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As i understand it an incident reading reads the light falling on the subject rather than the light reflected by the subject. You should be as close as possible to the subject and point the incident meter toward the light source. It does work but may not be best for every situation.
http://www.jeffreyglasser.com/
As the distance in question increases, the area decreases proportionately (I THOUGHT cube of distance relative to the inverse cube of the area - but, I don't know - possibly my memory cells are getting rusty).
The amount of light given off for each unit of area remains the same. An example: If a measurement limited to a one degree circular area of a gray card from one cm indicates an EV of 10, increasing the distance to the card to 100 meters and limiting the measured area to the same area as before will also indicate 10EV.
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Incident metering measures the light falling on the subject - the distance to the CAMERA has no effect. [...]
The inverse law is irrelevant in incidental light readings (after all, if the light comes from the sun -92 million miles- does it matter which side of the Grand Canyon your'e standing on in relation to your subject?)
However, what does matter is that both your subject and the meter are in the same light (not subject in the shade, you in the sun etc).
Point the dome on the meter towards the camera (away from subject) along a line parallel to that between the camera and subject.
Can't be any simpler!
No, point the incident meter at the camera lens, not the light source.
You need to measure the light falling on the subject from the direction of the camera.
The inverse law is irrelevant in incidental light readings (after all, if the light comes from the sun -92 million miles- does it matter which side of the Grand Canyon your'e standing on in relation to your subject?)
However, what does matter is that both your subject and the meter are in the same light (not subject in the shade, you in the sun etc).
Point the dome on the meter towards the camera (away from subject) along a line parallel to that between the camera and subject.
Can't be any simpler!
Just what I was going to write, the explanation is simple the light source the Sun is a fixed distance away, and since you are measuring the light falling on the subject not reflected of the inverse square law is irrelevant.
"Experience" seems to suggest that the piano is going to be well exposed.
My (shaky) understanding of physics seems to suggest to me that the light reflected by the piano should indeed obey to the inverse square law, and the reading I take with the incident light meter should produce underexposure. If we had placed the lamp at twice the distance, the light we would have measured would have been 2 stops less. Now we walk 300 m away from the piano reflecting our light in all directions, we should observe a fall of light reflected from the piano with distance.
Do that if you want to average the light that is falling on both sides of the subject (i.e., from the area covered by the dome). But when using light that is uneven, why would you want to average it as a matter of course? Sometimes you do, yes. But not always; not even often, I would say, for my own pix. Do this in deliberate ratio lighting, e.g. a side-lit portrait, and you overexpose. The more contrasty the lighting ratio, the more overexposed you will be.
Point it at the light for which you want to correctly expose - the "main light." You have gone to all this trouble to craft light, or choose a location and time of day, that will sculpt the subject the way you want it, why would you then average that light with the dark side, which you have intentionally made dark? You crafted, or chose to use, the light that way because you want the dark side to be dark. If you don't want it to be dark, then change the fill ratio. Don't average the exposures for the light side and the dark side as a matter of course; it does not make sense. For best results, one meters the main light source, unless in very flat, even light, in which case one could point the dome practically anywhere and get the same reading.
Y' pays y'er money and y'er takes y'er choice.
Yes, there ARE areas of the scene that I would want "dark", but the question is "How dark?"
If you measure ONLY for the Main, you wil get ... a density in the negative that correlates to 18% gray, and the "dark" areas will sink into some level of unmetered black.
All well and good, if you are a devotee of "It ain't good unless there is a *really* dense, black area".
It depends .. on the situation, the intent, the aesthetics (aesthetics - there is a cop-out).
Generally (read: exceptions would not be surprising) I would use out-of-the-box `Set the meter and dome to "Incident": At the subject, point the meter more or less at the camera' and blaze away.
Experience helps - a LOT.
We do!
There is less light, reflected off the piano, reaching the camera than at 150 m, or 10 m.
But don't forget that the image of that piano will be proportionally smaller.
As mentioned before: both image size and light intensity follow the same geometry. So though there's less light, that light has to fill in a smaller spot on film. The intensity per area unit will be the same at 300 m as it would be at 150 m, or 10 m.
So the setting to use is the same too.
According to the Encyclopedia of Photography, "The luminance of an extended source or surface is invariant with viewing distance. Luminance is also invariant within a lossless optical system because changes in image size are balanced by inverse changes in solid angle. The luminance of a Lambertian diffuser is invariant with viewing angle because the reduced power reflected off-axis is balanced by a reduction in projected area."
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