Understanding exposure: shutter speed, aperture, and ISO

When your camera is set on automatic, making a photograph is as simple as pressing the shutter release button. Somehow, the camera magically records just the right amount of light to render an image of the scene before it. But what is really going on? How does the camera know how to do that?

Read on to find out how a little knowledge about what goes into making an exposure can open up new worlds of creative possibilities.

There are essentially three factors that go into making an exposure whether on film or a digital sensor. These three things are the shutter speed, the lens aperture, and the film or sensor sensitivity called ISO. Each of these things individually and taken together affect how much light from a scene is recorded. And photography is nothing if not capturing light.

Shutter speed is probably the simplest to understand. Inside your camera is a movable screen in front of the film or sensor call the “shutter.” When you press the shutter release button, the camera opens the shutter and then closes it again. This controls the amount of time that light is collected. It is measured in fractions of a second from 1/8000 of a second to 30 seconds.

The aperture is a variable-sized hole inside your lens through which all light must pass before it reaches the shutter. The hole can be small (“stopped” down) or it can be large (“opened” up). The wider the opening, the more light that can be collected. Aperture size is expressed in “f-stop” (or “f-numbers”) like this: f/2.8. The smaller the f-stop, the larger the aperture is. When someone says they are shooting “wide open” they are referring to the aperture in their lens being open to the largest size. And when a photographer refers to “stopping down,” he means closing the aperture (setting the camera to a bigger f-stop number).

Everything in the frame is in focus.
1/320s @ f/9

Let’s talk about these a minute before moving on. To record a given scene, your camera has to capture light from the scene and record it on the sensor. This is called the “exposure.” You can do this by capturing as much light as possible for a short time (a wide aperture and a short shutter speed) or by capturing a little light over a longer time (a small aperture and a long shutter speed). But why on earth does this matter? Because these two simple factors give you a huge amount of creative control over the resulting image.

Shutter speed and aperture control how much light gets to the sensor. But they also control something else. Shutter speed also controls time. A very fast shutter speed will capture the briefest of moments on film, freezing action, suspending water droplets in mid-air. A very slow shutter speed will blur a mountain stream, capture star trails moving across the sky, and blur moving objects to give a powerful sense of motion.

The aperture also controls the “depth of field.” This is the portion of a photograph from front to back that is in acceptably sharp focus. A very small aperture opening will keep everything in the frame from the closest flower to the furthest mountain peak in focus. But a very large aperture opening (remember, a smaller f-stop!) will keep only a slice of the photo in focus: just the eyes of a portrait remain focused while the background is thrown into beautiful blur.

Narrow depth of field
Narrow depth of field.
1/60s @ f/2.8

These settings affect each other because you still need a certain amount of light to record a good exposure. So, if you widen the aperture letting in more light, you also need to shorten the shutter speed to compensate. And if you lengthen the shutter speed to let light fall onto the sensor for a longer time, you need to close down the aperture to let less light in.

Fortunately, most cameras come with Shutter Priority and Aperture Priority modes to make these computations simple for you. If you put your camera in Shutter Priority mode (often labeled “Tv”), you set the shutter speed for the effect you want and your camera will compute the correct aperture. You’ll do this when you want to control how much blur is in the photo or to make sure you can freeze the action. In Aperture Priority mode (“Av”), you set the aperture and the camera will compute the correct shutter speed. Do this when you want to control how much of the photo is in focus from foreground to background, either a sliver of a portrait (large aperture, small f-number) or an entire landscape (small aperture, big f-number).

Lenses usually indicate their biggest aperture setting. Lenses that are capable of very large openings like f/2.8, f/1.8 or even larger are called “fast” lenses because they let in a lot of light enabling fast shutter speeds in low-light conditions.

The last factor is ISO sensitivity. The more sensitive your film or sensor, the less light needs to be captured to record the image. ISO sensitivity also affects noise or grain. The higher the ISO, the grainier/noisier the image will be. But higher ISO also allows you to shoot with faster shutter speeds in low light, an option that is preferred by many photographers over using a flash.

Letting go (motion blur example)
Slow shutter speed, small aperture.
1/40s @ f/20

The last thing to remember is that shutter speed, aperture, and ISO are usually expressed in numbers that indicate a halving or doubling of exposure. For example, 1/30s lets in twice the amount of light as 1/60s but half as much as 1/15s. f/4 lets in twice as much light as f/5.6 but half as much as f/2.8. And ISO 800 is twice as sensitive as ISO 400 but half as sensitive as ISO 1600. This is useful to remember if you are going fully manual and want to control all three settings yourself. For example, if you slow down the shutter speed by half, you can either decrease the aperture size to keep the same exposure (1/60s @ f/5.6 is the same as 1/30s @ f/8) or decrease the ISO while keeping the aperture constant (1/60s @ ISO 400 is the same as 1/30s @ ISO 200 with constant aperture).

Shooting in aperture priority or shutter priority modes is an easy way to take advantage of this knowledge without having to remember formulas or do any math. If you want motion blur or frozen action, go with shutter priority mode and use a long or short shutter speed. If you want to control the depth of field, use aperture priority mode and set the aperture to the smaller f-stops for narrower focus (portraits) and larger f-stops for wider focus (landscape photography).

By John Watson

John is the original founder of Photodoto, but after running it for 4 years he had to focus on different things. If you're interested in what John has been up to recently, you can check is personal blog or browse his photo blog.