Shutter Speed, Aperture and ISO Explained

My name is David Taylor. I’m a professional landscape and travel photographer and author of 12 photography books, based in Northumberland, England. Welcome to an extract from my new MyPhotoSchool 4 week online course Beginners Guide to Digital Photography. In this course I’ll be introducing and explaining concepts, such as exposure and white balance, shutter speed and aperture.

Digital photography is not complicated!  Simply put, cameras rely on a combination of shutter speed, aperture and ISO to make a picture and once you mastered this, you can then focus on the creative side of the art-form.  Cameras are essentially light-tight boxes that contain a light-sensitive surface inside. In a digital camera this light-sensitive surface is the sensor. Welcome to my beginners guide to digital photography.

When you press the shutter button on your camera light enters the camera to fall onto the sensor. At that point the quantity and quality of the light is recorded and converted into data that is then processed by the camera to create the final photo.

Photography would be far easier if the level of available light was constant. However, the world is more complex than this and also more interesting. You’ll quickly learn as your photography skills develop that the amount of available light varies enormously, particularly when shooting outside when it can fluctuate from minute to minute.

This photo was shot just before the sun emerged from below a rain cloud. A few minutes earlier and the light levels were far lower, a few minutes later when the sun was fully out the light levels were far higher. If I’d shot a photo for each of these three different lighting conditions, three different exposure settings would have been needed.

To accommodate this variability in light levels cameras have three exposure controls. The three controls are the shutter speed, the aperture and the ISO setting. Altering just one of these controls will affect the look of the final photo, sometimes in very dramatic ways. Understanding how these three controls work and the affect they have on a photo is therefore the key to successful and creative photography.

Modern cameras are usually equipped with a variety of shooting modes. Some of these modes will automatically set the exposure for you so that you just have to point and shoot.

Although there’s nothing wrong with using these automatic shooting modes, they often allow at best a limited amount of control over the final exposure. If you want to take your photography further I’d recommend using one of the semi-automatic modes such as aperture or shutter priority. Or, if you really want to be in the driving seat, using manual exposure mode. I’ll mention which mode to use when later in this lesson.

Managing the amount of light that reaches the sensor is important. If too little light reaches the sensor the resulting photo will be underexposed. Underexposed photos look dark and muddy, with dull highlights and little detail to be seen in the shadow areas.

However, if too much light reaches the sensor then the photo will be overexposed. An overexposed photo is light and airy with bright, open shadows but the highlights will often be burnt out to pure white. Between these two extremes is a photo that is correctly exposed and has detail in both the shadow and highlight areas. This photo has achieved that balance, with no detail lost in either the shadows or highlights.

Although it’s possible to make some adjustments to under or overexposed images later this isn’t ideal and it’s always better to achieve the correct exposure in-camera if possible.

I’ll discuss a way to check whether the correct exposure has been achieved later.

The three exposure controls on a camera work on the principle that you can vary the amount of light reaching the sensor by either doubling or halving it. This is known as increasing or decreasing the exposure by one stop. So, when photographers talk about increasing the exposure by a stop it’s a shorthand way of saying that they need to double the amount of light that enters the camera to make the final photo. Or, if the exposure is decreased by a stop the amount of light is halved.

This principle of doubling or halving light is probably easiest to understand with the first control, shutter speed.

In front of the sensor inside your camera is a shutter. In its simplest form a shutter is made of two opaque metal or cloth curtains, one in front of the other. When you press the shutter button down on your camera to make an exposure the first or front curtain begins to rise, exposing the sensor behind to light. Then, after a precise period of time, the second or rear curtain follows, stopping light from reaching the sensor as it rises. The period of time between the first and second curtain rising is the shutter speed and is chosen either by you or set automatically by the camera.

Shutter speeds are measured in fractions of a second. The fastest shutter speed generally found on cameras is 4000th of a second, which is a fantastically short period of time - though some cameras can manage even faster shutter speeds. This photo was shot at an 8000th of a second - the fastest shutter speed available on the camera that I use.

Shutter speeds then get slower in a logical progression: 2000th of a second, 1000th of a second, 500th of a second and so on down to half a second. From that point on shutter speeds are measured in whole seconds, one second, two seconds, and so forth, usually to a maximum of thirty seconds.

If you have an aptitude for maths you’ll have immediately noticed a pattern. The shutter speed examples I’ve just mentioned double in length as the shutter speed gets slower or halve in length as it gets faster. So, a 2000th of a second is twice as long a period of time as a 4000th of a second but half as long as a 1000th of a second. Each time the shutter speed is doubled or halved, the amount of light reaching the sensor is also doubled or halved.

Setting the shutter speed is generally achieved by using either shutter priority or manual exposure. If you want to experiment with using different shutter speeds I’d recommend using shutter priority. This is a semi-automatic mode that allows you to alter the shutter speed, with the camera adjusting the aperture to maintain the correct exposure. Which neatly brings us to the next exposure control, the aperture.

Inside a camera lens is an iris, known as the aperture. The diameter of the aperture can be altered in a very precise series of sizes. This series is known as the f-stops. The most common range of f-stops, in sequence, are: f/4, f/5.6, f/8, f/11, f/16, and f22.

Slightly confusingly, the smaller the f-stop number the wider the aperture. So f/4 is a far wider aperture than f/22 and is thirty-two times larger in area. Although the f-stop sequence initially doesn’t look as logical as the shutter speed sequence each f-stop also represents either a doubling or halving of light let through to the sensor. So, f/5.6 will allow through half as much light as f/4 but twice that of f/8.

Different lenses have different maximum and minimum apertures. A lens that is referred to as being ‘fast’ is one that has a large maximum aperture, whereas a ‘slow’ lens has a relatively small maximum aperture.

Setting the aperture is achieved by using either aperture priority or manual exposure. Of the two, I’d recommend using aperture priority. Again, this is a semi-automatic mode that allows you to alter the aperture, with the camera this time adjusting the shutter speed to maintain the correct exposure.

The third, and final, exposure control is the ISO setting. The ISO setting regulates how sensitive the sensor is to light. The higher the ISO setting, the greater the sensitivity. This means that with a high ISO setting less light is needed to make an acceptable exposure.

However, the drawback to using a high ISO setting is a reduction in image quality due to an increase in noise. Noise is seen as random splodges of colour or as a distinctive grainy texture that is particularly noticeable in areas of even tone such as sky or in the shadow areas of a photo. This photo was shot at dusk. Even though the light levels were low I used the lowest ISO setting I could to keep noise to a minimum.