Exposure Metering | How Your Camera Sees the World

In camera exposure meter scale.
Exposure Triangle | Balanced Light Meter

In a previous article we examined the Exposure Triangle and how ISO, aperture, and shutter speed are used in concert to balance exposure for a scene. However, there is one thing that should not be overlooked. How does the camera determine what is the proper exposure in the first place? In this article I’ll discuss exposure metering to help you start to see like your camera.

Goals

  • Demystify exposure metering
  • Understand what your camera is telling you
  • Learn some pre-visualization techniques

Visualization Exercise

If you look at a blank piece of white, unlined paper, what do you see? Just a white rectangle, right? Now, reach for the nearest writing utensil and draw a shape on the paper. The shape you just drew shows up on the paper because it reflects a different amount of light to your eyes. Just like drawing on a blank piece of paper, differences in light are what allow us to see the world.

With this in mind, let’s talk about how your camera sees and interprets the light of the world. The term for this is exposure.

Start With Stops – Zone Metering

Woman in pink coat gazing upon Peyto Lake in Banff National Park, Canada.
EXPOSURE METERING | PEYTO LAKE, PROPERLY EXPOSED

When we evaluate a photograph we talk about whether or not it was exposed properly. What does that mean exactly? In general, a properly exposed image allows you to see details from the darkest to lightest part of the scene. Alternatively, an underexposed image is too dark. Finally, if the overall image is too bright, overexposed.

Underexposed image of woman gazing upon Peyto Lake, Banff, Canada.
EXPOSURE METERING | PEYTO LAKE, UNDEREXPOSED
An example of an overexposed image from Peyto Lake, Banff, Canada.
EXPOSURE METERING | PEYTO LAKE, OVEREXPOSED

Properly exposed, underexposed, and overexposed are the terms used to describe the light of the overall image.

If an image is overexposed or underexposed we need a way to measure the difference so we can fix it. The stop is the answer. But, what is a stop? A stop of light is either half or double the amount of light reaching the camera sensor or film. It is a measurement system that makes it easier to talk about exposure in a consistent way to every other photographer.

Next, what is Zone Metering? Ansel Adams created zone metering to teach his students about exposure. Zone metering is a visualization method for photographers to see the world around them. Mr. Adams said that you could visualize scenes to have +/- five stops of light from the average. With each increasing stop of light the scene is brighter until you cannot make out details. For every decreasing stop of light the scene is darker until you cannot make out details.

Terminology –

Blown out refers to loss of detail in the highlights of a photograph. Clipped refers to loss of detail in the shadows of a photograph.

Rather than just put this in writing let’s take a look at the concept visually.

Black and white scale that demonstrates Ansel Adams' zone metering system. Vertical scale with complete black at the top, continuing to lighten by one stop of light until reaching complete white at the bottom of the scale. Numbers on the individual bars indicate the number of stops from the average, or "correct" exposure.
EXPOSURE METERING | ZONE METERING SCALE

In the first example, I captured a series of eleven exposures from -5 to +5 of the same textured wall. What you should notice is that somewhere between -3 and -5 you lose the ability to make out details in the scene. The same between +4 and +5 stops.

Why is this important to you and your photography? To me, zone metering simplifies the relationship between different stops of light. I use it almost every day when planning a scene. This is the kind of tool you can keep with you all the time and make quick improvements to your results.

Next, let’s see how you can use the concept of zone metering and stops in real world scenes.

Recently, I had the opportunity to spend some time in Calgary and the surrounding area. Calgary has quite a few beautiful pieces of art, but this particular mural of a First Nations woman really caught my attention.

Beautiful mural of a native woman and section of downtown Calgary, Canada.
EXPOSURE METERING | CALGARY MURAL AVERAGE EXPOSURE

The original photo is in color, but I wanted to share this with you in black and white to simplify the discussion of evaluating the different stops of light in the scene. I’ve incorporated the zone metering scale from above to help you start to visualize the differences between the light and dark areas of the scene.

Two frame picture. On the left is a black and white photo of a mural and section of downtown Calgary. On the right is a vertical black and white scale that is used to compare the relative exposures for various portions of the picture on the left.
EXPOSURE METERING | CALGARY MURAL, ZONE METERING COMPARISON

When you look at this scene you see that there are areas of black, white, and gradations of grey. This is because there are different amounts of light in the scene. In the following image I set the camera to aperture priority, then metered various parts of the scene. Here are the results.

