Cameras are simply boxes with a lens at one end and a light sensor or film at the other. They have shutters which when open allow light to enter. The light comes in through the lens and whilst the shutter is open the light is recorded as an image on the film/light sensor. The image that is recorded during this process is referred to as an exposure.
This is perhaps the most primitive definition of the word exposure but this word gets used in several different contexts:
1. Exposure is often used to refer to the quantity of light that reaches the film/sensor, (dependent upon aperture and duration controlled by shutter speed).
2. A printed image can also be referred to as an exposure.
3. Sometimes photographers refer to the number of exposures that they have remaining on their film.
So how do you produce the correct exposure? Well, there is not really such a thing as a single correct exposure. It really depends upon what you aiming to produce as a photographer.
There are three main variables that control exposure: Aperture, Shutter Speed and ISO.
The aperture setting allows you to adjust the amount of light coming into the camera by controlling the size of the opening. You can choose a narrow aperture or a wide aperture. Narrow apertures let in less light, so are appropriate for scenes where there is simply too much light. Narrow apertures also have the effect of applying focus to a larger area of the photograph, which is often desirable for landscape and architectural photography. Wider apertures are often used for more selective focus, for example portrait photography when focusing on the subject and blurring the background. A telephoto lens can also help achieve this affect. Often selective focus is desired in the exposure and increasing the size of the aperture is often the best way to increase the amount of light.
Shutter speed determines the amount of time the film/digital sensor is exposed to the light.
Typical shutter speeds are between 1/300th of a second and 1/30th of a second.
If you are capturing a moving scene or object then you will need a faster shutter speed in order to avoid blur due to the motion. Similarly, if you do not have a tripod, and cannot hold the camera perfectly still, you will also see blur at lower shutter speeds, so for these shots you should generally use a shutter speed above 1/30th of a second.
For these reasons, it is normally advantageous to use a fast shutter speed. However, increasing the shutter speed has the effect of decreasing the overall light exposure. This can become an issue in low-light scenarios and a common technique to use in these scenarios is to remain on a low ISO setting, but decrease your shutter speed to still get the amount of light required for the exposure. This can help reduce noise and means you have less reliance on the ISO sensitivity of your film/camera sensor.
It is, of course, possible to use much more extreme shutter speeds varying from 1/1000th of a second to several seconds. Extremely long shutter speeds are often used to produce special effects, such as motion trails of cars and light. Long shutter speeds are also used in astronomy and celestial photography to help illustrate the motion of celestial bodies.
For these shots a narrow aperture can be used to avoid over-exposing the photograph to compensate for the amount of light hitting the film/sensor whilst the shutter is open for such a long time.
ISO is the other significant variable that can affect exposure. Your choice of ISO will affect which shutter-speeds and apertures to use. ISO is a measure of light-sensitivity of your digital camera or film. High ISO can be used to pick out detail, even in low-light situations. The disadvantages are that high ISO can make lights took unnaturally bright and also can lead to the camera capturing more noise and grain in the images. More sophisticated cameras are less susceptible to this, and there are good post-processing software tools for removing noise. However, generally speaking, increasing the ISO will increase the amount of noise.
These days most cameras have an automatic or “green box” mode that will attempt to automatically select the best settings for your exposure.
This is the easy way to take photographs. However, even if the automatic exposure is adequate, it will be just one out of many possible exposures that you could have selected if you had taken the photograph manually, and it is unlikely to meet your expectations all the time, no matter how advanced the camera. The point is, no matter how advanced the camera is, it cannot predict via telepathy how you would like the photo to appear. Furthermore, even these advanced automatic cameras will often produce poor exposures and will at least require some level of manual guidance.
To get consistently good exposures, there is nothing for it but to learn how to take photographs in manual mode. Once you have learnt how to do this you will understand your camera much better. You will then be really able to take control of your camera and understand when it’s best to shoot manually versus shooting in automatic mode.