Light Meter Frustration

A lot of my friends ask, "Why do some of my photos come out well exposed and yet some are dark or some are too bright?" In the main, most of my friends use a camera with a pretty good light meter built right into it. They are using some auto or default setting and letting the camera's meter determine the exposure without understanding the basics of how it actually works. Even in the college classes that I teach, lots of my students find it difficult to grasp the basics of "how a camera's light meter works".


Putting the camera aside, let's look at light meters in general. There are 2 types of light meter; the type that measures incident light and the type that measures reflected light. Incident light meters measure the light that is falling on the subject. It is held near the subject and measures the light striking the subject from one or multiple light sources. Incident light meters are used by most professionals in the studio where flash or strobe lighting is used; they may be used in natural light situations as well. Incident meters are not often used by amateurs as they add an equipment expense and they are not very useful for those candid shots. The person taking the photo would need to go close to the subject and measure the light, adjust the camera settings, and return to the spot from which to take the photo. Convenient for one person; it's not.


Reflected light meters measure the light that is being reflected from a surface on the subject. The light strikes the subject and the light that is reflected is measured by the meter. Different surfaces and colors reflect light differently. White or very light/bright surfaces reflect more light than dark surfaces. It's a physics thing. A reflected light meter is the type of meter that is built into cameras. It is measuring the light reflected from the subject to the camera position. This meter is actually measuring the luminance of the light that is reflected. It uses 18% gray as the baseline; so, the meter computes the exposure based on looking at something with a luminance of 18% gray. The meter considers all reflected light to be coming from a source that is 18% gray.  If the subject is reflecting light that is higher or lower in luminance as compared to 18% gray, the exposure will be less than optimum. Colors can have a luminance equal to that of 18% gray. Check out the following example.


The gray sample is 18% gray which indicates a 50% luminance (Why is 18% gray equal to 50% luminance? This is a topic for another time, but trust me it is the same). The red, green and blue samples have the same luminance as the gray sample. Therefore, the reflected light meter will see them all in the same way. But remember, if the luminance (think of it as brightness) is greater or lesser, the meter will not calculate an optimum exposure.


By now, 90% of the readers of this post have likely moved on to some other surfing activity. It is a bit difficult to wrap one's head around. Suffice it to say that the meter isn't all one needs to determine a proper exposure; An understanding of how to apply the information is essential.


In the next post, we will explore selecting light meter settings in the camera options menu. 


Note: The illustration may not display the precise luminance. This will vary with the monitor and claibration of that monitor.

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