Kate's Photo Journal

Before I got my Canon EOS 400D I had only ever used point-and-click cameras and so never had to think about the aperture value on my camera. Being non-technically minded I genuinely did not know what the term 'aperture' meant when I first heard it, so I have done some reading and experimenting with my camera to try to get a better (if still somewhat basic) grasp of what the term 'aperture' means, how it is measured and what effect it has on photographs.

What does 'aperture' mean?

The term 'aperture' refers to the opening in the lens that lets light into the camera. Adjusting the aperture value changes the size (diameter) of the aperture, which will affect the amount of light that reaches the imaging sensor in the camera (the digital camera equivalent of film). Obviously, if you set a wide aperture a lot of light will be allowed to reach the sensor, whilst setting a small aperture reduces the amount of light that reaches the sensor.

How do you change the aperture value?

On the Canon EOS 400D you change the aperture value by turning the dial behind the shutter button on the camera body. The new aperture size and any resultant changes in shutter speed will be shown on the LCD display in the camera's viewfinder before you take the photo. You can change the aperture value in the following advanced modes:

1. Av (Aperture-priority AE) Mode - aperture can be changed and the camera will attempt to set the appropriate shutter speed to correctly expose the photo
2. P (Program AE) - turning the dial changes the shutter speed and aperture setting combination (as part of a program)
3. M (Manual Exposure) - where both shutter speed and aperture values can be set as desired

Measuring Aperture Size

Aperture size is represented using f-numbers. A large f-number represents a small aperture - the lens will have a small opening and not let much light through to the imaging sensor. A small f-number represents a wide aperture - the lens will have a large opening and therefore let a lot of light through to the imaging sensor.

This is the case because f-numbers represent the ratio between the diameter of the aperture and the focal length of the lens. As an example:

• if the aperture value is shown as f/4 this means that the aperture's diameter is 1/4 (25%) of the focal length of the lens

• if the aperture value is f/32, the aperture's diameter will be 1/32th of the focal length of the lens - a much smaller diameter than the 1/4 size.

This is probably the very minimum you need to know about aperture and f-numbers in order to have a basic understanding of what you are doing with your camera and why. There is a lot of advanced information available on this subject on the internet (see for example wikipedia: http://en.wikipedia.org/wiki/F-number) and in books.

Aperture, Shutter Speed and Exposure

By adjusting the aperture value and shutter speed of a camera you change the exposure of the photograph you are taking.

'Exposure' is defined in Wikipedia as "the total amount of light allowed to fall on the sensor during the taking of a photograph" (http://en.wikipedia.org/wiki/Exposure_%28photography%29).

Clearly if you set a wide aperture a lot of light will be allowed to reach the sensor, which in turn means that the camera can use a faster shutter speed in order to obtain the correct exposure. This is because the sensor only needs to be exposed for a shorter period of time before it will have received enough light to expose the photograph.

Conversely, setting a smaller aperture reduces the amount of light reaching the sensor and means that a slower shutter speed must be used if the camera is to properly expose the picture - more time is needed to ensure enough light reaches the sensor.

Therefore if you are in a dark environment and want to take a properly exposed photograph without using a flash and with the shortest possible shutter speed (to minimise/avoid camera shake) you should:

1. Set the widest possible aperture (bearing in mind the affect on depth of field discussed below) and

2. Select a higher ISO speed (bearing in mind the potential affect on quality discussed in the previous article).

An example:

Whilst experimenting with changing the aperture value on my Canon EOS 400D in the AV Advanced Mode (Aperture-priority AE) I decided to see what would happen if I made the aperture as wide as possible. On my LCD display the shutter speed 1/4000th of a second (the fastest shutter speed on the camera) was flashing when I took this photo; this is a feature in the Canon EOS 400D that warns you that there is not a fast enough shutter speed to produce the correct exposure for the selected aperture (and ISO settings). As a result the following overexposed photograph was taken:

The settings for this photo were: Aperture - f/3.5 - very wide aperture (letting a lot of light in); Shutter Speed - 1/4000th second - the fastest shutter speed on the Canon EOS 400D (but not fast enough!); ISO Speed - ISO 1600 (much too high for sunny outdoor conditions, but for the sake of this illustration it works). 

You can see by the irregular colours and the lightness of the photo that it has been badly overexposed. This is because the camera could not set a fast enough shutter speed to produce the correct exposure. However, I then changed the ISO speed to 200 (and thus reduced the imaging sensor's light sensitivity) and without changing the aperture the camera was able to set an appropriate shutter speed (1/1600th sec) to correctly expose the photograph:

Aperture Size and Depth of Field

Making changes to the aperture value also affects the depth of field of your photograph:

• If you set a wide aperture the background of the photograph will be out of focus resulting in a shallow depth of field.

• If you set a small aperture, both the foreground and the background will be sharp so your photograph will have much greater depth of field.

Photograph taken with Aperture Value of f/36 (very small aperture)

In this photograph due to the f/36 aperture value all the candles are in focus, which gives a much better sense of depth. Due to the narrower aperture the shutter speed when taking this photograph was 1/15th second - relatively slow. This was to enable the imaging sensor to get enough light to properly expose the photograph in the relatively dark room (with ISO Speed 1600).

Photograph taken with Aperture Value of f/5.6 (wide aperture)

The wide aperture represented by the f/5.6 number has resulted in a blurry background and created a shallow depth of field. This does help the main subject of the photograph to stand out from the others. The wider aperture meant that the correct exposure was produced using the faster shutter speed of 1/800th of a second (with ISO 1600).