Guide Number vs Light Meter

Discussion in 'Lighting' started by bvy, Mar 19, 2017.

  1. bvy

    bvy Subscriber

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    I have various old flashes of which I wanted to verify the guide numbers. For instance, I have two Olympus flashes for the XA cameras -- the A11 and A16 (those are also the guide numbers in meters at ISO 100). So I set my light meter to ISO 100 and fired the flash at it (at full power). Both flashes registered one stop over what the guide number suggests. For example, if the guide number suggests f/11, the meter says f/8. I tried it with an accessory flash (Sunpak 383) and it's the same thing. At least it's consistent, but I have to doubt that all the flashes are off. The meter is a Sekonic L-308B. I have the dome over the sensor, and I'm shooting from one meter away. The meter is dead on, in my experience, for my large format portrait work -- even with the IP instant film I'm using that has very little latitude. So I'm not sure it's the meter either. Everything has fresh batteries. Is my methodology flawed?
     
  2. APUGuser19

    APUGuser19 Member

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    I have a 308 and found that with shorter duration flashes it has no accuracy at all, such as with using flashes in auto mode. I have further found that calculator dials on flashes tend toward underexposure. The only true way to find where the variables are is to test with reversal color film, which has the least latitude. The 308 might be a "flash meter", but imo it isn't much of one.
     
  3. MattKing

    MattKing Subscriber

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    All of the guide number calculations and auto functions on flash include assumptions about reflections from ceilings, walls and floors. If there are no such surfaces involved in your tests, then you need to adjust.
    I would also suggest checking your flash meter - it too may be calibrated based on similar assumptions.
     
  4. wiltw

    wiltw Subscriber

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    ^^
    A number of us on a different photo forum (a 'di*ital' one) have tested our speedlight flash units with our flash meters over the years, and we have found -- regardless of brand/model of flash and regardless of brand/model of flashmeter -- that flashmeter readings are very typically -1EV lower in exposure than suggested by the manufacturer Guide Number!

    I just tested my Metz 45CL4 and rated GN148 means f/14.8 at 10', and the calculator dial says about that same value. But the Minolta Autometer Vf says f/8.0 + 0.66EV or about GN100.
     
    Last edited: Mar 19, 2017
  5. AgX

    AgX Member

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    But thus without walls and a ceiling or with them but with direct metering without dome (but calculating for the lacking opacity of it) one would likely get underexposure. (Reflections are part of the lighting that makes up the GN.)

    The OP however gets overexposure.
     
    Last edited: Mar 19, 2017
  6. Sirius Glass

    Sirius Glass Subscriber

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    The dome is used for incident light readings. Try RTFM before posting. :wink:
     
  7. cowanw

    cowanw Member

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    I would have thought firing the flash at the metres was not the correct methodology. guide numbers are after all based on reflection of the light off the subject.
     
  8. AgX

    AgX Member

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    I know, but nevertheless one can use a meter for incident metering without dome if one, as I said, takes the lack of its opacity into account. And in same cases such metering makes sense.
    But when establishing a guide number one should use a dome.
     
  9. AgX

    AgX Member

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    You either meter:

    -) in reflected mode using the plain meter and a neutral grey card.

    -) in incident mode using the meter with a dome.

    The latter method is the better method.
     
  10. bvy

    bvy Subscriber

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    Sigh. Segal's Law.
     
  11. cowanw

    cowanw Member

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    My error!
     
  12. MattKing

    MattKing Subscriber

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    If the meter is being used at the subject plane, pointed back to the flash, the incident dome should be in place.
    The guide number and auto functions are calibrated for medium distances and in situations where reflection off surfaces is assumed.
    The auto function, in particular, depends on the subject reflecting a fair percentage of the light back to the flash sensor.
    The OP's test using a one meter distance most likely differs a lot from the "typical" - the conditions used to calibrate both guide numbers and auto functions. All I am saying is that both systems need to be checked in the relatively unusual circumstances that the OP is working in.
    By the way, with respect to the auto functions built into the flash, over-exposure is not uncommon if the subject is rather small and there aren't a lot of nearby surfaces to reflect light back to the sensor.
    Sort of like those shots of the wedding couple and their first dance at the darkened hotel ballroom.
     
