Exposure is the amount of light used in a photograph. Exposing as much as needed for a picture, is what we consider a “correct exposure”. There are three very important terms when it comes to exposure:
This page will explain these three words and how the influence your picture.
One way to influence the exposure of a picture, is to change the aperture. Simply stated, aperture is to the lens, what the pupil is to our eye: The lens opening. The bigger the aperture, the more light enters the lens (and therefor the more exposed). Aperture is noted as f (or f-stop). A large aperture has a small number (for example f1.4 or f4), while a small aperture has a high number (for example f11 or f16). Changing to a larger aperture from f2.8 to f1.8 is what we call a one stop increase of light. Changing you exposure by one stop, simply means you double the amount of light entering your lens (read this page).
However, the aperture does not only influence the amount of light entering the lens, but also the depth of field. Depth of field is the area in your photograph we consider sharp. In a picture with a large depth of field (created by a small aperture like f11), a large area of the picture is sharp (great for landscapes).
A larger aperture, will create a smaller depth of field. Have you ever seen a portrait where the person on the picture is nice sharp, but the background is gently soft? This is done by using a larger aperture like f2.8 or f4.
If you own a DSLR and what to play around. Set your camera on “A” using the Mode dial and set the aperture as large as possible (go for the lowest number on your display) and take a picture of any object. Now take the exact same picture, when changed to a smaller aperture like f13. Do you see the difference? Make sure to focus on the same spot though!
The other way to influence the exposure of a picture, is the shutter speed. The slower the shutter speed, the longer the shutter is open and more light enters the camera (thus the picture will be brighter). Shutter speed is measured in seconds or a thousandth/hundredth/tenth of a second. However, the longer the shutter is opened, not only more light comes in, but also the more movement is visible. So in dark situations when you need a shutter speed of seconds (or a quarter of a second), movement will be visible. A shutter speed of a hundredth of a second will freeze action, but not allow a lot of light to enter the camera. This means, if you want to freeze movement in a dark situation, you need a fast shutter speed. But because not enough light might enter the sensor you need a large aperture and higher ISO.
Finally, the third influence on exposure is ISO. ISO is an abbreviation for International Standardization Organisation. Since digital photography, ISO is a standardized indication on the light sensitivity of a cameras sensor (like ASA was on a film). ISO also is measured in stops. Changing from ISO 100 to 200 (or from 200 to 400), means you double the amount of light entering the cameras sensor. But there is a catch. The higher your ISO, the more sensitive your sensor becomes, meaning the more sensitive it is to electronic noise. This noise is visible as little spots on the photograph (100 is considered standard and most clean, for an example on noise check this Wiki page). The most common use for changing the ISO from 100 to higher is to reach a correct exposure, while not changing aperture or shutter speed.
These three terms combined, are the holy trinity of exposure. Tuning these three is essential for reaching the right exposure. If you don’t want to change your aperture, you play around with shutter speed and ISO or visa versa.