good way to expose flash. Not consistently as it depends on
    the meter and how the photographer uses the meter v. the scene.
    (Although in the Nikon F5/F6 I would say yes, but it works a bit
    differently).

    **(Next time you load film, set the camera to manual, speed to B and
    hold the shutter down with the back door open. You will see the sensor
    'pits' on the 7xi. Not sure about other models, but Max 9 is like this
    too).

    Hope that helps.
     
    Alan Browne, Jan 13, 2006
    #3
  4. boby777

    Bob Hickey Guest

    a flash pix, using the built in flash. Then go to a low f stop, like around
    3.5 or 4, and I take a flash picture of the same subject, using the built in
    flash...
    the speed stays at 1/200. And the aperature is set by me, so I can vary the
    depth of field.
    intensity or light output duration in order to provide the proper exposure
    (assuming I'm within the proper range of the built in flash, which is
    probably only up to 15 feet).
    will change DOF if you shoot ambient lite and stop down, or if you get a
    bigger lite and move it back. Bob
    Hickey
     
    Bob Hickey, Jan 13, 2006
    #4
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