EXPOSURE TRIANGLE

Exposure Triangle is made up of three things:

1. Aperture
2. Shutter Speed
3. ISO

Let us have a look how light enters a camera and makes a picture.

The light coming to the lens has to pass through something called Aperture and then passes through the Shutter and finally hits the Sensor.

Aperture: It controls the quantity of light should come to the shutter and then the sensor.
Shutter: It controls the duration of the light.

Aperture and Shutter Speed and very much related to each other. Let us looks how can we balance their values while taking a picture.
If we have a large Aperture opening,more light coming to the lens,  we can go for a fast shutter speed (1/2000). Same case reverses if we  are using a small Aperture(Shutter speed 1/15).

To help us regarding this matter, most of the cameras having Shutter Priority Mode and Aperture Priority Mode.  If we go for a Shutter Priority Mode, the camera will recognize the shutter speed that set by us and provides the best Aperture for the best result.   In Aperture Priority Mode, device will choose the best Shutter speed for a given Aperture for the bast result. In Nikon Cameras, Shutter Priority Mode is given as 'S', where as in Canon it is gives as Tv(Time value). Aperture Priority Mode is given as 'A' in Nikon and 'Av' in Canon cameras.

ISO:  In a low light situation, our eyes readjusts its the sensitivity to light accordingly. Same way, while shooting in low light situations, we can adjust the sensitivity of sensor of the camera. If we are shooting on a bright sunny day, we can go for low ISO values like 100 or 200. But at night or dark situation, we have to use ISO values like 1600 or 3200.