Metering for Flash?

Discussion in 'Exposure Discussion' started by mfratt, Dec 1, 2010.

  1. mfratt

    mfratt Member

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    I haven't done a whole lot of work with flash in the past, but I'm interested in exploring the option a bit, especially for portraiture work where I need to isolate movement in less than ideal lighting.

    My question is, and forgive me if this seems like a juvenile question for those of you with experience, how exactly does one determine exposure and flash power when shooting entirely analog?

    I have a Canon 580EX-II flash gun right now, and I'm going to set up a small mini-studio in my new apartment next month, for which I'd like to get a strobe or two, but I'm completely lost in terms of metering for this. Can someone help me out and/or point to some resources which may be helpful?

    Thanks, and sorry if this has been discussed before; I searched but couldn't find anything.
     
  2. dehk

    dehk Member

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    One of the universal Secret.

    f-number = Guide Number / Distance
     
  3. ic-racer

    ic-racer Member

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    So, if you have 2 lights, with a main and fill, use the above relationship for the main light. Use the light-to-subject distance (camera distance does not matter).
     
  4. mfratt

    mfratt Member

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    How do I determine the correct shutter speed?
     
  5. johnnywalker

    johnnywalker Subscriber

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    By far most of the light will come from the flash(es) and the shutter speed doesn't really come into play unless the shutter speed is very slow and/or the light in the room is quite bright.
     
  6. mfratt

    mfratt Member

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    Oh, I see, because the flash will be much shorter in duration than the actual shutter speed, right?

    So is something like 1/60" a good point to start with?

    Where does film speed play in?

    Also, is there any way to meter according to the zone system with flash?
     
  7. dehk

    dehk Member

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    Your shutter speed must not exceed the sync speed of the camera. Like Johnnywalker said, shutter speed doesn't really come into play. The f stop controls the exposure on your subject that is lit by the flash(es). The shutter speed controls the exposure behind your subject. The slower it is the brighter your background will be (saying if you're outdoor - day time ). Indoor its effect is minimal, if everything is going to be light up by your strobe anyways.
     
  8. dehk

    dehk Member

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    Film speed. The Guide number usually is stated with the ISO number, and usually with ISO100, if you're using film other than 100, do the math to open or stop down your aperture.
     
  9. mfratt

    mfratt Member

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    Awesome! Thanks for the help. I think this should get me off the ground level with this stuff.
     
  10. chriscrawfordphoto

    chriscrawfordphoto Member

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    Handheld flash meter. There really is no other way for multiflash studio work.
     
  11. Q.G.

    Q.G. Inactive

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    Though, as was mentioned, the flash's duration will be the shutterspeed for the flash exposure, you do have to check the ambient light levels, and make sure that the aperture-shutterspeed combinbation you set will render the bits exposed by ambient light the way you like them to be. (Because, opposed to what was suggested, ambient light levels do come into play even when not very bright.)

    So it's a double job: both flash and ambient light exposures have to be checked and regulated.
    You set the aperture to get the correct flash exposure, and use the shutterspeed to control the balance between flash and ambient light exposures.
     
  12. MattKing

    MattKing Subscriber

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    The flash you mentioned is designed to interface with all sorts of automatic exposure functions on Canon EOS cameras.

    Are you planning to use a Canon EOS camera with it?

    If so, your camera may be able to do a bunch of the metering for you.
     
  13. aluncrockford

    aluncrockford Member

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    Use a hand held meter with an invercone and point it at the lens , the flash is a constant so that will allow you to set the f stop then the shutter speed can be adjusted to control the ambient light .
     
  14. 2F/2F

    2F/2F Member

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    "[H]ow exactly does one determine exposure...when shooting entirely analog"

    It is the same with film and digital. (Why would it be different? We are talking only about light, not about cameras.) Incident flash meter is the most accurate way (and also usually the quickest). Guide numbers are another, but once you start adding modifiers, the amount of accuracy decreases (unless you have previously measured the effects of these modifiers with a flash meter...in which case you wouldn't need to be using guide numbers anyhow, because you have a flash meter).

    When using flash, there are always two exposures being made at the same time for each shot; one is made with flash and one with ambient light. For the flash part, aperture controls your flash exposure. Flash duration (set by the manufacturer and usually not changeable) is the closest thing to shutter speed in this equation. However, though it does determine exposure length like shutter speed, it does not affect exposure itself except in cases of reciprocity failure in short exposures. As long as flash duration is shorter than the shutter speed, shutter speed does not affect flash exposure. (If flash duration is longer than the shutter speed, exposure is affected, but this generally only happens with flashbulbs, which have a longer duration than electronic flash.) For the ambient part, both aperture and shutter control your exposure, just as they always do without flash.

    It follows that if you want to show as little ambient light as possible when shooting with flash, use the top flash synch speed when shooting. With a leaf shutter this is generally '400 or '500. With focal plane shutters, it ranges from '30 to '250 on most cameras. Blocking out the mild "leakage" that can occur is very important with color film, as every light source has a different color. It is important with b/w as well, but not nearly as noticeable.
     
    Last edited by a moderator: Dec 2, 2010
  15. benjiboy

    benjiboy Subscriber

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    Many modern digital meters (Sekonic L758, Gossen Starlite, Kenko KFM 2100) will tell you what proportion of the the reading taken is flash and how much is ambient light, they can even take 1 degree spot readings with flash. I don't own a digital camera, but the top of the range digital meters are magic.