Light

When we take a photograph we are trying to capture light, and we want just enough  to do the jobOtherwise we can get photographs with blown-out skies and shadows so dark, there is no detail to see.

Sometimes, this is our intent, but most of the time it is simply the result of a poor exposure.

The sad truth is that we see things one way, and cameras see it another. So we have to understand just how our camera is interpreting and recording the light in our scene. This way we know if that information is right or wrong, and what to do to correct things.

Most cameras come with a built-in light-meter which measures the light being reflected  off our subject(s). We, or the camera's programming, will use that information to come up with a formula to balance the 3 properties of ISO, Shutter-Speed and Aperture.

Reflected light-meters see a world not in color, but in shades of gray, and it searches for the halfway-point between full-black and full-white (something called 18% gray) to create an exposure formula.

The downside to reflected-metering is a very bright scene like snow, a sandy beach, a lamp or  the sun. The meter is actually measuring too much light and will adjust the exposure downwards  to compensate. Our white snow is now gray, and people against a bright sky are in silhouette.

Likewise, if we're photographing a black surface, such as a cat or a big black monolith. This also will not be represented accurately and the exposure will be shifted upwards  to a dark shade of gray, over-exposing our image. Our deep, rich black cat is now more gray than black.

Let's look at a common problem with reflected meters.

I was at the Keene Pumpkin Festival and took 2 photographs. With one, I just pointed the camera and took the photograph in full-automatic mode.

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

In the second photograph, I pointed the camera down so the meter was concentrating solely on the pumpkins. I set the ISO, Shutter-Speed and Aperture to what my reflected light-meter indicated was a good, balanced exposure and took the photograph. (I can also use a camera's "Exposure-Lock"  feature to do this.)

Notice how we can now clearly see everything in the photograph.

The same solution can be applied in bright daylight. Turn your camera away  from the sun, get a meter reading, adjust the settings, then turn around and recompose how you want your photograph to look before pressing the shutter-button. 

All-in-all though, a reflected light meter does a very good job. 

An incident light-meter is a hand-held device.

This kind of meter measures the amount of ambient light falling on our subject. We can be photographing something right in front of us, or a mountain miles away. If we are all in the same light, the exposure will be accurate.

I set the ISO value then press a button. The meter will come up with a formula for what the Shutter-Speed/Aperture values should be. Up and Down buttons allow me to move up and down the scale, showing me all the different values available to me. I pick the value balance that looks best to me, set my camera and take the photograph. 

Incident meters are not fooled by light, snow is rendered as being white and not gray. The same with a black monument. Unlike reflected meters, an incident meter sees the world in color.

The downside to an incident meter happens when shooting outdoors. If our light is constantly changing, it may require us to remeasure as needed and shoot for an averaged exposure, but that's not really a big deal. Many incident meters have the ability to also read reflected light, increasing their versatility.

Plug-in accessories are available to turn mobile devices into incident meters.

This graphic shows a reflected light-meter display in a typical digital SLR camera (DSLR). The meter uses individual lighted-bars to display the amount of light (or lack thereof) being detected.

If the only bar illuminated is right in the center ("0"), then according to the meter, we have a good balanced  exposure. It's not too dark and it's not too light, either. We even have some  latitude with a bar or so (+/-) without it causing too many problems.

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

However, the more lighted bars in the positive (+) end of the meter, the brighter my exposure. I am over-exposed, so I need to cut back on the light hitting the sensor to bring things into balance. I will adjust either the ISO, the Aperture or the Shutter-Speed, whichever causes the least negative impact on things.  

Lighted bars in the negative territory (-), indicate I am under-exposed. Meaning I have too little light, and the solution is to allow more light in. Moving the ISO upward will usually resolve the problem. Sometimes I adjust just one property, while other times I need to adjust two.


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