Black and white picture of a portion of downtown Calgary. Image has various shutter speed values overlaid atop respective areas. This is used to illustrate the different exposure values within the scene.
EXPOSURE METERING | CALGARY MURAL SCENE EVALUATION

I converted the measured shutter speeds to show the number of stops between each of the different values. As seen in this table, there are six stops of difference between the brightest and darkest areas of this scene.

6x6 table that captures the differences between the lightest and darkest areas of a photograph from Calgary. The chart shows that there are six stops of light difference between the lightest and darkest areas of the scene.
EXPOSURE METERING | SCENE METERING TABLE

What does this mean?

This means that the camera must be able to capture six stops of exposure in order to show detail in all areas of the scene.

Exposure Metering – How Your Camera Sees the World

Your camera can measure the light from any given scene in a variety of ways. The names vary by manufacturer, but the functions are similar. The methods are evaluative, center weighted average, and spot metering. The following image shows a comparison of exposures based on the three different metering modes.

*Note: All photos were taken in Aperture Priority mode on a tripod. The only thing that changed was the metering mode.

Three images of the same scene stacked side by side. The images demonstrate the different results from using the three unique exposure metering modes for my camera. The camera settings for each image are displayed for the reader.
A. Evaluative Metering | B. Center Weighted Average Metering | C. Spot Metering

Let’s take a look at the reason for the changes between the images.

Camera Metering Modes

The following images represent the areas of the scene the camera uses to determine the correct exposure. When using the evaluative metering feature the camera takes a measurement of the light from the entire scene to determine the exposure. Switching to center weighted average reduces the metering area of the scene to a box around the center of the frame. Finally, spot metering measures the scene in a very localized area at the center of the frame.

Evaluative Metering

Image from downtown Calgary featuring mural of native woman and Calgary tower. The image has a red overlay that demonstrates the evaluative exposure metering mode. The overlay represents the area from which the camera takes light readings to determine exposure.
EXPOSURE METERING | CALGARY MURAL, EVALUATIVE METERING

Evaluative metering averages the light from the whole scene to determine the average. The average value tries to include details in the brightest and darkest parts of the scene.

Evaluative (or average) metering is a great tool for most scenes. It is an easy and straightforward method of capturing what you see. You simply put your camera up to your eye, frame your scene, check focus, and click. No fuss. No muss.

Center-Weighted Average

Image from downtown Calgary featuring mural of native woman and Calgary tower. The image has a red box and dot overlaid in the center of the image that demonstrates the center-weighted area exposure meter. The overlay represents the area from which the camera takes light readings to determine exposure.
EXPOSURE METERING | CALGARY MURAL, CENTER-WEIGHTED AVERAGE METERING

Center-weighted average is the next metering mode available. Instead of averaging the light from the entire scene, the camera takes a measurement from a certain amount of the scene around the scene center. This is a good method to use when your subject is near the center of the scene and you want to expose it properly.

Spot Metering

Image from downtown Calgary featuring mural of native woman and Calgary tower. The image has a red dot in the center of the frame to demonstrate spot metering. The overlay represents the area from which the camera takes light readings to determine exposure.
EXPOSURE METERING | CALGARY MURAL, SPOT METERING

Spot metering is the third way your camera can evaluate a scene. It is also the trickiest of the three modes because it measures light in very specific locations. This means you can get unexpected results easily if you are not careful. I usually use spot metering with manual mode to exercise full control over my exposure.

I used spot metering to obtain the shutter speeds for the various areas of the Calgary scene.

Black and white picture of a portion of downtown Calgary. Image has various shutter speed values overlaid atop respective areas. This is used to illustrate the different exposure values within the scene.
EXPOSURE METERING | CALGARY MURAL SCENE EVALUATION

Go Forth and Meter Confidently

Now that you, hopefully, have a better understanding of how your camera sees the world it’s time to get creative. I’d encourage you to pick a scene to recreate the exercises in this article.

Set your camera to aperture priority. Select spot metering. Meter different areas of the scene you wish to capture to record the different shutter speeds. This will help you start to visualize the dynamic range of your scene.

Then, place your camera on a tripod. Still in aperture priority mode take a series of photographs using the different metering modes. This part is important because it will help cement the results of using the different modes for a given scene.

Finally, import your photos to your favorite editing software to evaluate the different results on a bigger screen. After one or two experiments, the lessons of this article will become valuable tools in your photography toolbox.

Thank you

Thank you for letting me share in your photographic journey. As always, please let me know if you have any questions. Also, feel free to post some of your images in the comments below. See you all on the trail.

EXPOSURE METERING | END

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