All cameras, including iPhones and other smartphones, have built-in exposure meters. The job of the meter is to measure the light and return an exposure setting that records the brightness of a scene faithfully. The exposures are usually close but almost never right on.

How camera light meters work

The exposure (a.k.a. light) meter built into cameras measure light that has bounced off of the subject being photographed. The amount of light the meter sees is influenced by how much light a subject reflects. A dark subject, like a black cat on a deep brown sofa, reflects a lot less light that does a bright subject like a white horse. Meters that read reflected light translate the amount into a reading that results in a gray subject.

Is the meter in your camera accurate? Do this for yourself to see

photograph a black subject

  1. Set your camera on Manual
  2. Set your ISO at 800 or so
  3. Set the White Balance to the color of light where the subject is
  4. Find a black subject like a black suede coat
  5. Set the aperture on f/4.0 or f/5.6
  6. Move the shutter dial until the indicator shows a “correct” exposure by centering between the + and – marks
  7. Fill the viewfinder with the subject then take the picture
  8. Resist the temptation to look at the monitor

photograph a white subject

  1. The camera is still set to Manual and ISO 800.
  2. Set the White Balance to the color of light where the subject is
  3. Find a white subject like a white wall or shirt
  4. Set the aperture on f/4.0 or f/5.6
  5. Move the shutter dial until the indicator shows a “correct” exposure by centering between the + and – marks
  6. Fill the viewfinder with the subject then take the picture
  7. Resist the temptation to look at the monitor

Expectations

Before looking at the results, check your expectation of what the results will be. The first photo will show a black subject and the second will show a white one. Right? These are perfectly reasonable outcomes to anticipate.

Reality check

The result will be two nearly identically gray pictures. The black door is on the left. The white door is on the right.

Reflected light meters have a tough time rendering the true tone of a subject unless it reflects close to a middle gray value. An exposure that shows the true tone of the subject is difficult to achieve with the meter in our cameras as this experiment shows.

The (not so) black door

Here’s what happened. The black door photo is much darker than the middle 12.5% gray the meter says is the “correct” exposure. To move the exposure from almost black to gray, the meter adds exposure. The dark photo is “overexposed” when the desired outcome is a picture of an almost black door.

The (kinda dark) white door

Once again, the camera’s meter did its job to perfection if not to expectation. It “saw” the white door as being way too bright to be middle gray so… It provided a reading that underexposed the white door to render it as gray. All the meter did was only what it is programmed to do. It cut the exposure time to make the door brighter.

One solution — exposure compensation

Knowing that the camera’s meter is going to make dark subject brighter and bright ones darker, the question is “How do I get a correct exposure, for goodness sake???” Well. There are two methods.

The reflectance chart helps understand how exposure relates to reality.

This chart helps to figure out where a subject’s real tone lies in relation to how the in-camera meter sees it. It’s really straightforward to use. The exposure from the camera’s reflected meter is represented by the 12.5% — 0 patch. Remember, our camera’s meters provide exposure information that makes what it sees a middle shade of gray. That gray is usually close to 12.5% reflectance.

Let’s start with the black door

The meter sees a door. “Yup,” it thinks, “That door is 12.5% gray. Here are the settings to use.” The chart tells us the difference between the gray version and what we actually want. The black door needs to be at least 2 f/stops darker (less exposure) than the meter suggests. That’s shown as 3% reflectance or -2 on the chart. The bottom of the chart shows the f/stop changes for each patch. Darker stops are represented by the minus ( – ) numbers and brighter ones by the plus ( + ) numbers.

Now the white door

The meter looks at the white door and gives an exposure when used on the camera gives a gray door (again). That exposure is the gray patch on the chart labeled 12.5% on the top and 0 at the bottom. It’s easy to see that at least two additional stops of light — +2 on the chart — need to be added to get closer to the exposure that makes the door white.

The easier solution — incident metering

Occam’s Razor states that the simplest solution is usually the best choice. In this case, it’s totally true. The reflectance chart is a good rule-of-thumb exposure estimator. Getting an accurate exposure no matter how dark or bright a subject appears to be requires measuring the light falling on the subject. Light falling on the subject is invisible until it is reflected off of a subject. This invisible light is named incident light. Knowing the amount of incident light coming from the source of light at the subject position will give an accurate exposure reading. Placing these settings on the camera assures that the subject will be recorded faithfully. Let’s revisit the black wall and white door using an incident meter.

The Illuminati meter bluetooths to your smartphone to measure the light falling on a subject.The Illuminati meter reads incident light

One of the things I love about this meter is its size and ease of use. The meter has a magnet built in that allows the meter to be attached to metallic objects. It connects via Bluetooth to my iPhone (Android phones too). This means I can make readings from the subject while I work with the light.

Point the dome at the light

The key to using an incident light meter is to place it at the subject pointing directly at the source of light hitting the subject. I put the Illuminati meter on the black door where it read the light hitting it. The reading went to my phone. I transferred the setting to my Canon 1Dx Mk II. I did the same with the white door. Note: Both doors were in the same room under the same light. Here’s the result:

Incident metering gives the exposure that shows the subject’s true tone

Here are both doors. They are both lit the same. These two photos show the results of the same exposure reading. Each door is its proper tone. The black door is black. The white door is white.

The correct exposure shows each door with its true color.