Various ways to use an incident light meter

Discussion in 'Lighting' started by Diapositivo, Feb 9, 2011.

  1. Diapositivo

    Diapositivo Subscriber

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    Suppose we have a portrait with two light sources.

    When we use an incident lightmeter to give us a correct average exposure for a tridimensional object which is non-evenly illuminated, we use the dome and we point the instrument to the camera taking it in front of the subject. Part of the dome will receive more light, part less. Inside the dome there will be a great shake of photons and the result will be the reading of the light meter, a form of averaging. We will call this method A.

    When we want to determine the illuminance ratio, or how you call it, I mean the difference in lighting on let's say the two sides of the face, we use the flat cover, not the dome, and we point the light meter to the light source, not to the camera. First we measure light source A, then we measure light source B, than we see that they are let's say in 2:1 relation. If you want more contrast you diminish light on the "darker" side etc. So no dome, meter pointed at the light source, and two readings in our case. We will call this method B.

    A third way is to use the dome, but on two different sides of the face. You first use the light meter on one side, so that the dome is invested only by one light source. You put it "by the cheek" rather than "by the nose" so to speak, but pointed toward the camera. Then we use it on the other side, so that it only takes the other light source. Again using the dome, and pointed toward the camera. So dome used, meter pointed at the camera, two readings in our case. We will call this method C.

    My incident light meter doesn't have the flat cover, only the dome. At the moment I am not doing the kind of work that would require method B, but I might begin soon.

    The questions:

    Can't I use method C to investigate the illuminance ratio of the scene? Do I really need the flat cover over the sensor?

    Or to be clearer: What is the difference between method B and method C?

    Which also begs the question: should I buy another incident lightmeter, that has the option of the flat cover?

    Fabrizio

    PS I specify I don't have the flat cover lest somebody tells me to try and find by myself :sad:
     
  2. Mainecoonmaniac

    Mainecoonmaniac Subscriber

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    No need to complicate things. Meter the main light source. Take a reading and take note of it and turn off the main light. Turn on you fill, take a reading. If it's a 2:1 ratio the fill is one stop less than the main light. The fill ideally should be a broader i.e. a flatter light source. Don't get hung up with numbers and ratios. See how it looks to you. A dome type of incident meter works best with 3 dimensional subjects. No need for a flat diffuser for your meter if your doing portraits.
     
  3. Diapositivo

    Diapositivo Subscriber

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    Well, I intend to do some still life, product shots etc. Initially with two light sources or one and a bounce surface. I thank you for your answer but my question was not about the "look" of it (whether 2:1 is good or not, how to judge visually etc.) but about the difference in results between method C and method B, it really was about metering technique so to speak.

    Is there any application that requires method B, or, instead, can I just ignore the existence of the flat thingy?

    Fabrizio
     
  4. ic-racer

    ic-racer Member

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    Good question. The two measurements with the dome can then be averaged or weighted to give a value you can use to make an EXPOSURE. Whereas the two values obtained with the flat disk, pointed right at the light source, will only give you a RATIO. The absolute numbers won't easily compute to an exposure value you can use.
     
  5. 2F/2F

    2F/2F Member

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    You'll do just fine using method B with the dome. It is actually preferable to using the flat disc IMHO. It gives you both an exposure and a ratio quickly and easily.

    I go through the so-called "trouble" of using an incident meter to avoid many of the pitfalls of reflective metering, one of which is that reflected meters average things. If I am trying to avoid averaging, why average? The other reason I want to use an incident meter is that reflected meters are subject/composition-based, not light based. If I am trying to avoid composition-based measurements of reflected light by using a hand-held incident meter, why would measuring the light coming from the direction of the camera make sense as a general rule?

    There are instances in which I do want an average reading, and in those instances (primarily uncontrolled lighting), I take an averaged reading. But pointing the dome at the camera as a matter of course just makes little sense, even though it is in all the instruction manuals. The dome gives you 180 degrees of coverage to point any way you would like - not just at the camera. Choose where to place those 180 degrees wisely, and you are set.
     
