believe flash meter or believe flashlight sendor?

Discussion in '35mm Cameras' started by Steven Woody, Jul 16, 2006.

  1. Steven Woody

    Steven Woody Guest

    my flashlight has a table on its back, it say, in the automatic mode, i
    should use f5.6 at 3.6m. but within subject distance of 3.6m i did
    some tests using the flashlight and my handheld flash meter,

    1, put the meter in the subject location and measure incident flash
    light
    2, put the meter in the same location with the flashlight pointing to
    the subject, meter is in reflective mode

    in any case above, the meter show different reading ( f-stops ) other
    than f5.6, some times larger sometimes smaller. my meter is a Sekonic
    DulalMaster 558 and i believe it is accurate, what i can not understand
    is where the differences come from and how exactly a automatic
    flashlighter works

    thank you.

    -
    woody
     
    Steven Woody, Jul 16, 2006
    #1
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  2. Steven Woody

    Ken Hart Guest

    An automatic strobe (flash) puts out a certain amount of light based on how
    much is bounced back to it's photocell. In other words, the strobe lights up
    and immediately measures the light it sees. When it sees a certain amount of
    light, it turns off. Keep in mind, this happens _very_ fast, perhaps 1/1000
    of a second or less.

    How much difference did you observe in your meter readings? Just changing
    where you stand when you fire the strobe can change the amount of light
    reflected back to the strobe. Incident and reflected readings can differ
    based on how reflective the subject is. If you reflective meter a light
    color subject, you may get a higher reading than you would get reflective
    metering a dark color subject.

    If the reading variation is less than two stops and you are shooting print
    film, don't worry about it. Do some actual tests to see what looks best on
    the finished photos, and go from there. Take some pictures in a small room
    with light-colored walls and ceiling at a couple different f/-stops; in a
    big room with dark-colored walls and ceiling (or high ceiling), and see what
    exposure gives the best results. You will find that you need more exposure
    when there is less reflective surface. How much more can only be determined
    by actual testing.

    Ken Hart
     
    Ken Hart, Jul 16, 2006
    #2
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  3. Steven Woody

    Steven Woody Guest

    there is one to two stops difference. what i interested is not how
    much the difference, but why the difference happen. if your answer
    explained where the difference come from when using reflective meter,
    it seemed not explain why the difference exist when when incidently
    metering. actually, i found, the incident meter reading changes when
    the flash put near or close to the subject ( the distance does within
    the flashlighter limit ).
    you said the flash sensor cut off the light if and only if it receives
    enough amount of bounced light. it can be deduced that for what kind
    of reflective surface, the flash light hitting on the subject will be
    the same, so i should need the same exposure. if true ( i think so ),
    there is obviously a conflicts with what you said "You will find that
    you need more exposure when there is less reflective surface"

    -
    woody
     
    Steven Woody, Jul 16, 2006
    #3
  4. Steven Woody

    Bob Hickey Guest

    I'm not sure about yours, but many meters are set to meter at a certain
    time, say 1/1,000 sec or slower. Many automatic flashes fire much, much
    faster, giving a false reading. At the same time many manufacturers give
    their lites a higher rating than they have. If you bracket a few once, then
    you'll know where it works best.
    Bob Hickey
     
    Bob Hickey, Jul 16, 2006
    #4
  5. Steven Woody

    no_name Guest

    What did the test IMAGES look like. That's what I'd use to determine
    what settings to use.

    The table on the back of the strobe is generic. Actual conditions will
    vary a whole lot. If it's got a thyristor, in automatic mode it will
    shut the flash off when it registers enough light reflected from the
    subject for an aperature at f/5.6 (based on whatever ISO is set in the
    flash).

    For flash meter readings, I'd use incident readings instead of
    reflective. You want to know how much light is going onto the subject,
    not how much is bouncing off.

    Reflective readings are going to be influenced by the darkness of the
    subject. The meter will tell you how much exposure is needed with a
    given flash to make the subject 18% gray.

    If it's a wedding gown, 18% gray, a gray flannel suit, 18% gray ... a
    slinky, little black dress, 18% gray.

    The other thing that comes to mind is the mode the Sekonic is set to
    when testing. It might be set to hold the shutter speed constant and
    give variable f/stops. You also should be able to set it to read a
    constant aperature and give you variable shutter speeds.
     
    no_name, Jul 16, 2006
    #5
  6. Steven Woody

    Steven Woody Guest

    but there are cases that ask me to use reflective reading, in which i
    want to know what part of subject will fall in what zone in the
    nagative and ready to shift 18% gray to include my prefered contrast.
    i am so interested the Sekonic mode you mentioned. how can i set it to
    read a constant aperture and give me variable shutter speeds ( in flash
    measuring ) ? it seems not appeared in the manual.

    -
    woody
     
    Steven Woody, Jul 17, 2006
    #6
  7. Steven Woody

    no_name Guest


    Well, like I said, look at the test images that went along with the
    flash readings. They're your best indicator of how to adjust.
    I can do it with the L-408, but I had to figure it out since it's not
    specifically addressed in the manual for that one either. On the L-408
    there's an EV/multi button & a mode button. You have to fiddle both
    controls until you get the meter to where it's reading flash & keeping
    the f/stop constant.
     
    no_name, Jul 17, 2006
    #7
  8. Steven Woody

    Steven Woody Guest

    still can not figure it out on my L-558 :-(. meter always allows me to
    select shutter speed and give the apperture reading based on the flash
    strength, i then can change the shutter speed but get no control on
    apperture.

    -
    woody
     
    Steven Woody, Jul 17, 2006
    #8
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