Manual and auto-aperture flash usage (non-dedicated unit), help needed

Discussion in 'Lighting' started by Alexz, Oct 10, 2004.

  1. Alexz

    Alexz Member

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    Just ordered Sunpak 383 Super unit for my 6x7. Never used any but TTL flashes in the past, so no experiences with manual flash exposure, auto-apertures and whatsoever. I'll appreciate any help clarifying how to use all that.

    After reading a general stuff I understood few things: 1. All the calculations are made based on GN provided for particular flash.

    2. Since the standard GN is for aperture of f/1.0, to obtain a real distance coverage of the unit I have to divide that GN by the particular aperture used to the shot, right ? For instance, shooting at f/8 by a flash gun having GN = 120 feet (about 40 m) I get 40/8 = 5 meters actual distance coverage. What does that mean ? A full flash power at 5 m distance ?

    4. What is the principle of using auto-aperture ? Setting f/8 on the flash unit, what it will result to ? Does that mean the flash unit will adjust its output to provide its maximum power for 5m only thereby concerving the remaining power (faster recycling ?) ?

    5. What general differences between fully manual and auto-aperture modes ?

    6. If I need a gentle fill-in only, how what I set the flash in auto-aperture and fully manual modes ?

    Thanks in advance, Alex
     
  2. 127

    127 Member

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  3. John Koehrer

    John Koehrer Subscriber

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    Keep in mind the calculator dial on the flash unit is only a GUIDE and with your particular film/flash combination it may not be the same as the manufacturers advertised GN.
    1.Manual operation always gives full power & you must adjust the aperture to give proper exposure.
    2.Auto operaton allows the flash to determine the exposure based on light reflected back to the camera/flash position.(if you're using flash in a crowd it will ALWAYS see the most reflective surface first. usually the bald guy off to the side)
    3.To establish YOUR GN for a particular film/flash combination, shoot a test roll.
    .set up an "average" subject 10 feet from the flash.
    .make a series of exposures in 1/2 stop increments.(you can use the calculator on the flash as a starting point & go two stops either side) Either take notes or have subject hold card w/f stop used to take that frame for reference.
    .process the film & make judgement call on best exposure.

    You are corrrect in your question #4. You set the aperture for a recommended value & flash controls exposure.
    Depending on your synch speed using fill flash may not be the simplest project in auto. I have used auto fill myself by setting the flash to the widest aperture available and using correct aperture as metered on the camera. Once again, shoot some tests.
     
  4. Ed Sukach

    Ed Sukach Member

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    The "Guide Number" (GN) is a value calculated to be used in determining the aperture setting, using "manual" operation. It is usually given as a "strength" for a given film speed, (usually ISO 100), as is appropriate for distances in either feet or meters.
    Back in the "old days" when we had slow "flash bulbs" there was a guide number for each shutter speed .. in these days of fast electronic flash and single synchronization speeds, only one number is really necessary.
    In use: Set the flash to "manual". Determine the ""flash-to-subject" distance. Divide the guide number by that distance to obtain the aperture setting. Modify the result for the film speed and set the f/stop.

    An example:

    GN = 100 (@ ISO 100)
    Distance = 9 feet
    Film Speed = 400

    100/ 9 = 11.1 - f/11 is close enough. Close the aperture by two stops, to f/22, to compensate for film speed, and fire.

    "Auto Aperture" mode, is probably a Thyristor-controlled flash that measures the light reaching the subject from the flash and returning to the flash unit via a sensor, and regulates the unit's output. There are controls on the flash unit to set it for film speed and aperture ... simply match the aperture on the camera lens and the aperture setting at the proper film speed on the flash unit.
     
  5. Alexz

    Alexz Member

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    Thanks guys.
    Hopefully will have the flash in my hands within a week or two, then will arrange a test session implying all your advises.

    Regards, Alex