  6. Galah

    Galah Member

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    If all you want to do is establish a lighting ratio between the two sides of your subject (using lights), you don't need to mess with the meter: just move the light on the less lighted (darker) side further from the subject.

    For example, twice as far away will give you a ratio of 1:4. Work it out from there. :smile:
     
  7. markbarendt

    markbarendt Subscriber

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    Yes
     
  8. stillsilver

    stillsilver Member

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    Use the dome for A and B. I don't see a need for C. Just my opinion.

    Mike
     
  9. benjiboy

    benjiboy Subscriber

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    You would appreciate the need for the flat receptor if you were using it in a studio and you wanted to measure the contrast level between the various lights which by pointing it at each light from the subject position you can, or photographing flat copy or artwork.
     
  10. Diapositivo

    Diapositivo Subscriber

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    Ok, I distill from your answers that if I want to take a measure of the ratio, I can use method C (separate measure with the dome at each side of the object, pointed at the camera) and it will work just as if I had used method B (separate measure with the flat thingie pointed toward the light source).

    I can ignore the existence of the flat thingie, and live happy with my incident meter which only has the domed thingie.

    Many thanks.

    Fabrizio

    PS Stillsilver wanted to give me this answer I guess but inverted method C and method B. Otherwise it is clear. I'll do as Mark Barendt says. I just use method A for average exposure, and C for checking ratios, and that's all. And yes, I can skew final exposure toward one of the two readings with method C if I want to expose in a non-average way.
     
  11. benjiboy

    benjiboy Subscriber

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    That's a very good point, the flat plane receptor is an ideal for calculating the contrast ratio when pointed from the subject position at each light not the exposure, to estimate the exposure use the normal hemispherical dome pointed at the camera.
     
  12. Diapositivo

    Diapositivo Subscriber

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    Let's say I have two lamps. By using the dome, and pointing from both sides of the face to the camera, I get those values:

    I lamp: 1/125, f/8;
    II lamp: 1/125, f/11;

    Those measures tell me both what the average exposure would be (1/125, f/9.5) and the illuminance ratio (2:1).
    So why would somebody need the flat thing if the with the dome thing one can kill both birds with one stone?

    The dome can be used for ratio inspection. Shouldn't there be some application in which the flat thing is necessary? Or more reliable? Which is the reason for the flat thing to exist?

    Fabrizio
     
  13. 2F/2F

    2F/2F Member

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    The flat disc is a bit more precise in determining the lighting ratio of the lamps themselves, as it reads a narrower field. It is not influenced by the other lights as much. It more truly reads just the lamps themselves, while the dome will give you a better idea of how the light is actually falling on the face. If there is flare/spill from one lamp to the area mostly lit by another lamp, the dome will read that, while the disc will not. IMO, you want to meter the flare, as it is what you will actually capture when you shoot.

    In your example, to capture the lighting ratio just like you designed it so that it prints as intended at your normal printing time and filter, you would expose for the brighter lamp number two (which I would actually call lamp number one, or the main light). If you expose by pointing the lamp at the main light, then the fill light will be captured at the intended value, not too brightly, like with the averaging/toward-the-camera method.

    Now, if you were stuck with a certain lighting ratio (natural light, e.g.), and you would prefer if it were less contrasty on the print than it is in reality, that is the time to either average down the middle or meter the darker side, and probably adjust development as well.
     
    Last edited by a moderator: Feb 13, 2011
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  15. benjiboy

    benjiboy Subscriber

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    In practice if you don't have a flat plane receptor you can shield the incidental light dome with your hand, or make a shallow cardboard tube that fits over it so it only takes frontal light into account..
     
  16. markbarendt

    markbarendt Subscriber

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    Actually, IMO there is absolutely no point in figuring or finding an average reading.

    The lighting on the main, the most important side of the subject, needs to match the camera setting.

    That camera setting places the important parts of the subject's "zones and tones" in all the intended places.

    At that point your camera and main light settings, are "etched in stone".

    The measurement and adjustment of lighting on the the other side of the subject is done "after-the-fact" to control the ratio.
     
  17. 2F/2F

    2F/2F Member

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    exactly!
     
