Reader's Comments - Index of - Free

Exposure
When using a reflected light meter, the most important source of error is
that the subject's reflectance may not match the meter's assumption about
the subject's reflectance. Suppose that you're taking individual portraits of
Alex and Mia (at right). You measure the light being reflected off Alex's
white fur and set the camera to whatever the meter recommends. Repeating
the image with Mia as the subject you find that much less light is reflected
by her black and brown fur. So the reflected light meter recommends a
wider aperture or a slower shutter speed than it did for Alex.
Does this make sense? With negative film, perhaps. Mia is darker and if you want to get her tones into the linear
portion of the film's curve you'll need a longer exposure. But consider that if you'd used an incident light meter it
would have recommended the same exposure for both dogs. After all, the same amount of light was falling on
them. If you'd used color slide film and the incident meter's recommendation you'd get one slide with a white dog
in it and one slide with a black dog in it. What if you'd used the reflected meter's recommendation with the slide
film? You'd get two slides exposed with an identical amount of light and therefore both would be the same shade.
Exactly what shade do you get when you follow a reflective meter's recommendation? 18% gray. This is a tone
midway between 0% gray (white) and 100% gray (black). Reflected meters are calibrated to assume that the
average scene is 18% gray. The reflected meter couldn't know that Alex is a white dog and that Mia is a black
dog. When you pointed it at Alex it assumed that the day had gotten brighter. When you pointed it at Mia it
assumed that the sky had become cloudier.
Is this 18% gray assumption reasonable? If you take portraits of Caucasian people and meter off their facial skin
you'll probably find that your slides come out a bit too dark. Typical Caucasian skin is about 1/2 f-stop lighter
than 18% gray. So the reflected meter thinks that the subject is lit somewhat brighter than in reality.
Here are some examples:
http://www.photo.net/making-photographs/exposure (10 of 13)7/3/2005 2:22:14 AM
A dream scene! All roughly the same tone. All
roughly 18% gray. (from Italy)