Exposure means the amount of light that reaches your cameras sensor at a given time. Depending on amount of light it can be correct, less or more. Correct exposure will produce correct exposure. Same way “less” mean “under” and “more” means “over” exposure. Every camera has a metering system. Taking your camera settings into account, it will measure & calculate the exposure or light amount that hitting your sensor. Then, it will indicate that on exposure meter located at camera’s viewfinder info bar. Exposure meter is a scale with “zero” at the middle, means correct exposure. Less than zero or minus means under exposure and more than zero or plus means over exposure.
To correctly expose the scene, our aim will be to keep the indicator at the middle or “zero” position by controlling the amount of light. There are three things that control the amount of light. They are (1) Aperture (2) Shutter Speed and (3) ISO. Together they form the Exposure Triangle. In “Manual” mode, by tuning them individually you have to bring the exposure indicator to “zero”. In “Auto mode the camera will automatically adjust them to keep it at “zero”level.
By changing the size of the diameter Aperture can control the amount of light that flow through the lens. Bigger diameter allows more light flow. The shutter speed determines how long the light will be allowed to pass through the aperture or lens. ISO determines the light absorption power of the sensor. Higher ISO number means higher absorption power & vice-versa.