  13. wiltw

    wiltw Subscriber

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    As I said earlier, I did a test today: Metz 45CL4 and rated GN148 means f/14.8 at 10', and the calculator dial says about that same value. But the Minolta Autometer Vf -- with white hemisphere in place -- says f/8.0 + 0.66EV or about GN100....IOW my testing -- at the identical distance listed for published manufacturer specs for GuideNumbers (10' or 3 meters) reflects what the OP observed, and what a lot of others have confirmed in testing of their own.
     
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  15. APUGuser19

    APUGuser19 Member

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    Since we're talking about the Sekonic 308, strictly speaking, there is an accessory attachment for use with flash metering, as the regular incident dome is not really the correct thing to use for the purpose.
     
  16. Sirius Glass

    Sirius Glass Subscriber

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    Which is what I said. RTFM and follow the directions. If one thinks that they know more than the manufacturer, then they are just wrong.
     
  17. APUGuser19

    APUGuser19 Member

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    For what it's worth, I have found the (my) 308 can be dismally inaccurate with any flash other than a standard studio strobe, like a monolight. In either the case of flash duration falling below half power on a monolight, or use with an automatic hand held flash, or for that matter with any flash where either light level drops or flash duration shortens; the 308 (my 308) is worthless. It may be a flash meter by advertisement, but it really only is a fair to middling daylight meter with some flash meter capabilities. The only place where it shines above CdS meters is as a ambient tungsten-light meter. And that is because of the silicon cell having a bit flatter color temperature sensitivity curve than CdS. For truly professional use, I believe I'd put more stock in a high-dollar flash meter. When you're a pro, you can't afford to fool around with flash metering. (In not a pro, so the 308 is all I can justify the expense of owning). If I were a professional, I wouldn't trust it as a flash meter any farther than I could throw an anvil.
     
  18. John Koehrer

    John Koehrer Subscriber

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    Manufacturers ratings are usually ASA 100 @ 10' or 3 meters.
    I believe it's based on a typical ceiling height and an "average" room. As everyone else says.
    I don't think I've ever seen a GN and a flash meter agree.
     
  19. bvy

    bvy Subscriber

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    No one is second guessing the manufacturer. I've read the manual. I have no idea what you're talking about.
     
  20. APUGuser19

    APUGuser19 Member

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    I m certainly not disparaging the sekonic company. But the 308 is a consumer product compared to the big dollar pro meters.
     
  21. wiltw

    wiltw Subscriber

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    Sorry, Sekonic documentation contradicts what you just said.
    The hemisphere is used for ambient incident or flash incident readings,
    the Lumidisc is only for flat art or to measure lighting contrast.
    Flash exposure metering is done with the ordinary hemisphere

    [​IMG]
    [​IMG]
    [​IMG]
    There is no optional accessory that you can buy to serve any purpose related to flash metering per se...the gray card is for either ambient or flash
    [​IMG]
     
  22. bvy

    bvy Subscriber

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    Yes, the lumidisc, which I don't have but which is described for metering flat surfaces or calculating lighting ratios. In this case, I'm after a reading which simulates the "average room" scenario as @John Koehrer describes above. The dome seems to be the best tool for this. Even then, though, this is starting to sound like a futile exercise.
     
  23. APUGuser19

    APUGuser19 Member

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    The luminisc is directional which allows for metering individual lights in a multi light setup. Translation: contrast. I will always contend that using reversal color film as as testing material will help the studio professional KNOW his flash setup. That's why I'm not a professional. I would never sleep nights worrying about my flash setup.

    Yes, it does seem futile or at least not particularly inspiring. Flash is tough stuff. Not like continuous light at all.
     
  24. RalphLambrecht

    RalphLambrecht Subscriber

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    I would trust your meter.guide numbers can only be very rough estimates at best.They ignore the surroundings and thereby the bounce light from walls and ceiling but a flash meter measure the incident light actually arriving at the dome;much better approach.
     
  25. AgX

    AgX Member

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    To the contrary, GN's include reflections.
    BUT as the average user does not know the test-space they indeed will be vague to some extent.
     
  26. benjiboy

    benjiboy Subscriber

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    Japanese flash guns always have optimistic guide numbers they must test them in rooms with white walls and ceilings. In my experience German flashguns like Metz have more accurate guide numbers because they have to conform to D.I.N. standards.