Exposure

Exposure is basically getting the correct amount of light onto the film and although it can be confusing I'll try and make it as easy to understand as possible.

In the section on the camera I explained that we control the light entering the camera in two ways, 
                    1. The time that the shutter is open to allow light to reach the film - Shutter speed.
                 2. The size of the hole through which the light passes - Aperture.

Shutter speed is gauged in fractions of a second  i.e. 1/60,  1/125,  1/250,  1/500  1/1000 etc. It is important to note that the step between each of the shutter speeds doubles or halves the amount of light falling on the film. 

Shutter speed gives the following effects on the photograph. A fast shutter speed will freeze movement and therefore a slow shutter speed will blur or show movement. Slow shutter speeds will also show movement of the camera during exposure. This is known as camera shake. As a rough guideline to prevent camera shake the minimum shutter speed used should be 1/focal length of the lens. i.e. if a 500mm lens is used the minimum speed should be 1/500 sec. unless the camera is supported on a tripod.

                  
These 2 photographs show the effects of using fast and slow shutter speeds. The photograph on the left was taken at 1/1000 sec and has frozen the movement of the water. The photograph on the right was taken at 1/8 sec and shows the movement of the water while the shutter is open. A much smaller aperture was selected for the second photograph to keep the exposure the same. (see below) this has brought more of the foreground into focus.

Aperture is gauged as f stops. i.e. f2.8,  f4,  f5.6,  f8,  f11,  f16,  f22 etc. It is important to note that like the shutter speeds the amount of light entering the camera halves or doubles from one f stop to the next. The smallest f stops are the larger numbers. It's easy to remember this by thinking of them as fractions of the lens diameter. i.e. 1/2.8  is bigger than 1/22.

Aperture gives the following effects on the photograph. A large aperture will give a shallow depth of field. (This is the the distance in front and behind the object that is focused on that is also in focus.) A small aperture will give an extensive depth of field.

These photographs show how depth of field can be used for different effects in photography. In the photograph on the left a busy, uninteresting background would detract from the portrait and therefore was thrown out of focus using a large aperture. You can see that the grass in the foreground is also out of focus. The photograph on the right was taken to record a trip to a special place (high force in Teesdale) therefore the background was kept in focus using a small aperture.

Now that you understand shutter speed and aperture you will see how it all falls into place when selecting the correct exposure. Lets think of light as water.

If with 200asa film the correct amount of light to fall on the film to get the correct exposure is 1 cup full then you can see that that cup can be filled in many ways. Turning the tap on fast, for a short amount of time to turning the tap on very slowly for a long time. Each gives correct exposure though the effects on the film are different. 

Example - If the meter on the camera says that the correct exposure is -   1/250 second at f8   then if we half the time the shutter is open 1/500 but double the size of the hole through which the light is passing  f 5.6 we still have exactly the same amount of light falling on the film.

I mentioned that 200asa film would require 1 cup of water, 400asa would require only half a cup full for correct exposure and 100asa 2 cups full. If you find because of light conditions that you need a faster shutter speed to freeze movement and the aperture is fully open then each time you move to the next fastest film you will gain another stop.

It may take some time for you to get your head round this, but once the 'penny drops' you'll find it quite easy to understand. If you have a camera that has a manual exposure setting then using this is a good way to get used to understanding exposure.
                               

Camera / Lens / Film / Exposure / Composition