Read on for more explanations of some of the most common and useful terms.
All cameras have a diaphragm inside them that can change size to admit more or less light. This is very similar to how the human eye works a circular opening that changes size depending on light levels. For many, the most confusing part about aperture is the nomenclature used. This is quoted in what are commonly referred to as ‘f-stops’. To better explain, a smaller f-stop number such as f/1.8 means the aperture will open wider while a larger f-stop number such as f/22 means the aperture will open very slightly. Unless it is an SLR, your camera will have a range of available f-stops, for example f/2.8-f/8. This means that the widest the aperture can open is f/2.8 and the narrowest, f/8.
Your digital camera contains a light-sensitive panel that records the light that hits it when you press the button to take a photo. This performs the same function that film does in older cameras. The sensor remains hidden behind the closed shutter (see below) and is only exposed when a photo is taken. Your camera records the light hitting the sensor when you take a picture, and stores the result on the memory card in the camera.
A cameras shutter sits just in front of the image sensor. It is normally closed so that no light is getting through to the sensor, but when you press the button, it opens for a certain length of time to expose the sensor to light. The shutter in most cameras will be comprised of two curtains. For longer exposures, the whole sensor might be exposed for a period of time before the second curtain closes it. In the case of fast shutter speed however, the first curtain opens only a tiny gap between it and the second curtain, and travels across the sensor with the second curtain following close behind. The speed of the shutter is expressed in whole seconds as well as fractions of a second. A shutter speed of 1/640 means the shutter will open and the sensor exposed for only one-six-hundred-and-fortieth of a second.
The term ‘exposure’ refers to exposing the sensor or film to light in order to take a photograph. In addition, a photograph is referred to as an ‘exposure’ for this reason. When you take a photograph, how much light the light-sensitive part of the camera is exposed to is determined by the aperture and shutter speed settings, as explained above. Darker conditions will require an exposure with a lager aperture setting, a slower shutter speed, or both. The term ‘correct exposure’ refers to the approximation of what could be seen by the photographer’s naked eye at the time of the photograph, however this is truly a matter of taste. It is possible however, to take a photo in the dark of night at a shutter speed so long that the photo appears to have been taken in the daytime.