Back to basics

ISO - the digital camera sensor’s sensitivity to light
Aperture - the size of the opening in the lens
Shutter Speed - the length of time a camera's shutter is open

If you would like to ditch Auto mode on your camera and try out other manual modes such as A, S, P, and M mode, understanding of the three elements above is crucial. These three elements determine the exposure of the picture.

In layman's term, getting a correct exposure is like filling a glass of water under a running water. you can't control the running water, but you can set the size of a opening where the water can flow through it as well as the time you left it under the running water. Now you just need to make sure that you get a full glass of water (correct exposure) but not too little water (underexposed) or too much water (overexposed) in the glass.

A changes in one of the elements will affect the others.


ISO 
ISO determines how sensitive is the camera sensor to light. It is measured with numbers (100, 200, 400, etc). The lower the ISO, the less sensitive is the sensor to light. High ISO is normally used in dark situation (indoor) or to achieve high shutter speed (to freeze movement). However the use of high ISO (normally beyond 800) tend to introduce noise into the image.

Aperture
Aperture is the size of the opening in the lens. The larger the opening, the more light can get into the camera and vice versa. Aperture is measured in f-stops (F/1.4, F/2.8, F/5.6, etc). The smaller the f-stop number, the bigger the opening. Thus F/1.4 has a bigger opening the F/8. Moving from one f-stop to the next doubles of halves the opening of your lens.

Changing the aperture will also affect the depth of field (DOF). Depth of field refer to the amount of area that will be in focus (the nearest and furthers from your camera). A large depth of field (deep focus) means that a wide area will be in focus while small depth of field (shallow focus) means only a tiny bit of area will be in focus. The bigger the f-stop number, the bigger the depth of field and vice versa.

Shutter Speed
Shutter speed is length of time a camera's shutter is open. When the shutter is open, the camera sensor captures the light coming from the lens. As mentioned, shutter speed is the time the shutter opens, thus it is measured in second (1/1000, 1/160, 3). The faster the shutter speed (shorter time) the less light can be captured by the sensor. With this, shutter speed of 1/1000 capture less light than shutter speed of 1/500 (1/1000 second is smaller than 1/500 second)

Shutter speed can also be used to capture the motion. By using a slow shutter speed, a motion can be captured. The reason why a motion can be captured is, when the shutter is opened, the sensor will captures all the incoming light until the shutter closed. Thus if a slower shutter speed is used, the sensor will captured the movement of an object from the time the shutter opened until the shutter is closed. "Freezing" a motion can also be done by using a fast shutter, a shutter speed that is faster than the motion speed.