Photography, like playing a musical instrument, is a lot of fun. Anyone can make noise on a piano but to really make music, it is important to read music, learn proper fingering and practice your scales. Learning how to control exposure is a similar discipline. It's not exactly fun but, if you ever want to get beyond 'click and hope for the best' it is absolutely essential.

What is exposure?

The fundamentals of a camera are very simple: a box with a door (the shutter) that allows light in to burn an image on a light-sensitive surface. The term 'exposure' refers to the amount of light that the film - or the sensor - is exposed to.

The most basic photographic errors come down to allowing in too much light (over-exposure/too bright) or too little (under exposure/too dark).

A word about light

Understanding exposure makes one marvel at the human eye's capacity to adjust to the huge differences in light we experience every day. There is around 1,000 times more light outdoors on a bright sunny day than in an averagely lit room indoors. Our eyes can simultaneously process a sunlit lawn and a log in the deep shade of a tree. Cameras are becoming better at this but the light 'latitude' of a sensor is still nowhere near what we are capable of. To correctly expose the sunlit lawn (ie make it appear as it does to the naked eye), the log would be lost in blackness. To correctly expose the log would leave the lawn a blaze of white.

Controlling exposure: ISO, aperture and shutter speed

These are the holy trinity of photography. They work in tandem and to master them is half the battle of becoming a true photographer.

ISO. Don't worry about what the acronym stands for (it's 'International Organization for Standardization', if you must know). ISO refers to the 'speed' at which film - or a sensor - absorbs light. Outdoors on a bright sunny day you would use ISO 100. This is a 'slow' ISO but since there is so much light it can absorb it very easily. A 'fast' ISO (eg 1600) such as you would use at dusk, would be overwhelmed or 'burnt out' by so much light. Slow ISOs give minimal grain (for film) or noise (for digital). Generally this is desirable. Shots taken in low light tend to be grainy or noisy - a trade-off for the being able to shoot when it's so dark.

ISO is the first thing I set on my camera when starting a shoot. Unless the light changes, I can the forget about it and move on to...

Aperture. Put crudely, the aperture is the size of the hole through which the light passes on the way to the sensor. It belongs in the lens rather than the camera body. Making the aperture larger allows in more light; making it smaller allows in less. Aperture is measured in 'f stops' with slightly odd numbers attached. f2.8 is relatively wide. If we halve the size and therefore the amount of light we talk about 'going down a stop' to f4. If we go down another stop, we're at f5.6 etc. Aperture is universal - it doesn't matter if your lens is tiny and retractable on your point-and-shoot, or needs two men to lift, f8 is f8 on both lenses and the results in terms of exposure will be the same.

Aperture not only controls exposure (how bright/dark an image is) but determines depth of focus - one of the most creative tools available to the photographer. But more on that in a later tutorial.

Shutter speed. The shutter is like a door. Most of the time it's closed but every now and then, when we press the shutter button, it opens. The longer it stays open, the more light it lets in. Shutter speeds vary greatly - for portraits, 1/125 of a second is fairly normal.

Aside from exposure, shutter speed is important in allowing the photographer to blur or freeze movement. Again, that will be discussed in a later tutorial.

ISO, aperture and shutter speeds all work on the principal of 'stops' a standard measure of light that is most easily tracked in half or double increments.

For ISO and shutter speed, this is fairly straight forward. ISO 400 (a good ISO for a heavily cloudy day) is clearly a stop 'faster' - or twice as light absorbent - as ISO 200.

A shutter speed of 1/250 is twice as fast 1/125 and therefore lets in half the amount of light.

For aperture, only the numbers are confusing - the principal remains the same. f11 allows in half as much light as f8 since it is one stop 'smaller'.

If you take a shot that is too dark, you could try increasing the exposure (letting in more light) by a) opening up the aperture by a stop (say f8 to f5.6) or, b) slowing the shutter speed by one stop (eg from 1/500 to 1/250). This way you double the amount of light entering the camera. Either adjustment, of aperture or shutter speed, will result in the same exposure.

If all that has left you a little befuddled, fear not, in the next tutorial, I'll take another look at exposure - this time how it can be applied in a more practical sense using the different camera modes (M, TV and AV).

I'm Tom Greenwood- I'm originally from England, now happily residing in Sydney. Not so long ago I was a journalist in Southeast Asia. After I made the switch to photography I worked as a photojournalist as well as shooting for organisations such as the United Nations World Food Programme, Oxfam and Save the Children.

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