Photography is about capturing light, and a light-meter evaluates the light in a scene in order to create a formula for what the ISO, Shutter-Speed and Aperture values should be.

Cameras come with a built-in light-meter that measures the amount of light being REFLECTED  off our subject(s). The reflected light-meter sees a world not in color, but in tones of gray.

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A reflected light-meter searches for the halfway-point between full-black and full-white, something called 18% gray.

Based on the light being measured, the meter will then create an exposure formula for us. When we take the picture, this formula should have a good, tonal balance.

The downside to a reflected-meter is when there's a very bright scene like snow, a sandy beach, a lamp or the sun.

The meter factors in all this reflected bright light, and will adjust our exposure downwards  to compensate.

Our beautiful white snow is now gray and people against a bright sky, are in dark silhouette.

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

The exposure will be shifted upwards  to a dark shade of gray, over-exposing our image and losing its richness.

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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 pointed the camera downward.

With the reflected light-meter now concentrating solely  on the pumpkins, I set the ISO, Shutter-Speed and Aperture values to what my meter indicated were good values. 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 suggested 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-stop.  That's an increase of almost 2 full-stops of light!

The result is a photograph where we can now see everything much 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, recompose and take the picture. Many cameras allow us to over-expose and under-expose things 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). 

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.

Neither too dark, nor too light. 

If I take a meter reading and I have more lighted bars in the positive (+) end of the meter, I am in danger of being over-exposed. The solution is to adjust the last property value I set to cut down on the light, so I can bring my meter into balance at 0.

Lighted bars in the negative territory (-), indicate I am in danger of being under-exposed, so I need to somehow allow more light to come through.

I may have to adjust the last property and/or 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. 

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

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.

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 property value options available. I simply decide which formula will best fit the situation, then set my camera accordingly and take the photograph.

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

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

A digital camera's LCD display allows for instant feedback of our photos, nothing more.

NEVER trust it to determine if the exposure is good or not.

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Instead, if your camera (or the app) has a Histogram display, use that.

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.

Pixels are then 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 where the problems areas are and that a correction to our exposure 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 image to reality in the form of a final photograph.

Remember, when we take a picture, there is a very small chance that the camera's auto mode will deliver the exact photograph we are imagining.

It probably has a good tonal balance to it, but is there a better Shutter-Speed  we could be using? Do we want to blur the background with Aperture, so our friends stand out more in the photograph.

Once we determine what settings deliver a good exposure, we can fine-tune things.

As I've repeatedly said, I always set the ISO first. 

This is a throwback to my early days when we'd put a roll of film in the camera and set a dial to whatever the roll's ISO was. It worked then, and it applies to digital. ISO is the foundation on which our exposure-house is being built.

Next, I set the next property (Shutter-Speed or Aperture) to whatever value will provide the biggest impact for my image. 

The remaining property value is set to whatever will bring my light-meter into balance at 0. It will also be the first one to change if I need to make any changes. If that fails to get me the result I need, I go to ISO.

The Exposure Triangle

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

W. Eugene Smith

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