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  1. #21
    Steve Smith's Avatar
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    Quote Originally Posted by Diapositivo View Post
    EDIT: I read now on Wikipedia the EV concept (I would say convention) was developed in the '50 in Germany. That would mean that before that epoch light meters did not indicate EVs
    Light meters have never indicated EV (or if they claim to, they were mistakenly marked). EV is Exposure Value and relates to a combination of aperture f No. and shutter speed. LV or Light Value is exactly that, the Value of the level of Light.

    Any meter marked up as EV is actually measuring LV

    With ISO 100 film, the EV required is the same number as the LV measured.


    I suspect that at one time, Mr Adams had a meter which only measured in foot candles so this system was perfectly workable for him. Now there is not much point as you have to use a meter either way so it might as well be what we now consider to be a normal photographic light meter.

    EDIT: Just to take issue with myself. When I wrote "Light meters have never indicated EV" that was not 100% correct. A lightmeter can indicate which EV to set the camera to if it has a means of entering the ISO value but it does not give an EV level as a measure of light.


    Steve.

  2. #22
    Diapositivo's Avatar
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    Yes I agree, that's why in post #8 I specified "EV at ISO 100". In fact, the use of LV instead of "EV at ISO 100" as a measure of illuminance is not really standard as well:

    http://en.wikipedia.org/wiki/Exposure_value
    where is says:

    Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100. However, this term does not derive from a standards body, and has had several conflicting definitions.


    I agree that LV is faster - and more sensible - than saying "Ev at ISO 100" but it's not widespread.
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  3. #23
    Bill Burk's Avatar
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    I think I know the root of the problem.

    Sekonic L-758C reads directly in foot candles (Incident with dome retracted, Advanced function 16 setting 2)

    Weston Master scale is labeled with brightness units in a series similar to foot candles but I believe it is an arbitrary scale about 4 stops higher.

    My Sekonic L-28C is a true incident meter with a scale that claims "foot candles" on the faceplate. Setting ASA 64, rotate dial L indicator to 64, the computer recommends f/8 between 1/2 and 1/4 second.

    But on a Weston Master if you try to dial in the same number (for example 50 which is close to 64) you get a recommended combination of f/stop and shutter speed that is four-stops higher. For example setting Exposure Index 64, rotate arrow to 50, computer recommends f/8 at 1/50.

    Ansel Adam's Exposure Formula is not valid for real foot candles. It is valid for the old Weston Master meter brightness units instead.

    p.s. SEI has a scale for Log Foot Lamberts

  4. #24

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    1. Many meters do readout in EV not LV.
    2. Ansel Adams was talking about cd/ft^2 ( a luminance measurement ) not Foot-candle which is an Illuminance measurement.
    3. 100 cd/ft^2 = 1076 cd/m^2 which is equal to LV 12.9 with a K factor of 14 or LV 13.1 with K factor of 12.
    4. Ansel exposure setting of f/10 @ 1/100 which is EV 13 so his formula is right on the money.
    5. His method is only applicable if your meter only measures in cd/ft^2 otherwise it's easier to do it with EV or direct aperture and shutter speed readout.

  5. #25

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    Quote Originally Posted by davidave View Post
    Yes modern light meters use foot candles. I use a Sekonic 758C and it uses foot candles.

    I hope someone who actually understand the formula can offer some advice instead of people trying to say it's not useful; you're just making it for harder for other people to find out the answer. Just because you don't find it useful doesn't mean it's NOT useful.
    The Sekonic 758c can make measurement in foot-candle but not candle / foot ^2.
    the ft.cd is an illuminance measure or incident. the cd/ft^2 is a luminance value or reflective reading. It does however readout in cd/m^2 and foot lambert (FL).
    I believe the confusion between foot candle and candle per foot squared led the original poster to come up with the exposure time of 1/8 second.

  6. #26
    Bill Burk's Avatar
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    Chan Tran,

    I think you got it.

