Photography Blog

Camera guides, technology tips

Technical Terms – Shutter Speed Basically Explained

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I’ve referred to shutter speed in previous articles, but for the sake of completeness I thought I’d write a quick entry on the subject.

What does ‘shutter speed’ mean?


The term ‘shutter speed’ refers to the time for which the camera shutter is held open while a photograph is being taken in order to allow light to reach the image sensor (or film in a non-digital camera). Shutter speed is measured in seconds – on the Canon EOS 400D the available shutter speeds range from a maximum duration of 30 seconds (very slow) to a minimum duration of 1/4000th of a second (very fast).

Shutter speed and exposure

A slow shutter speed will potentially allow more light to reach the image sensor than a fast shutter speed. In order to correctly expose the photograph you need to balance the selected shutter speed with the appropriate aperture size. Basically, if you are using a slow shutter speed the shutter will be open for longer, so you should use a narrower aperture to avoid overexposure, whereas if you are using a fast shutter speed a wider aperture is needed to avoid underexposing the photo.

The relationship between shutter speed and exposure is useful to know about if you are trying to take a photograph in low light conditions where a flash would not be appropriate, for example if you were trying to photograph London at night. In this situation you would need to use a slow shutter speed so that the shutter will be open for as long as possible in order to allow the maximum amount of light to reach the imaging sensor.


Shutter Speed and Camera Shake

The slower the shutter speed the more likely it is that camera shake will occur since it is incredibly difficult to hold a camera perfectly still for even a second. Therefore if you want to use a slow shutter speed without compromising on picture quality you will need to use a tripod to keep the camera steady.

Shutter Speed and Moving Subjects

When you are photographing moving subjects/objects (e.g. moving cars or running water) the choice of shutter speed can make a real difference to the type of photograph produced.
 Previously I only wanted to take photographs that simply capture the reality of my chosen subject and thereby preserve a scene or event for future enjoyment. However, whilst on holiday my boyfriend experimented with some of the settings on the Canon EOS 400D and produced some really interesting images – they no longer directly reflected reality, but were more ‘artistic’.

I shall certainly be playing around with shutter speed in the future!
In order to minimise/avoid overexposure of the photo, the camera compensated for the slow shutter speed by selecting a very narrow aperture size (f/36). Since this aperture value is towards the maximum size available on my current camera setup, the photograph may be slightly overexposed, which would account for the slightly irregular colours.

Technical Terms – Aperture Basically Explained

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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).

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.

Technical Terms – ISO basically explained

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My current photography goal is to get comfortable using the Advanced Modes on the Canon EOS 400D. However, for me it is helpful to know some of the theory behind relevant technical terms before I actually try to put anything into practice, so in the next few blog entries I am going to attempt to get a basic grip on some of the technical features I have encountered when playing around with the various Advanced Modes.

ISO – changing the camera’s sensitivity to light

The ISO figure indicates the sensitivity of the camera’s imaging sensor to light. In the Basic Modes examined in the previous blog entry the ISO speed is automatically set within ISO 100 – 400 as appropriate to the light levels. However, when using the Advanced Modes the Canon EOS 400D has the following ISO Speed options: 100, 200, 400, 800 or 1600. Basically the higher the ISO number you select the more sensitive to light the camera will be and vice versa. Therefore if you are taking photos in a dark room then you should select an ISO figure towards the top of the range (since higher light sensitivity will enable the camera to make the most of any available light there is), whereas if you are in a sunny park you should select a lower ISO figure. This is the very minimum you probably need to know to effectively use the ISO setting in Advanced Modes.

ISO and shutter speeds

Additionally, changing the ISO will affect the shutter speeds that are available for taking the photograph. For example, if you are taking a photo in a low light and you select a high ISO number the camera will allow you to use a faster shutter speed than would have been the case with a lower ISO number. This is because the increased light sensitivy of the camera’s imaging sensor means that the shutter can be open for a shorter period of time and still get enough light to properly expose the picture. Having a faster shutter speed in such situations is good since it means that the camera will take the photograph more quickly, which really reduces the likelihood of camera shake occurring (particularly useful if, like me, you lack the steady hands of a surgeon and do not carry a tripod around with you wherever you go!). The shutter speeds (example) you can use in the photos:

  • Photo (ISO 1600) – a shutter speed of 1/15th of a second, which is very fast and accounts for the much sharper image produced.
  • Photo (ISO 100) – a shutter speed of 0.8 seconds – this is quite a slow shutter speed – I actually had to wait for the camera to take the photograph after pressing the button – in that time clearly my hands weren’t still enough!

ISO and picture quality

One disadvantage to using a high ISO speed is that it may result in slightly grainy images – therefore it is important to experiment with the whole range of available ISO speeds in order to find the lowest one that works in the light conditions rather than just opt for the highest ISO all the time.

Additionally, according to the Canon Manual using a high ISO speed may cause irregular colours in the image. I don’t think there is such an effect in the photos above, but when playing about with ISO settings previously I have noticed that if you set the ISO speed too high in already light conditions it does affect the colours. I’ll try to get an example up here soon.