In order to record an image, the camera film (or sensor, in the case of a digital camera) needs to be exposed to light. The exposure can be controlled in two ways; by shutter speed, which determines how long the film or sensor is exposed and the aperture, which determines the width of the hole through which the light passes. There is also a third factor that needs to be accounted for, namely the sensitivity of the film or sensor to light (see: ISO)

Exposure is commonly measured in terms of EV (for exposure value). EV1 is defined as the exposure obtained with a film or sensor rated at ISO100, a 1 second shutter speed, and an aperture of f/1.4. Each step up on the EV scale represents a halving of the amount of light that falls onto the film or sensor. Thus in bright conditions, such as strong sunshine, a higher EV is necessary to avoid the photograph being over-exposed and burnt out, like this:

The EV can be adjusted in three ways: by changing shutter speed, aperture size or ISO. The table below shows how different combinations of these can produce the same EV.

In short, the following principles are worth remembering:

Because these three factors - aperture, shutter speed and ISO - have a relationship with each other in this way, you can achieve the same exposure in a variety of different ways. For example, suppose it is a windy day and your camera’s automatic exposure program suggests a shutter speed of 1/25s and f/14 at ISO100. Now you’re worried about the risk of camera shake at this shutter speed and would rather shoot faster. No problem. You could:

• Open up the aperture one stop to f/11 which would allow you to increase the shutter speed to 1/50s.
  
  
• Increase the ISO to 200 which would allow the same shutter speed, but without reducing aperture.

As with all things in photography, there are some trade-offs with each approach:

• Changing the aperture will effect the depth of field. The higher the f number, the wider the depth of the field. If you are shooting landscape photographs, you usually want wide depth of field (high f number) in order to produce a sharp image.
  
  
• Increasing the ISO will increase the graininess of the print (for film) or the digital noise (if shooting digitally).
  
  
• Decreasing shutter speed risks camera shake or, if you are shooting outside and there is wind, blurr in things likes trees and plants.

Learning to handle exposure and the factors effecting it is arguably the hardest but also the most rewarding aspect of photography. Four tips:

1. Keep a note of what settings you used to take each photograph. Digital photography makes this really easy, since most cameras will record these settings with the picture.
   
   
2. If you’re shooting an important scene that you can’t revisit (e.g. the view from the top of Everest!) then bracket your shots.
   
   
3. Don’t trust your digital camera’s LCD preview to tell you if a shot was correctly exposed. Instead, you should learn to make use of the histogram if your camera offers one.
   
   
4. Keep experimenting. The more you experiment the more you learn. If a shot comes out well, make a note of what you did to produce the result. If a shot comes out poorly, try to work out what went wrong.

Back to glossary