    Luminance : candela/square foot

    Illumination : foot-candle

    So the old Weston meters do show candela/square foot. You really need a convenient way to get candela/square foot to really enjoy the Ansel Adams Exposure Formula. Old Weston meters would be a way to go.

    The L-758C can't be programmed to display candela/square foot. It can display foot-lamberts, but then you would have to convert to candela/square foot which would take some of the fun out of it.

  7. #27
    2F/2F's Avatar
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    It's an $800+ light meter. You shouldn't have to do any $#!+ remotely like that to find an exposure or a luminance range.

    Foot candles are no better or worse for figuring out luminance range than are f stops. Both are symbolic/arbitrary when used for this purpose anyhow. You don't need to know how much light there is when measuring a luminance range. You just need to know how far apart lights are from each other. One full step on a meter with either an f number scale or a foot candle scale means the same thing: either a doubling or a halving of light intensity read by the meter. You don't even need numbers. All you need are hash marks showing the full steps.

    What does make visualizing luminance range much, much easier is having an analog scale with a full ring-around of equivalent exposures. If this is how you plan on using your light meter, sell the $800+ one while it still has value, get a Studio Deluxe used for under $100, and enjoy $700 worth of hamburgers or something.
    2F/2F

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  8. #28

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    Quote Originally Posted by 2F/2F View Post
    It's an $800+ light meter. You shouldn't have to do any $#!+ remotely like that to find an exposure or a luminance range.

    Foot candles are no better or worse for figuring out luminance range than are f stops. Both are symbolic/arbitrary when used for this purpose anyhow. You don't need to know how much light there is when measuring a luminance range. You just need to know how far apart lights are from each other. One full step on a meter with either an f number scale or a foot candle scale means the same thing: either a doubling or a halving of light intensity read by the meter. You don't even need numbers. All you need are hash marks showing the full steps.

    What does make visualizing luminance range much, much easier is having an analog scale with a full ring-around of equivalent exposures. If this is how you plan on using your light meter, sell the $800+ one while it still has value, get a Studio Deluxe used for under $100, and enjoy $700 worth of hamburgers or something.
    Well, if you are truly interested in measuring luminance then there are luminance meter like that of Konica Minolta (KM still makes and sell them unlike the exposure meters) LS100 or LS110 for only around $3,500 each. Your choice.
    What I meant is that to accurately measure luminance requires a much more expensive meter than an exposure meter.

  9. #29
    2F/2F's Avatar
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    Quote Originally Posted by Chan Tran View Post
    What I meant is that to accurately measure luminance requires a much more expensive meter than an exposure meter.
    I was responding to the OP's stated reasons for wanting to read in foot candles because he thought it was better for determining a luminance range. It was not in response to anything you wrote.

    The best tool on that meter for determining the brightness range of a composition is the spot meter, set at it's narrowest setting. As long as you aren't also using it to determine exposure, the actual units used are irrelevant. Only the number of steps between the measurements are meaningful.
    2F/2F

    "Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."

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  10. #30
    Bill Burk's Avatar
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    Quote Originally Posted by 2F/2F View Post
    Foot candles are no better or worse...
    I know you're point is about measuring the range. That's fine.

    I just want to reiterate what Chan Tran pointed out to me today that what messed me up was confusing Candela per square foot versus foot candles.

    Reflected light is measured differently than incident light. You can't convert between the two.

    (If you arrive at a difference around 4 stops you probably confused reflected vs incident measures.)

    But this thread has also revolved around the question... Why bother?

    I would say Ansel Adams' Exposure Formula was meant to be a practical way to get to the point where you can predict your f/stop and shutter speed before you pull out the meter because you instinctively know what the meter will read anyway.

    EV or LV numbers aren't as practical for that game. I would recommend picking up a Weston Master II III or IV meter if you want to experiment with Ansel Adams' Exposure Formula.

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