Cameras come with a built-in light-meter which measures the amount of light being REFLECTED off whatever we are photographing.

With this information, 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.

Any time we have ever taken a photograph in automatic mode, our camera's light-meter was at work, whether we had access to it or not.

So we have to understand just how a reflected light-meter actually works or suffer the consequence of bad exposures and missed opportunities.

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The reflected light-meter sees a world not in color, but in tones of gray.

It is designed to search 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 that it's easily fooled 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 this extra light (whether it's direct or reflected) and will adjust the exposure-formula downwards (under-exposed) to compensate accordingly. 

(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, that 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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In this case, 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 would expect.

In the second photograph however, 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 to accomplish 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-stop. Gadzooks! That'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.

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 these faults though, 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 an SLR camera (film or digital). 

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).

After setting the ISO and the second property, I will check my light meter (which I fully expect to be way off).

If the meter has more lighted bars in the positive (+) end, it's too bright and I am over-exposed. To fix this, I would first adjust that last property value I had 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 probably has the least negative consequence for my photograph. If I can adjust the ISO without significant impact, 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 adjusting that last property first and 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 AMBIENTlight 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 both in the same light, the exposure will be accurate.

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

Don't like it? Press the Up and Down buttons to cycle through all the different exposure-formula options available. Decide which formula will best fit your situation, then set the camera accordingly.

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

Incident light-meters are that good, but we need to have it with us.

The Histogram

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Do not trust what you are seeing on your camera's LCD display to determine if the exposure is good or not. What we are seeing is a greatly reduced, compressed version of what we took, it is not an accurate representation.

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

In a histogram, the brightness value of each pixel in an image 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, and the more pixels there are in any given area, the taller it will be.

This right here, is a photograph's real exposure.

Histograms are a product of digital photography, it doesn't exist with film.

Many people, myself included, were originally told that we wanted a histogram resembling a mountain range, where the pixels are concentrated predominantly towards the middle of the graph.

That having a histogram graph pegged too-far left (under-exposed) or too-far right (over-exposed), was something to be avoided. Well, that is partly true.

Actually, we want to have the pixels slanted over more to the right, so it covers the Midtones, and especially the Highlight and White regions as much as possible without clipping.

This process is referred to as "exposing to the right."

Of course, the world is an imperfect place and there are times when we will need to clip slightly in the brightest spots in order to get our photograph just the way we want it.

It's all a matter of what we are willing to sacrifice. 

We expose-to-the-right because the Black and Shadow areas are where we begin to have noise. So the more pixels we have in those areas, the more noise our recorded image will have.

With analog film this is not really a consideration, as grain/noise is perfectly acceptable and often adds to a photograph's creativity.

Digital cameras are different. They have a luminance noise (which isn't too bad) and a color noise (awful). We look in a nice shadow area and see a slight pattern of barely-visible colors of blue, red and green. Something that shouldn't be in our image.

Color noise also has the added benefit of including "hot pixels," which is one single random pixel, say a noticeable bright red pixel in the shadows.

So to recap, film noise is good and digital noise is to be avoided if possible.

The moment of capture is a time of magic, when the creative and technical sides of our brain comes into play. We imagine the photograph first, even before we begin to pick up a camera.

We can let the camera do the work for us, or we can take a few moments and use our knowledge of ISO, Shutter-Speed and Aperture to get the best photograph possible and really, isn't that WHY we take photographs? 

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

W. Eugene Smith

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