Not a member yet? Register now and get started.

lock and key

Sign in to your account.

Account Login

Forgot your password?

Basics – Part 2: Exposure

22 Jan Posted by Matt in Basics, Guides |
Basics – Part 2: Exposure

Following on from part 1 of our Basics series focusing on Composition, today we’re looking at a more technical aspect of photography – exposure.  Don’t worry, this isn’t going to be a technical ramble that you’ll need a physics degree to understand – we’ll just be focusing on the basic ideas.

Even if you don’t consider yourself a photographer and you use a basic point and shoot, having a little knowledge in this area can be a real help.  If you have a more advanced compact, a bridge camera, or a digital SLR then a small amount of knowledge here will be invaluable in getting better results, especially in tricky situations.

So what is Expsoure?

Exposure is just the amount of light that is let into your camera – it can be affected by a number of factors detailed below.  In short, on nearly every occasion a correctly exposed photo is what you are aiming for, i.e. one that is neither too dark or too bright.

Exposure Comparison

What affects Exposure?

In its’ simplest form taking a photo involves light being let through a lens, for a set period of time, so that it hits the image sensor (the digital equivalent of film).

Camera Basics Exposure

To increase or decrease how much light gets into the camera and is seen by the image sensor we have two options – we can

  • Vary the size of the hole letting the light in (the aperture)
  • Vary the length of time the hole is open (the shutter speed)

There is a sneaky third tool we have at our disposal too; being able to adjust the sensitivity of the image sensor, known as the ISO value.

Lets look at each of these variables in slightly more detail…

Aperture: The Size of the Hole


Image: Mohylek on Wikipedia

The aperture of a lens is the size of its opening; a large aperture equates to a big hole so more light can get into the camera, and a small aperture equates to a small hole that will let less light in.

Picture 1 on the left shows a lens with the aperture opened wide, so more light can get in, whilst picture 2 shows the same lens with a much smaller opening.

So why is this useful? If there is limited or poor light available (a situation that can make taking good photos very hard) being able to open your lens wider and let more light in can be a big help.

Even in good light using a larger aperture may be an advantage, as in order to get the correct exposure you will need a shorter shutter speed – more on this in a minute!

Changing the aperture has side effects mind…

Aperture Side Effects: Depth of Field

Depth of Field Effect - Insect

Depth of Field Effect

Aside from trying to make the most of a dimly-lit room, the main reason for changing the aperture of your lens would be to create (or eliminate!) the soft background (depth of field effect) favoured by many when taking portrait photos.

A large aperture (big opening) will, for complicated reasons we won’t get into, reduce the depth of field and create images where either the background or foreground appear blurred and out of focus.  Naturally, the amount of blurring you get will increase as you increase the size of the opening, although other factors such as how far away from your subject the background or foreground is will also play a big part.

Changing the Aperture

Aperture Priority DialSo now you’ve got an idea of what aperture is, how do you go about changing it?  If you have a digital SLR, compact system, or bridge camera then usually you will turn your mode dial to Aperture Priority Mode, signified by the letter A (or letters Av on Canon models).  On compact cameras it’s slightly trickier – if you have an advanced compact model you may either have a dedicated Aperture Priority Mode or a mode designated A/S, a hybrid aperture priority and shutter priority mode.

On some models you may need to delve into the menus to access these settings, and on basic models you may be unable to change the aperture at all.

Once you have worked out how to change the aperture it’s time for the slightly confusing bit!  Aperture values are measured in ‘f stops’, and you may see them written or on your camera display as F3.5, F5.6, F8 etc. Confusingly the numbering is the other way around to what immediately would seem logical, so to increase the size of your aperture you need a small F number and vice-versa.

In Aperture Priority Mode your camera will automatically set the shutter speed to ensure that your photo comes out correctly exposed.

The range of aperture settings available will vary depending on your lens, but as a general rule the smallest opening available will be F32, and the largest opening around F2.  The benefits of a lens capable of a large aperture, and the technical difficulties in producing them, mean they can command a hefty price premium.

Shutter Speed: How Long the Hole is Open

Waterfall Slow Shutter Speed

Slow shutter speeds can show movement

The shutter speed of your camera is just the length of time that the lens opening (the size of which is determined by the aperture, explained above) is open.  It shouldn’t be a shock then that a longer shutter speed will let more light fall onto the image sensor, and therefore result in a brighter image.

There is an obvious and potentially irritating side-effect to using a long shutter speed in that any movement which happens when the shutter is open will show up as blurriness in your photos.

