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Exposure (part 1)
The term 'exposure' refers to the total amount of light allowed to fall on the image sensor of the camera.
There are three controls on a camera that enable the photographer to control the exposure:
- the shutter
- the lens aperture
- the sensor sensitivity - ISO.
Every modern camera has a light meter that measures how much light is entering the lens and, combined with the electronics in the camera, will compute how much light is needed to make a correct exposure (ie. a photograph with a good balance of shadows, mid-tones and highlights).
The Shutter
The shutter is simply a curtain that blocks light from hitting the sensor. When the photographer hits the shutter release button the shutter opens, lets the required amount of light in, and closes again.
The camera will allow you to control how long the shutter is open for: the longer it is open the more light gets in. This control is termed shutter speed and the value can vary from fractions of a second to whole seconds. Fast shutter speeds are those in which the shutter is open for a short period of time; slower shutter speeds are, unsurprisingly, when the shutter is open for longer periods of time.
Check your camera. You will see that your shutter speeds are labelled in terms such as 1/60, 1/1000 and so on. These measurements are fractions of a second. A shutter speed of 1/1000, for example, is where the shutter is open for one thousandth of a second. A shutter speed of 1/1000 is a faster shutter speed than 1/60 which, in turn, is faster than 1/8.
Note that sometimes a camera will show the shutter speed on the display or in the viewfinder by just showing the lower number. For example 1/500th second might be shown as 500.
Very slow shutter speeds (i.e. from 1s to 30s or more) are identifiable in books by not having the '1/' fractional part preceeding the number and usually having 'seconds' or 's' following it. You probably won't be surprised to learn that 30 seconds is a slower shutter speed than one second. In the viewfinder slow shutter speeds might be shown using a double dash to represent a decimal point. For example, half a second might be shown as 0''5 and 2 seconds as 2''0.
When hand-holding a camera, the slower the shutter speed the more likely you are to have visible camera shake in your image - it is much harder to handhold a camera perfectly steady for 1/2 second (near impossible in fact) than it is to do so for 1/1000 second.
You may also see a shutter speed indicated by a 'B'. This stands for bulb and means that the shutter will stay open for as long as your finger is on the shutter release button. This facility is useful when you need a shutter speed longer than the maximum shutter speed available on your camera (often around 30 seconds).
The Lens Aperture
Your camera lens has a controllable opening at the back. This opening is called the aperture. The lens aperture is in front of the shutter (at the rear of the lens) and controls the amount of light entering the camera at any moment in time. By making the hole larger you can let more light in; make it smaller and the amount of light is reduced.
Apertures are measured in terms of the diameter of the hole as a fraction of the focal length of the lens. You will see them referred to as f/2, f/5.6 or f/16 and so on. Initially this labelling may seem very confusing especially after we tell you that the bigger the number the smaller the aperture. However, bear with us and we'll explain why this is.
The letter 'f' when referring to aperture sizes represents the focal length of the lens. For example, on a lens with a focal length of 50mm, an aperture of f/2 means 50/2, i.e. 25mm. On the same lens, an aperture of f/16 means 50/16, ie. around 3mm. A hole with a diameter of 25mm is bigger than one with a diameter of 3mm therefore f/2 is a wider aperture and will let more light in than an aperture of f/16.
Bear in mind that you do NOT need to know the diameters of lens apertures. It is mentioned in this book just to show you why f/2 lets more light in than f/16. All you need to remember is that lower aperture numbers like f/2 let in more light than higher aperture numbers such as f/16.
Despite the fact that an aperture of f/4 on a 50mm lens is half the diameter of f/4 on a 100mm lens, they both let in the same amount of light. This is explained below but, to take photographs, all you need to know is that if your exposure requires f/4 on one lens it will be the same on any other lens. This is why photographers like using the f-number system rather than actual diameters. f/4 = f/4 regardless of the lens (ignoring the fact that some lenses transmit light a little better than others).
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Just a Little Maths
It may appear rather counter-intuitive that f/4 on a 50mm lens will result in the same amount of light being transmitted as f/4 on a 100mm lens when the aperture sizes are physically different. This can be explained by a law of physics - the inverse square law.
The law states that intensity is inversely proportional to the square of the distance between the light source and the object it is illuminating. This is because the light spreads out more as you get further from the source.
In simple terms this means doubling the distance results in quartering the intensity.
The focal length of a photographic lens is the distance a certain part of it (the nodal point) needs to be from the sensor (or film) in order for it to be focused on infinity. Increasing the focal length means increasing the distance between the nodal point of the lens and the sensor, and, as far as the sensor is concerned, the light source is the lens.
So, in this case light source = lens; object = sensor.
If you double the distance between the lens and sensor, you quarter the intensity of the light hitting the sensor. Therefore, as we move from a 50mm lens to a 100mm lens, we are quartering the amount of light hitting the sensor because the image is twice the size and the light is spread over twice the area 1/(22) = 1/4.
However the aperture is physically bigger in the 100mm lens, in fact it has twice the diameter. Doubling the diameter of a circle will quadruple its area, allowing four times the light to get through. So we can see that the doubling of the focal length reduces the light intensity by a factor of four but also increases the area of f/4 (and all apertures) by a factor of four and these things cancel each other out.
50mm lens at f/4: diameter = 50/4 = 12.5mm; radius (r) = 6.25mm
Area of the circle = pi x r² = 123mm² approx
100mm lens at f/4 : diameter = 100/4 = 25mm; radius (r) = 12.5mm
Area of the circle = pi x r² = 490mm² approx
These figures have been rounded to whole numbers so aren't exact but you can see, taking that rounding into account, that the 100mm lens has an aperture at f/4, four times that of a 50mm lens.
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The ISO Setting
ISO is the International Standards Organisation. Back in the days before digital, when film was the primary choice of all photographers, a standard mechanism was devised for measuring how sensitive any particular film is to light. It started off as an American standard, using the term ASA (American Standards Association) before being adopted as an international standard.
When purchasing a roll of film, a photographer would have a choice of film sensitivities. These sensitivities would be labelled with ISO followed by a number, eg. ISO 100, ISO 200, ISO 400, etc. The higher the ISO the more sensitive the film. By sensitive we mean the amount of light that has to hit the film to create an image of a certain lightness. For example, to create a particular shade of grey on a film of ISO 400 you would only need half the amount of light to create the same shade of grey as you would on film with ISO 200 sensitivity.
Higher ISO films will have more noise/grain in the image than a low ISO film. Therefore there is a trade-off: if you need to shoot in low light without flash you could use a higher ISO film. This is convenient as you don't need as much light to hit the film to make an image so you can use a faster shutter speed if you wish. The down side is that the image will be of a lower quality than that taken with a low ISO film.
You may now be wondering what this has to do with digital photography. In the digital world we have the same concept of ISO ratings except now we have the luxury of choosing a different ISO rating for each picture if we wish, rather than being stuck with a whole roll of film with the same ISO. The trade-off is the same though: the higher the ISO you choose, the greater the noise in the image and the lower the quality.
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