A correct exposure is determined by the following three controls:
aperture
shutter speed
sensitivity (ISO)
The following items will also affect the look and feel of your photograph:
distance of the subject
focal length (wide angle or telephoto)
Other photographic terms:
depth of field
Exposure differences are measured in stops. Doubling (or halving) the ISO or shutter counts as 1 stop. Doubling (or halving) the aperture counts as 2 stops. The aperture, shutter speed and sensitivity controls are related to each other in that they control how much light (i.e. the 'exposure') your camera gets. The ground rule for this is: if you adjust 1 control to receive more (or less) light, one other (or both) controls have to be adjusted to maintain the same exposure (the same amount of light that the camera receives) to avoid over exposure (too much light, the photo looks too bright) or under exposure (too little light -- the photo looks dark).
Examples:
If you increase the shutter speed from 1/200 to 1/400 (which is one stop), the camera receives half the light it should get. To adjust for this, either: increase the ISO by 1 stop (double the ISO value) or increase the aperture opening (lower aperture number) by 1 stop.
If you increase the ISO from 200 to 400 (which is one stop), the camera gets twice the light it should get. To adjust for this: decrease the shutter speed by 1 stop (for example: from 1/200 to 1/400) or decrease the aperture opening (higher aperture number) also by 1 stop.
Combinations are also possible: in the above examples both other controls can be adjusted by ½ stop each.
Settings on your camera will determine whether the aperture can be changed by ⅓ stop (recommended) or ½ stop.
Controls the depth of field (the range in which objects are sharp):
Increasing the aperture number (closing the aperture -- less light coming through) = an increase in the depth of field.
Decreasing the aperture number (opening up the aperture -- more light coming through) = a decrease in the depth of field.
For most cameras, keep the aperture below F11 to avoid diffraction. This causes the image to becomes less sharp, despite the increased depth of field.
Controls how moving objects are captured:
Increasing the shutter speed (more light coming true) = the lesser amount that objects can move before they appear blurry.
Decreasing the shutter speed (less light coming true) = the larger amount that objects can move before they appear blurry.
The light sensitivity controls the quality of your photograph:
Increasing the ISO (less light needed for a correct exposure = more grain visible.
Decreasing the ISO (more light needed for a correct exposure) = less grain visible.
The goal is to keep the ISO as low as possible.
The distance (as measured from the camera) between the nearest and furthest object that will appear acceptably sharp in the image. The depth of field is controlled by:
Focal point: The distance between the subject and the camera. The further the focal point is away, the larger the depth of field will be.
Focal length: Wide angle or narrow angle (telephoto). The wider the angle of the lens, the larger the depth of field will be.
Aperture: The more the aperture is increased (closing the aperture), the larger the depth of field will be.