On some occasions, like in this example on the left, a slow shutter speed can be used creatively to give the impression of movement.

Sadly, using a longer shutter speed to compensate for limited light is often not practical, especially if you are using your camera hand-held and/or taking photos of people or other subjects likely to move.

As well as occasions when you want an especially long shutter speed there will be times when a very quick shutter speed is needed, for example when you want to freeze the action of a fast moving object.

Changing the Shutter Speed

Aperture Priority DialIf your camera has a mode dial like that pictured here then turn it to S, or if you’re using a Canon model – Tv (which stands for time value), to put it into Shutter Priority Mode.

As with changing the aperture, on some models (especially compacts) you may need to delve into the menu to access these settings, and on basic models you may be unable to change the shutter speed at all.

Thankfully there is no confusion in how we measure shutter speed – all cameras should measure it in seconds. Typically cameras will have a minimum speed of at least 1/1000th of a second, and a maximum of 30s or more.  For some compacts, very long shutter speeds (say over a second) may only be available in specific night modes – check your manual if you’re unsure.

In Shutter Priority Mode your camera will automatically set the aperture value to ensure that your photo comes out correctly exposed.

If your camera doesn’t have a Shutter Priority mode, or you’re in a rush, you may be able to use the Sports Mode (usually designated by a picture of a man running) to guarantee a short shutter speed, or the Night Mode to guarantee a long shutter speed.

ISO: The Sensitivity of the Image Sensor

The ISO value of your camera is just its sensitivity to light. The term ISO is actually a hangover from the days of film, but the same measuring system is used in digital cameras.

ISO sensitivity is simply referred to as a number, with a higher sensitivity being expressed as a higher number. Most cameras will start with a low sensitivity value of around ISO 100, and may go up to ISO 3200 or even higher on some models, especially digital SLRs.

Increasing the ISO on your camera can help you take photos in low light when increasing either the aperture or shutter speed is not possible or impractical.  However there is a very noticeable trade off in all digital cameras between a high ISO and an increase in digital noise, which appears as multi-coloured speckles.

On most cameras, particularly compacts, an ISO value of more than around ISO 800 can lead to very noisy pictures that look quite poor even at a small size – these examples were taken on a Fuji compact camera (click to enlarge).

ISO 100

ISO100 - Low Noise

ISO 200

ISO 200 - Noise still low

ISO 400

ISO 400 - Noise now visible, but passable

ISO 800

ISO 800 - Noise very noticeable, detail is lacking

ISO 1600

ISO 1600 - Photos here aren't very usable

Adjusting the ISO

Adjusting the ISO varies between cameras; on digital SLRs there is usually a dedicated ISO button that provides instant access to preset values.  On a compact camera you will probably need to explore the menus to find the setting.  ISO adjustment may not be available on basic compacts, but is a common feature found on most mid-range and high-end compact cameras and on all digital SLR, bridge and compact system cameras.

A Last Resort: The Flash

Using the Flash can ruin a photoYou may have wondered why, in an article about getting properly exposed photos, we’ve left it this long to mention the device built-in to practically every digital camera designed specifically for taking pictures in the dark – the flash.

Well, put simply, in most cases using the flash for photos in low-light won’t give you great results – the picture on the left here for example really doesn’t capture the atmosphere of the gig.

We’ll cover ways in which you can make the most of your built-in flash in a future article, but for now here are a couple of quick tricks…

Using the Flash in bright light

Ever taken photos in bright sunlight and been disappointed with harsh shadows? Turning on your cameras flash manually may give you just enough extra light to make your photos much more even… give it a try and you might be surprised.

Night Portrait: Slow Shutter Speed and Flash

Flash and a slow Shutter Speed creates a vibrant imageOne way you may be able to maximise your chances of getting a nice “full” image in bars and clubs or at night is to use a slow shutter speed and a flash.  The idea with this is to let enough natural light into the camera with a long shutter speed so that you capture the ambience of the room or area, but freeze the foreground action (and light it properly) using the flash.

You need a steady hand, and you run the risk of getting pictures that have a fair amount of blur in them if your subjects don’t stay still, but for that mandatory Facebook set of paparazzi-style night out shots it might just give your photos a bit of an edge…

On many compact cameras and some digital SLRs you can create this effect by using the Night Portrait mode.

If your camera doesn’t have this feature then you can simply set the flash to come on in shutter priority mode, setting the shutter speed to something between 1/10 and 1/30 of a second.


Comments are closed.