Unit 3: Photography Basics
Exposure makes the photo — so learn to make the exposure.
Introduction to Exposure
After covering the basics of how cameras work, focal length, and image file types, it's time to look at one of the most fundamental aspects of photography: exposure.
Since exposure is a big topic, we’ll break it down into manageable parts. In this lesson, we’ll define exposure, introduce the three settings that control it, and learn how to assess it using the histogram. In later lessons, we’ll go deeper into each setting—shutter speed, aperture, and ISO—so you can understand their individual roles.
What is Exposure?
A photograph is a record of light. Exposure refers to the amount of light that reaches your camera’s sensor and how the sensor processes it to create an image.
You've probably seen photos taken indoors without a flash that look too dark—that’s underexposure, meaning not enough light reached the sensor. Conversely, photos that are overwhelmingly bright, with large areas of pure white, are overexposed because too much light was captured.
There’s no single "correct" exposure for a scene. Your creative choices determine how you expose an image. A silhouette against a sunset is intentionally underexposed, while a bright, high-key portrait might be slightly overexposed to create an airy, cheerful effect. Generally, a well-exposed image has a balanced range of tones from dark to light, with details preserved in both shadows and highlights.
How Your Camera Measures Light
Modern cameras have built-in light meters that measure the available light and suggest what the camera considers a "correct" exposure. What happens next depends on your camera mode:
Auto mode: The camera chooses all settings.
Semi-automatic modes (Aperture or Shutter Priority): You control one setting while the camera adjusts the others.
Manual mode: You set everything yourself, using the light meter as a guide.
Three key settings determine how much light reaches the sensor: aperture, shutter speed, and ISO. Let’s break them down with an analogy.
Think of your camera as a window with blinds, where light is coming into a room. Your goal is to let in just the right amount of light to make the room comfortable—not too dark, not too bright.
Aperture (Window Size) – A big window lets in more light, making the room brighter. A small window limits light, making the room dimmer.
Shutter Speed (How Long the Blinds Are Open) – If you open the blinds for a long time, more light enters. If you close them quickly, less light gets in.
ISO (Wall Color and Reflectivity) – Bright white walls (high ISO) reflect light well, making the room seem brighter even with a small window. Dark walls (low ISO) absorb light, keeping the room darker unless a lot of light comes in.
These three settings work together. If you have a small window (low aperture), you might need to keep the blinds open longer (slow shutter speed) or use brighter walls (higher ISO) to maintain the right brightness. If you have a big window (wide aperture), you might need to close the blinds sooner (fast shutter speed) or use darker walls (lower ISO) to avoid excessive brightness.
Exposure in Numbers
In manual mode, your camera displays three numbers, such as:
f/8 (aperture)
1/50 (shutter speed, measured in fractions of a second)
ISO 400
Changing any of these values affects the exposure. The details of how each setting influences the final image will be covered in later lessons, but for now, it’s important to understand how they interact.
Try This on Your Camera
Set your camera to Aperture Priority Mode (this may be labeled differently on your camera).
Turn the control wheel to change the aperture (f-number). Notice how the camera automatically adjusts the shutter speed to maintain proper exposure.
Observe the light meter indicator—it should stay centered if the exposure is balanced.
Now, switch to Manual Mode:
Adjust all three settings (aperture, shutter speed, ISO) yourself.
Check how the light meter changes as you modify settings.
Take a correctly exposed shot, then intentionally overexpose and underexpose an image. How does the light meter reflect these changes?
Online Practice: Canon Outside of Auto
To see exposure in action, visit Canon Outside of Auto and experiment with different settings. While the example camera is a Canon, the concepts apply to all brands. Try making a well-exposed, overexposed, and underexposed image, and observe how the light meter reacts.
This lesson lays the groundwork for understanding exposure. In the next lessons, we’ll break down shutter speed, aperture, and ISO in detail, exploring not just their effect on brightness but also their creative impact on your photos.