Light-Meters

Photography is about capturing light.

A light-meter evaluates the light in a scene and creates a formula for what the ISO, Shutter-Speed and Aperture property values should be.

Cameras (even a simple mobile device) already come with a built-in light-meter, and this kind of meter measures the amount of light being REFLECTED off a subject.

The meter is there, whether we have access to it or not. So we have to understand how this meter works or suffer the consequences (bad exposures).

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The reflected light-meter sees a world not in color, but in tones of gray. It searches for the halfway-point between full-black and full-white, something called 18% gray.

Based on 18% gray, the meter then creates a formula so when we take a picture, we have a photograph with good, tonal balance. It does a pretty good job at it, too.

The downside to a reflected-meter is when there's a very bright scene like light being reflected off snow or a sandy beach, perhaps an indoor lamp or if we're shooting in the direction of the sun or some other bright object.

The meter factors in ALL this extra light and will adjust our exposure-formula downwards (under-exposed) to compensate. Our beautiful white snow is now gray and people who are against a bright sky, are a dark silhouette.

Likewise, if we're photographing a black surface close-up, such as a black cat or a large dark statue, that also will not be represented accurately.

The exposure will be shifted upwards (over-exposed) and black is now a dark shade of gray.

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I was at the Keene Pumpkin Festival and took these 2 photographs...

With one, I just pointed the camera and took the shot in full-automatic mode. 

Since the bright street lights are part of the metering equation, the camera took a very dark, under-exposed photograph for me.

In the second photograph, I went full manual and then pointed the camera down to the ground.

With the reflected light-meter now concentrating solely on the pumpkins, I kept the same ISO, but changed the Shutter-Speed and Aperture values to what my meter suggested. I then reframed the scene and took the shot. (I can also use a camera's "Exposure-Lock"  feature to accomplish this.)

By metering away from the bright lights, the formula had a Shutter-Speed that allowed a full-stop of light  more to come through. Not only that, the Aperture was also opened up by about a full-stopThat's an increase of almost 2 full-stops of light!

The result is a much better photograph where we can now see everything more clearly. The same solution is easily applied to bright daylight.

Turn the camera away from the sun, get a new meter reading, adjust the settings, then turn around to recompose and take the picture. Many cameras allow us to over-expose and under-expose at will to accomplish this same task.

Despite its faults, a reflected light meter does a very good job overall.

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This is how a typical reflected light-meter display might look in a digital SLR camera (DSLR) such as mine. 

The meter uses individual lighted-bars to display the amount of light being detected.

If the only bar being illuminated is right in the center ("0"), then according to the meter we have a good balanced exposure (based on 18% gray).

If the meter has more lighted bars in the positive (+) end of the meter, I am over-exposed.

The solution is to adjust that last property value I set, in order to cut down on the light and bring my meter into balance at 0. I almost always adjust that last property first, because it has the least negative consequence for my photograph

Lighted bars in the negative territory (-), indicate I am under-exposed, so I need to somehow allow more light to come through. Usually this is resolved by either adjusting the last property first, adjusting the ISO or both, all depending on the circumstances.

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An incident  light-meter is a hand-held device which measures the amount of AMBIENT light falling on our subject. 

We can be photographing something right in front of us, or a mountain range miles away. As long as we are ALL in the same light, the exposure will be accurate.

Unlike reflected meters, an incident-meter sees a world in color and is not fooled by snow or a black cat. Blacks are rendered as being black, and white is rendered white.

Such meters are far more accurate than reflected ones, and plug-in accessories are available to turn a mobile device into an incident meter.

Just set an ISO value and press a button. The meter will come up with a formula for what the Shutter-Speed and Aperture values should be.

Up and Down buttons on the meter allow us to cycle through all the different exposure-formulas available. Simply decide which formula will best fit the situation, then set the camera accordingly.

If things look good, take a picture and judge the results.

Yes, it's just that good. It's also pricey AND we need to have it with us, too.

The Histogram

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Don't trust what you are seeing on a digital camera's LCD display to determine if the exposure is good or not.

If your camera (or the mobile-app) has a histogram display, use that instead.

In a histogram, the brightness value of each pixel in the photograph you just took is measured, organized and stacked one-upon-another. The graph it creates goes horizontally across the bottom, from left (black) to right (white) and covers the entire tonal range in-between. This is your photograph's exposure.

The pixels are stacked vertically, the more pixels there are at any given point, the taller the graph. Our aim is for a balanced exposure, something resembling a mountain range, where the pixels are concentrated more in the middle of the graph.

Having a graph pegged too-far left (under-exposed) or too-far right (over-exposed), will show the problems areas and a correction to our exposure-formula needs to be made.


The moment of capture is a time of magic, when our creative and technical sides come into play. We imagine a photograph first. Then we use our knowledge of ISO, Shutter-Speed and Aperture to bring that imaginary-image to reality in the form of a final photograph.

"The world just does not fit conveniently into the format of a 35mm camera."

W. Eugene Smith

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