Understanding Reflective Light-Metering And The 18% Reflectance Phenomenon
[I took this shot with the macro setting on my camera. The shot was taken in a semi-dark room with a pink candle (seen in reflection) by the right side. This is my very first flickr macro shot! The screwdriver was put through hell when I tried unscrewing some of the tight fitting screws on my tripod . If you pay close attention to the way the hair are arranged at the tip, you’ll realize that they are actually aligned along the magnetic field lines that developed there due to static electricity! Neat .]
Howdy all! I’ve been studying digital photography lately and totally enjoying it. Today, I’ll share with you folks essentially what had me in one of those ‘aha! so that’s it!’ moments.
Most digital cameras have an in-built light meter whose job it is to discern the brightness of incident light falling on an object, so that the camera can adjust its aperture, shutter speed or ISO settings to allow into the camera, just the right level of light to yield a proper exposure. These light meters usually are of the reflective category, i.e. they try to predict the incident light’s brightness based on the
assumption that reflective light equals 18% of incident light.
In other words, if
x = incident light
y = reflective light
0.18x = y or x = y/0.18
It so happens that most objects satisfy this empirical rule and the camera judges incident light correctly.
In the case of white objects such as snow, etc. the actual equation is different. Assuming white reflects 90% of incident light:
0.90x = y or x = y/0.90
Because the camera calculates x as y/0.18 rather than as y/0.90 , it perceives incident light as being brighter than it actually is. The camera thus tries to decrease the amount of light falling on the film/digital sensor and as a result, underexposes the object. White objects turn into gray. And by extension, all highly reflective objects end up underexposed.
With black objects, the opposite happens. Assuming black reflects 4% of incident light:
0.04x = y or x = y/0.04
Because the camera calculates x as y/0.18 rather than as y/0.04 , it perceives incident light to be dimmer than it actually is. It thus tries to increase the amount of light falling on the film/digital sensor and as a result, overexposes the object. Black objects turn into grey. And by extension, all low reflective objects end up overexposed.
A great discussion about this topic and ways to solve this problem can be found on this website.
Any thoughts or tips of your own? I’m waiting to hear them! Adios for now !
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