Question regarding light meters

Discussion in 'Photography Beginners' Forum' started by hulk, Sep 23, 2009.

  1. hulk

    hulk TPF Noob!

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    Hi,

    I don't understand why one would want to measure the intensity of the incident light (onto the subject), as opposed to the intensity of the reflected light (off of the subject).

    Since reflected light is the actual light that's gonna hit the sensor, isn't that the light to which you should always be calibrating? Also, given the variance of reflectance of subjects, how can incident light be any relevant at all?

    In what situation would you want to calibrate to incident light?
     
  2. Derrel

    Derrel Mr. Rain Cloud

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    Incident light metering has been a proven method of exposure metering since the 1940's, with the development of highly accurate,precise incident light meters. It is the perfect way to calculate the actual amount of light falling on a scene,and is actually quite "relevant",to use you words, because it allows the photographer to measure the amount of light falling onto different parts of a scene or set with reliable accuracy,regardless of subject color or reflectivity.

    Precisely because incident light meters do *NOT* read the amount of light reflected from a subject, that allows the photographer to meter the strength of two, or three, or four, or even six lights,and to compare the strength or quantity of illumination that is distributed within the scene. This allows a photographer to establish lighting ratios, even with the subject still in the dressing room or off-set. Like in Hollywood filming or portrait work.

    A typical incident light meter reading will allow the photographer to "peg" his highlight exposure; what the meter reads is pretty close to the correct exposure for a highlight tone value when using color slide film, or digital capture. An incident light meter will also allow the metering of the shadow values, without being influenced by the reflective value of the subject; by metering the shadows and the highlight areas in incident mode, one can make a direct comparison in lighting ratios, without the need to compensate for subject color.

    An incident light meter will accurately tell you how much light is falling on a black cat and a white cat, or how much light is hitting the shadowed side of a face, and how much light is hitting the main light side of a face. A reflected light meter's readings always are influenced by the reflectance value of the subject--the black cat and the white cat will give vastly different light readings. Even though the same,exact amount of light is shining upon both cats. A reflected light meter's reading doesn't tell you anything about "the light" level that is at the scene--it tells you about the reflectance value of the subjects under the light at the scene.

    At times, one wants to meter the subject in incident light mode, on the highlight value side, and then meter the background's reflection value in reflected light mode, and compare the two readings, in order to produce a background that is white, medium gray,or pure black. Using this method, it is possible to make a true black-colored paper background render as a gray,or a white background.

    Both incident and reflected light metering have their uses and benefits/drawbacks. Greater personal experience and understanding would help you grasp the differences between incident and reflected light metering,so if you do get the chance, try using an incident meter.
     
    Last edited: Sep 23, 2009

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