The exposure of an image is dependent on 3 variables/elements…

Shutter Speed

Aperture 

ISO

These 3 variables must be set depending on the environment in which the photograph is being taken.

Shutter Speed – A faster shutter-speed lets in less light but takes a still freeze motion image of the subject. And a higher shutter speed lets in more light and gives the image a blur if motion is present.

Aperture – A lower aperture lets in more light and creates a smaller depth of field within an image and a higher aperture lets less light in and gives the image a larger depth of field.

ISO – A low ISO has less light sensitivity and causes the image to have a less grainy aesthetic, and a higher ISO has more slight sensitivity and gives an image a more grainy aesthetic.