Understanding Exposure

Getting the right amount of light for a correct exposure is like filling a car up with fuel.  Just as the car won't run properly if you don't put in enough of the right substance, an exposure will fail if there is too much or too little light - and if the light is the wrong kind, as well.

Lens aperture settings and shutter speeds

Every time a photo is taken  a photographer has used an aperture (f stop) setting and a shutter speed to control the amount of light making the exposure; the aperture governs the quality of light admitted and the shutter speed controls the length of time  for which that light is admitted.

Both controls are directly related, so they can be used in different  combinations and achieve exactly the same exposure.  Basically, each time you set the lens aperture to the next biggest f/number in the scale - e.g. f/11 to f/16 - you are halving the size of the aperture and therefore halving the amount of light admitted.  Conversely, if you set the lens aperture to the next smallest f/number in the scale - e.g. from f/8 to f/5.6 - you are doubling the size of the aperture and doubling the amount of light admitted.

The same principle applies to shutter speeds.  Each time you double the shutter speed number - e.g. going from 1/60 sec to 1/125 sec - you have the length of time for which the shutter is opened.  However, if you halve the shutter speed number - e.g. 1/15 sec to 1/30 sec - you will double the amount of time that the shutter remains open.

Stops

These units are referred to as 'stops'.  One stop represents a doubling or halving of the exposures and can be achieved by adjusting the aperture, the shutter speed, or both.

To understand this relationship more fully, imagine you are filling your car with fuel.  The amount of fuel required (correct exposure) is provided by opening the valve in the hose (lens aperture) for a certain length of time (shutter speed).  If the valve in the hose is small it needs to be kept open for much longer to fill the tank, whereas if the valve is large the tank will fill up very quickly.  Either way you will end up with the same amount of fuel in the tank.

Exactly the same principle applies to photography.  If you set the lens to a small aperture (big f/number) you need to keep the shutter open for longer, whereas if you set a wide aperture (small f/number) you can open the shutter for less time and still admit the same amount of light.  E.g. if your camera sets an exposure of 1/60 sec at f/11, you could use any of the following combinations to achieve exactly the same exposure -

• 1/8 sec at f/32
• 1/15 sec at f/22
• 1/30 sec at f/16
• 1/60 sec at f/11
• 1/125 sec at f/8
• 1/250 sec at f/5.6
• 1/500 sec at f/4
• 1/1000 sec at f/2.8