Photography: Guide to Exposure

Introduction

This Beginners Guide is designed to give a basic understanding of exposure. Before reading this document, you should read the Beginners Guide to Film.

The Stop

From the Beginners Guide to Film, you will know that a 100ASA/ISO film requires twice as much light to be properly exposed as a 200ASA/ISO film. This concept of changing the amount of light by a factor of 2, is very common throughout photography. It is known as a change of one stop. So we say that the 100ASA film requires one stop more light than the 200ASA film. Or we could say that the 200ASA film is one stop faster.

Control

Shutter Speed

High shutter speeds 'freeze' the subjects movement as the subject has little time to move during the exposure. 1/1000sec will stop a racing car in its tracks. At the other end of the scale, using a slow shutter speed can result in 'camera shake', caused by the photographer being unable to hold the camera still. Shooting at less that 1/60 may result in camera shake. To avoid this, a tripod can be used.

Aperture

The aperture is the hole in the lens that the light passes through. Unlike shutter speed, the apertures available depend on the lens, and are usually changed by rotating a collar on the lens close to the camera. In the above photograph of a lens seen from above, the aperture is the lower ring, the focussing ring is the upper. The larger the aperture number, the smaller the hole and so the less light can get through. Unfortunately, unlike shutter speed and film speed, a doubling of the number is not equal to one stop. A progression of aperture numbers (or 'f' numbers) in one stop intervals looks like this:

The aperture controls the 'depth of field' in the photograph. When you focus on a particular point, all points that distance from the camera are in focus. Points closer and further away may also be in focus, depending upon the aperture. At narrow apertures, more of a scene is in focus, useful in a landscape where you want the foreground and background in focus. On the other hand, when taking a portrait, using a wide aperture will put the distracting background out of focus, isolating and emphasising your subject.

The following table illustrates different depth of field with a 50mm lens

	With the len focussed on the side of the bench, the aperature of f1.4 throws just about everything else out of focus. Notice the benches in the distance and the plant in the bottom left corner. A wide aperature is often used in portraiture or whenever a subject needs to stand out form it's surroundings.
	An aperature of f16 results in everything being in focus. Narrow aperatures are often used in landscapes to make everything from the foreground to the sky in focus.

Exposure

Most cameras can be set so that the photographer sets the film speed and aperture and the camera uses a light meter to calculate the shutter speed required that will correctly expose the film. If the photographer sets an aperture of f5.6, the camera may suggest a shutter speed of 1/125 of a second. If the photographer changes the aperture to f8 (reducing the amount of light getting in by one stop), the camera will suggest a shutter speed one stop lower, 1/60 of a second, doubling the amount of light getting in, and resulting in no nett change in exposure. This means that for any photographic situation there are a huger number of different shutter/aperture settings that can be used. For example:

Compensation

What we want to do is make the camera give the film more exposure, to make the resulting snow white. There are many different ways of achieving this (note that only one has to be done!):

• Manual Mode: If we can do a bit of maths in our head we can work out the correct exposure and enter it into the camera. By using the camera in manual mode so that we set the shutter speed and aperture manually, we simply work out the correct exposure. If the camrea is suggesting a setting of 1/1000th at f8, we know that we want approximately two more stops of light to get to the film. So we can use a setting of 1/250th f8, or 1/1000th f4.
• Alternative reading: If there is an area of medium grey tone nearby in the same light, we can use the meter reading suggested by that area. Good examples of a medium grey tone are grass, some roads, stone and pavement. Note that they might not be medium grey in colour, they just have to be about as bright as it. Any bright red, green or blue is likely to be medium grey in tone. Most cameras have an Auto Exposure Lock button (sometimes called AE lock). This can be used to freeze the meter reading. We simply point the camera at the grass, press the button, recompose and shoot. This is much simpler, and usually the best method!
• Exposure compensation dial: Some cameras have a dial marked -2, -1, 0, +1, +2. The modifies the camera meter by the indicated number of stops. So to make the photograph two stop lighter, we would set this dial to +2 (two stops more exposure).
• Film Speed: Another way to do this would be to fool the camera into thinking that the film inside it required more light. As slower films require more light than fast ones we would decrease the ISO setting in the camera by 2 stops. For example, we sould set the ISO number for a 100ASA film to 25ASA. The camera would then give the film two stops more light than it needed and so turn the snow white. It is important to remember what speed the film is, and to reset it later!

By far the simplest method most of the time is to simply point the camera at something of grey tone, press the Auto Exposure Lock button and recompose. This is why you might see people point their camera at the ground on occassion. They aren't taking a photo of grass, mearly taking a reading.

Obviously the same process must be done when taking a photograph of something fairly black, As an exercise for the reader, work out what to do for taking a photograph of a black cat in a coal cellar.