All digital cameras create an image by focusing light through a lens onto a sensor. When the perfect amount of light reaches the sensor, there is enough light to show details in the darkest areas, but not so much light that the lightest parts are too bright. The goal of a good exposure is to have details in both the lightest and the darkest parts of the image. Today’s cameras have fantastic built-in light meters, and letting the camera determine the exposure is very easy.
Because the light meter in the camera is always on, even when shooting in Manual mode, the camera lets you know what the light meter thinks is the correct exposure. Understanding what affects exposure enables you to be in control, which in turn
allows you to improve your photography skills.
The four main things affecting exposure are:
- ISO.
The sensor in your digital camera acts like film does in traditional cameras. The sensitivity of both film and digital sensors is measured using the International Organization for Standards (ISO) rating. The higher the ISO rating, the lower the amount of light needed. When the light hits the sensor, the information gets captured electronically, and the resulting data will be stored on a memory card. When the amount of light is not high enough to achieve proper exposure, the sensor’s output can be amplified to match the film ISO standards.
- Shutter speed.
Shutter speed is the amount of light passing through the shutter. Before the light reaches the sensor, the shutter has to be moved out of the way. The length of time that the shutter is open is referred to as shutter speed. The brighter the light coming through the lens, the less amount of time the shutter has to be open.
- Aperture.
This concerns the amount of light passing through the lens. The light which reaches the lens may be reduced before it reaches the sensor. A hole in the lens—called the aperture (or diaphragm), which can be made larger and smaller— controls the amount of light passing though a lens. The relative size of the aperture is often called the f-stop.
- Light.
The amount of light reaching the lens affects how you approach a shot. This can be any light that you see, from the sun lighting up an outdoor scene to a table lamp lighting up a room. It makes no difference to the sensor what the source of the light is or how much light is there.
These four factors work together to produce an exposure.
See also: Controlling Exposure, Understanding focal length.
And of course the Photography tips page with a list of articles that might help you improve your photography skills.