Exposure is essentially the amount of light that falls on the sensor of your camera, resulting in the overall lightness or darkness of a photo. It is common to hear exposures being discussed using stops. The difference between each stop is pretty noticeable. Exposure is controlled by : shutter speed, aperture, and ISO speed.
Aperture - Control the amount of blur or sharpness around your subject.
Aperture is the size of the hole of the lens through which light travels, and is measured in f-stops. When the aperture opens up (lower f-stops: f1.4, f2, f2.8, f4), more light enters the camera, resulting in more light being captured. When it’s reduced (higher f-stops: f16, f22, f32), less light enters. More importantly, aperture also affects how blurred or sharp certain areas of the photo are, also known as the depth of field. For a uniformly sharp image — especially in shots of landscapes or architecture — a higher f-stop is used. This keeps both the foreground and background of the image in focus. However, because the aperture is reduced, you’ll require a much longer shutter speed to capture the same amount of light, so that your photo does not get too dark. One important thing to remember here, the size of the aperture has a direct impact on the depth of field, which is the area of the image that appears sharp. A large f-number such as f/32, (which means a smaller aperture) will bring all foreground and background objects in focus, while a small f-number such as f/1.4 will isolate the foreground from the background by making the foreground objects sharp and the background fuzzy.
Shutter Speed Show the movement of a fast moving subject or freeze it in action.
Shutter speed is the length of time the shutter of the camera remains open. All other factors constant, the longer this is, the more light is captured, and the blurrier moving subjects become. Shutter speed, also known as “exposure time”, stands for the length of time a camera shutter is open to expose light into the camera sensor. If the shutter speed is fast, it can help to freeze action completely. If the shutter speed is slow, it can create an effect called “motion blur”, where moving objects appear blurred along the direction of the motion. Shutter speeds are typically measured in fractions of a second, when they are under a second. For example 1/4 means a quarter of a second, while 1/250 means one two-hundred-and-fiftieth of a second or four milliseconds.
ISO - Sense the right amount of light for the visual effect you want.
With all other factors constant, a low sensitivity (ISO 100 to 800) results in darker images, while higher sensitivities (ISO 3200 onwards) result in brighter images. High ISO pictures are often “noisy,” in that they become grainy or contain discoloration. As a rule of thumb, ISO is set to the lowest possible value that can achieve the desired shutter speed and aperture. When there is plenty of light, you should always use the lowest ISO, to retain the most detail and to have the highest image quality. There are some cases where you might want to use low ISO in dim or dark environments.