Exposure is the process of choosing the correct settings on your camera to produce a picture that “looks right;” not too bright, not too dark, and sharp where it should be.

While your camera will figure all of this out for you, it’s good to know how to take control away from your camera, and make sure it’s making the right choices in each situation.

In this series, you will learn what each setting means, how they affect the way your images look, and how they work together to create an image that matches your creative ideas.

The process of exposing an image, simplified

The correct exposure is the amount of light controlled by the shutter speed, aperture and ISO. This is called the exposure triangle.

For every photo you take, there’s a correct amount of light that must make it down the barrel of your lens on to your sensor (or film) to make the photo look the way you want it to. When you point your camera at something, light is passing down your lens, either emitted from a light source or reflected off the objects in front of you. There are three settings which control how much of this light is allowed to hit your sensor and how that sensor reacts to the light:


  • Inside every lens is a mechanism to control the size of an opening, known as an aperture. This controls the amount of light allowed to pass through the lens to your camera’s sensor.
  • AKA: F-stop, usually a number like 2.8, 5.6, 8.0, 16, etc.
  • As you change this number, the size of the hole is changed.

Shutter Speed

  • A shutter is a like a door in front of your sensor (or film) inside your camera.
  • Press the button on your camera to take a picture, the shutter opens and lets light hit the sensor (or film).
  • How long it’s open is called the shutter speed.


  • ISO is how sensitive your sensor or film is to light.
  • In terms of the numbers, the higher the ISO number, the more sensitive the sensor, so the less light it takes to make the same exposure.

When we photograph something, we are capturing light from the scene in front of us for a moment. Let’s say you see something you want capture a photo of, so you press the shutter button.

  • The shutter opens for a split second.
  • Light traveling through your lens now hits your sensor.
  • After a moment, the shutter closes.
  • The camera saves a file that contains the data on the characteristics of the light hitting the surface of the sensor during the time the shutter was open.

That’s it, you just exposed an image! Sure, that’s a simplified outline, and there’s a lot going on in the actual process of creating a properly exposed image. But, the process itself is the same regardless of camera make, model or species.

What is an aperture?

Inside most lenses is a mechanism called a diaphragm, which blocks light from continuing down the lens to your camera’s sensor or film. This is where the term “stop” comes from, as it “stops” light from traveling through and “exposing” the sensor to light.

Also, known as an “iris diaphragm,” like the iris in your eye, this is a complex part of your lens consisting of an array of small opaque blades that close on a central point. By opening or closing these blades, a hole of varying size is made at the center of this diaphragm. In photography, aperture is the name for the hole the diaphragm, or iris, forms — specifically the size of that opening. Aperture isn’t the name for the diaphragm, but for the opening it creates that allows light to pass through it. It’s with the aperture setting that we can change the size of this hole to let more or less light in, making an image brighter or darker.

The best example to compare this to is your own eyes. The iris (colored part) is the diaphragm, the pupil (black spot in the middle of your iris) is the aperture, a hole allowing light into your eye. Your iris expands or shrinks your pupil to control how much light is entering your eye and hitting your “sensor,” the retina at the back of your eyeball. When the eye doctor “dilates your pupils,” what they are doing is causing your iris to open your pupil wider to allow more light into your eye. By changing the aperture in your camera you are telling it to open or close the iris diaphragm and increase or decrease the amount of light hitting your camera’s sensor.

The next time you have an eye doctor appointment, and are asked if you want your pupils dilated, tell them, “Heck no, I want you to open up my aperture.” The awkward silence following is sure to be from their being struck speechless by your clever wit and optical knowledge.

The aperture setting

Also known as an f-stop, or f/number, on your camera aperture appears as a number like 2.8, 5.6, 8.0, 16, etc. Where many budding photographers, and a few experienced ones, get confused is in understanding what increasing or decreasing this number actually does. As you move up this scale from smaller numbers to larger numbers you are actually shrinking the size of this opening and letting less light in.

Large aperture = small number. Small aperture = large number.

The reason for this is the aperture number is actually a ratio, comparing the size of the opening to the focal length of the lens. For those of you who like equations and the physics of light, here is a great article explaining in depth the science and math behind the settings. For those who are less inclined to study that, the affect is a wide or open aperture allows more light to pass through the diaphragm, a small or narrow aperture allows less light to pass through.

To change the aperture on most cameras you must be in Av (Aperture Priority) or M (Manual) mode. This will allow you to change this setting, either through a wheel or other button.

Depth of field

Not only does the aperture control the volume of light entering the camera, but it also helps control the depth of field or DoF. Controlling the DoF is one of the key composition techniques in photography.

Looking through the camera, you have focused on your subject. The depth of field is, at its simplest terms, what is in focus in your image.

  • A large depth of field means things both near and far away from you are in focus.
  • A shallow depth of field means only the thing you focused on is sharp. Anything else starts to blur, becoming more blurred the farther away it’s from the subject you focused on.
  • This is true for things closer to you than the subject, as well as objects further away than the subject.

This is how photographers get the shots with the bird in front of a blurry background (shallow depth of field, like f/5.6) or a majestic landscape with wildflowers in focus in the foreground and a distant mountain in the background (large depth of field, like f/16).

Depth of field = What is in focus in your image.
Small aperture = large number = large depth of field = “stop down”
Large aperture = small number = shallow depth of field = “open up”

Lenses and apertures

The aperture number we see on our camera is really a comparison of the diameter of the aperture to the focal length of the lens, written as a ratio. Every lens will have a minimum aperture — the smallest diameter hole that can be formed — and a maximum aperture — the largest diameter the hole can be. While it’s important to know both, the maximum aperture is generally what is printed on the lens and used as a selling point of one lens over another.

“Fast lenses” are generally ones that can open wide, typically a small number like f/1.4 or f/2.8. These lenses can produce shallow depths of field, and work well in lower light conditions.

Prime, or fixed focal length lenses, will have a single maximum aperture setting, while zoom lenses may have a range for their maximum aperture setting. This is a common point of confusion; it isn’t the range from maximum to minimum aperture for that lens. Rather, it’s the maximum aperture when the lens is zoomed out and what it changes to when the lens is zoomed in.

But what about….?

There are many terms and equations thrown around when talking about depth of field like circle of confusion, diffraction, hyperfocal distance, etc. Don’t get stuck on having to understand all these at the beginning. While they are important, they are not necessary concepts at the start to make great images. The purpose of this article is to understand why aperture is important, not delve into the mysteries of light and optics.


The correct exposure is the amount of light controlled by the shutter speed, aperture and ISO. This is called the exposure triangle.

Depth of field = what is in focus in your image.

Small aperture = large number = large depth of field = “stop down”

Large aperture = small number = shallow depth of field = “open up”

Up next … shutter speed!