  18. MattKing

    MattKing Subscriber

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    One point, for clarity.

    If the light source illuminating the shadows on one side of the subject is in the nature of a diffused "fill" light, it will also be contributing illumination to the other, highlight side. Thus a change to that fill light will not only change the ratio, it will also necessitate a change to the camera exposure settings.

    So it is important to differentiate between lighting setups that have two sources that illuminate separate sides, from lighting setups where one source is more of an all-over "fill".
     
  19. markbarendt

    markbarendt Subscriber

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    Maybe an example would help.

    (Just FYI I use strobes.)

    First, I decide on the DOF I want and set the aperture that will get me what I want, say f4 a classic short DOF portrait. This does not change once it has been set.

    Second, I measure ambient light and set the shutter speed to get exactly the amount of ambient light I want, say 1/60th to allow the back ground in or 1/400th to keep the background out. (With strobes, as long as we are in the sync range, shutter speed has no effect on the main subject's exposure.) This setting also does not change once set.

    Third, I measure and adjust the main strobe to match the camera aperture setting (f4). This is then left alone.

    Forth, I measure and adjust the secondary strobe to get me the ratio I want, maybe f4 for a 1:1 fashion shot, f2.8 for a gentle 2:1, or f2 for 4:1.
     
  20. eddym

    eddym Member

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    True, and since most portraits fall in that category, it's a very simple process:
    Turn on main light, point the dome at it and take a reading. Turn off main light.
    Turn on fill light, point the dome at it and take a reading.
    For a 2:1 ratio, fill should meter one stop less light than main.
    Turn main light back on. point the dome at the camera, and take a reading.
    Set the camera and take a picture.

    More complex lighting setups will call for more complex metering procedures, hence the need for the flat panel.
     
  21. markbarendt

    markbarendt Subscriber

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    Theoretically true, but the real effect I've seen in practice is limited.

    I do normally take a final reading under the nose and that combined reading may bump up a 1/3 or 1/2 a stop. For most all my setups that bump can be ignored.

    YMMV :smile:
     
  22. MattKing

    MattKing Subscriber

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    Eddy:

    Correct me if I'm wrong, but I believe that the lighting setup you describe actually creates a 3:1 ratio.

    In order to end up with a 2:1 ratio, one needs to have a fill light and a main light that, individually, provide the same amount of light.

    That way, the shadowed side of the subject receives one unit of light (light from just the fill), whereas the other side receives two units of light (light from the fill, plus the same amount of light from the main).

    I believe
     
  23. Diapositivo

    Diapositivo Subscriber

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    I think Eddy refers to a case, which I made reference to, where the two light sources are at the two sides of the subject, while you (Matt) refer to a case where the main light is lateral, and the fill light is frontal, near the lens axis, so that the fill light embraces both sides and "adds" its light to the one projected by the main (a more typical setup for portraits I suppose).

    Fabrizio
     
  24. Diapositivo

    Diapositivo Subscriber

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    Oh, and I begin getting the usefulness of the flat cover.

    If one uses the dome for determining the ratio, in the case implied by Matt with the fill light "frontal" and the main light "lateral", one must turn off one light to measure the other light.

    If instead of using the dome one uses the flat cover, the different power of the two lamps will be read precisely, without need to turn off the other light. With hot lamps that might even avoid stress to the lamps (turning on and off in rapid succession).

    Fabrizio

    EDIT In fact this was the conclusion that was already implied in post #13 by 2F/2F even if I did not catch it immediately.
     
  25. 2F/2F

    2F/2F Member

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    The dome, covering 180 degrees, takes that flare/spill into consideration. This is why I prefer it to the disc for measuring lighting ratios.
     
  26. benjiboy

    benjiboy Subscriber

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    The dome can't "see 180 degrees" 2F/2F because it can't see behind the meter body, there were however in the late 1950s a proposed version of Western Master meters with two hemispherical receptor domes one facing forward, and one back to take 180 degree light into consideration in one reading that was never manufactured, but this Duplex method can be achieved by taking two readings one in each direction and averaging them .