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Thread: Profisix -luna pro relationship between EV and ISO speed

  1. #1

    Join Date
    Oct 2006
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    ITALY
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    25

    Profisix -luna pro relationship between EV and ISO speed

    Hello, I've just discovered that my Gossen Profisix meter (Luna pro) produces the same couple f number-shutter speed once I set the EV. For Example if I set EV 4 at 100 ISO I get f 16/ 15 seconds. If I change sensibility to 400 ISO but select EV 4 I get the same f 16/15 seconds.
    On my Pentax digital spotmeter, in the second case I get f 16/4 seconds. It seems that the Profisix always considers EV as if it was set to 100 ISO. Have I discovered hot water?
    Luigi

  2. #2

    Join Date
    Jan 2002
    Location
    Besanšon, France
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    1,585

    Re: Profisix -luna pro relationship between EV and ISO speed

    Hello from good ol' Europe, Luigi!!

    As strange as it may seem, Exposure Values (EV) are not at all photometric units!

    They are simply a numerical index qualifying all couples (shutter speed - f-stop) supposed to deliver the same illumination to the film or sensor. For film, EV are useful within the range of speeds where reciprocity holds; for silicon, exhibiting in principle no reciprocity failure, EV are useful for the whole range of speeds (and f-stops, but reciprocity failure deals with long exposre times only; f-stop are not directly relevant to the question of reciprocity failure).

    EV scales are built on the following basis

    1/ EV 0 corresponds to the couple: speed = one second - f/stop = f/1
    2/ by changing shutter speed t by a factor 2X in time units, and stopping-down the aperture (f-number) N by one f-click, the EV index remains the same.
    For those who prefer the compactness of mathematical definitions:
    (reference: https://en.wikipedia.org/wiki/Exposure_value)

    EV = log2 (N2/t)

    As you can see, in the definition of EV, you can find no number of photons, no luminance, in short : no photometric units at all.

    Now, of course, people like to correlate EV with photometric units. For example: what is the lowest light level that my faithful Luna ProTM can detect?

    For this you need eventually to specify an ISO value, which makes the link between EV and photometric units.
    The good ol' Lunasix / Luna Pro with a CdS photoresistor has a sensitivity of EV -4 @ 100 ISO.

    For example, the legendary " sunny-16 rule " can be re-formulated as follows.
    Looking at the exposure table printed at the back of classical GOSSEN Lunasix/Luna-Pro exposure meters, one finds that the the “Sunny-16” rule is EV=15 @ ISO 125, and is actually valid when the amount of incident light is 70000 lux, i.e. when the absolute Gossen meter reading is 19+2/3 on the absolute light scale.

    I hope that his is clear! Photometry is something exceedingly cryptic

  3. #3

    Join Date
    Oct 2006
    Location
    ITALY
    Posts
    25

    Re: Profisix -luna pro relationship between EV and ISO speed

    Hi thank you for the clear explanation. I've also discovered that my 2 lightmeters work differently. Of course one is a spotmeter (pentax digital) the other is not (profisix-luna pro). What is more is that the sensitivity of the cell gives different readings (for the same subject) when I change the film speed on the Profisix (example: EV 7 at 50 ISO, EV 8 @100 ISO, EV 11 @30 ISO). With the Pentax the reading doesn't change (it's always EV 11 @50,100,200,400 ISO).

  4. #4

    Join Date
    Jan 2019
    Posts
    193

    Re: Profisix -luna pro relationship between EV and ISO speed

    An EV is an exposure value, a combination of aperture and shutter speed. EV doesn't depend on ISO or subject, it's just a number that tells what the camera is set to.
    An LV (Light Value) is a measure of light brightness reflected from a subject. LV also doesn't depend on ISO - it's the light reflected from the subject. But the correct EV for a given LV does depend on ISO.

    Guessing that one of your meters actually gives readings in LV, and you then use the calculator dial to determine the camera settings, ie the EV. See https://www.kenrockwell.com/tech/ev.htm

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