Why do I meter under the chin with a lightmeter?

Started Jul 21, 2014 | Questions thread
DecibelPhoto Contributing Member • Posts: 791
Re: Why do I meter under the chin with a lightmeter?

When you meter using an incident reading (light meter pointed at light source from subject location), you are not metering how bright the subject is - you are metering how much light is hitting the subject.  The meter merely tells you what to set your camera at so that medium grey will appear as medium grey.

Your light meter may indicate f/8.  But if that subject is bright white, and you were to reflectance meter your subject, it would actually read closer to f/16.  White is approx 2 stops brighter than medium grey.

The thing is, most matte opjects (like people), only have about 5 stops worth of tones in them when evenly lit (it could be closer to 7, but 5 is a better rule of thumb to follow).  So if your camera is set to f/8, and your light is set accordingly, your whites will only be 2-stops brighter and your blacks will only be 2 stops darker, for a total dynamic range of 5 stops (which is well within the dynamic range of a digital camera).  So nothing will blow out.  This is why you don't have to worry about reading brights and darks.  As long as you know what medium grey will read, everything else will fall into place.

Of course, this is only for your key light... the shadow areas that are not lit will read darker than 2 stops down, which is why you may want a fill light.  But if you use lighting ratios correctly, you can easily maintain everything within the range of your camera (or chose to let things go to pure white or pure black if that's your intention).

Now if you were lighting a car, or something shiny, you can not count on no blown out highlights by metering in this fashion.  Shiney clothes may also cause problems, but they still won't show with an incident meter reading.  But people are easy to meter... don't over complicate things.

Of course, all of this assumes that the amount of light hitting different parts of the face are even.  You should be able to put you meter anywhere on someone's face and get the same reading.  That's also why it doesn't matter where you read.  I guess if your light was someplace EXTREMELY close (not recommended), like touching their forehead, then this wouldn't work.  But in general, the light reading won't change from chin to forehead more than a couple tenths of a stop.

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