Cameras come with a built-in light-meter that measures the amount of light being REFLECTED off whatever we are photographing, near or far.

The meter will then create a formula for what the ISO, Shutter-Speed and Aperture property values should be, then applies this formula when we press the shutter button.

The meter is there, whether we have access to it or not. So we have to understand just how a light-meter works or suffer the consequence of bad exposures.

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The reflected light-meter sees a world not in color, but in tones of gray. It is designed to searche 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 (hopefully) a 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 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 any extra light (either direct or reflected) and will adjust the exposure-formula downwards (under-exposed) to compensate. That's why our beautiful white snow is now gray and people who are against a bright sky, are in dark silhouette.

Likewise, if we're photographing a black surface close-up, such as a black cat or a large black monument, they will not be represented accurately, either. The exposure will be shifted upwards (over-exposed), and blacks are 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. Just what I expected.

In the second photograph, I went full-manual on the camera, then pointed it down towards 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 was in balance. I then reframed the scene and took the shot. (Some cameras feature an Auto-Exposure lock for this.)

By metering away from the bright lights, the formula opened the Shutter-Speed to allow a full-stop of light  more to come through, PLUS the Aperture was also opened up by about a full-stopThat's an increase of almost 2 full-stops of light!

The result was a much better photograph where we can now see everything more clearly.

This same solution is easily applied to bright daylight.

Turn the camera away from the sun and get a meter reading. Use the Auto-Exposure lock or adjust the settings manually. Turn around to reframe, then take the picture.

Some cameras allow us to over-expose or under-expose at will to accomplish this same task. Despite any 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 dead-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, it's too bright and I am over-exposed.

First, I would adjust the last property value I set, in order to reduce 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. If I can adjust the ISO, I may do that as well.

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. Again, I may also be able to adjust the ISO, 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 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 (they can also be expensive), and plug-in accessories are available to turn a mobile device into an incident meter.

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

Press the Up and Down buttons to cycle through all the different exposure-formula options available. Decide which formula will best fit the situation, then set the camera accordingly.

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

It's that good, but we need to have it with us.

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. The pixels are stacked vertically and the more pixels there are in any given area, the taller it will be.

This is your photograph's real exposure.

Our aim is for a balanced exposure, something resembling a mountain range, where the pixels are concentrated more to 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 so 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 the photograph first. Then we use our knowledge of ISO, Shutter-Speed and Aperture to turn imagination into 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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