Posts Tagged ‘correct exposure’

Metering 101

September 4, 2012

Let’s talk metering for proper exposure!

Excited?

I’ll bet not. Any talk of metering and proper exposure sends most photographers into a state of uncertainty.

After all, why can’t we just point our sophisticated cameras at our subject and have all that whiz-bang computer stuff inside the camera give us a correct exposure?

The answer is, even though today’s cameras are extremely sophisticated, the reflected light meter is still a one trick pony when it comes to metering light.

The light meter in your camera measures the light values reflected from your subject, and then sets an aperture and shutter speed combination that will produce an 18% gray tone.

Why 18%, and why gray?

Let’s answer the gray part of the question first. A light meter does not actually measure color. It measures light reflectance.  We use a “Gray Scale” to measure light reflectance. This scale has ten steps from 0 to 10.

Zero on the scale measures 100% reflectance. If an object is absolutely white it will reflect 100% of all light. At the opposite end of the scale is a reflective value of 10. An object with a reflective value of 10 is absolutely black as it absorbs all light.

The tone halfway between pure white and pure black is mid-tone gray. While it would make sense that this tone would equal 50% light reflectance, scientifically it actually measures at 18% reflectance.

Therefore we call the mid tone in any photograph 18% gray. You may be asking how this applies to color photography. Remember, your camera meter is measuring light reflectance — not color — to calculate a correct exposure. This measurement applies the same to both black and white and color photography.

A camera’s reflective meter works on the assumption that every photograph you take will have a range of tones from light to dark. It assumes the amount of light reaching the metering diode is the equivalent of 18% gray. When that’s the case, your camera meter will calculate a perfect exposure.

Here’s the problem; the camera always assumes it is measuring an average scene of 18% reflectance. The camera does not know when you are photographing a field of white snow, or a pile of black coal. It will deliver an average meter reading for both. The results will be an underexposed field of “gray” snow, and an overexposed pile of “gray” coal.

Let’s look at two examples:

The first set of images shows photographs of a white sheet. In Photo #1 the camera set an exposure based on its assumption that the image averaged 18% reflectance. As you can see the 18% averaging by the meter underexposes the white sheet recording it as gray.

In Photo #2 the camera’s exposure values were set manually based on the readings from an incident light meter. An incident meter measures the amount of light falling on the subject instead of measuring reflective values. As you can see, using the incident meter settings will record white as white.

The second set of images shows the same scenario for a black sheet. Again, the camera’s reflective meter gives an 18% average reading that overexposes our black sheet recording it as gray. The incident meter is not fooled by the absence of an average tone in the photograph. Once again, setting the camera to the values measured by the incident meter records the black correctly.

The final two images are of a black/gray/white target that represents an image that averages 18% gray reflectance. As you can see each value is recorded correctly. Both the camera’s reflective meter and the hand-held incident meter delivered exposure values that recorded almost identical images.

This is proof that the camera’s meter is very accurate provided the scene being metered averages 18% gray. However, when presented with a scene that is primarily bright tones, or primarily dark tones, the camera’s reflective meter will likely be fooled.

Does this mean you should invest in an incident meter and quit relying on your camera’s reflective meter?

Absolutely not. What it does mean is you need to understand how your camera’s meter works. You need to evaluate the tonal values in the scene you are about to photograph.

Is there a good range of tonal values from light to dark? If so, your meter should be spot-on. As a general rule of thumb, when your scene is primarily dark tones you will need to use exposure compensation to let less light in than the meter reading recommends. The opposite is true for high key (primarily light tones) in your scene. Here you will need to use exposure compensation to let in more light.

Stay tuned. In a future post I will discuss fine-tuning the tonal values your meter reads by using your camera’s various metering patterns.

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