In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens aperture, and scene luminance (ISO).
So, there are three variables to create the exposure we need. Shutter speed, Aperture and ISO. In different situation we choose different values of these three variables. That's why it is also known as exposure triangle.
Shutter Speed:
Shutter speed defines how much time light will fall on the film or sensor. It is measured by the unit of second(s), like 30 s, 15 s … 3 s, 2 s, 1 s, 1/2 s, 1/4 s … 1/250 s, 1/ 500 s etc. There are another mode of shutter speed called 'Bulb mode'. In this mode the shutter will remain open until you hold the shutter button. The longer the shutter speed the more exposed the image will get and the shorter the shutter speed the less exposed the image will be at fixed aperture and ISO.
If you are photographing a subject that is in motion, you will get different effects at different shutter speeds. High shutter speed will give you freeze motion of the subject. In other hand a slow shutter speed will give you a blurry image from two way. One is camera shake (if the camera is hand held) and the another is motion blur of the subject.
For star trail kind of photography you have to use lower shutter speed like 30 s or more using Bulb mode.
Sometimes motion is required in the image to show the object is moving. This time you have to choose correct shutter speed according to the speed of the object.
1/80 s is the lowest shutter speed you can hold the camera in hand. Lower than that can cause camera shake. To prevent that tripod might needed.
Aperture:
Aperture controls the amount of light that passes through the lens and falls on the film of digital sensor. There has an adjustable iris diaphragm, a thin opaque structure of some blades with an opening in center, controls the amount of light entering. It is expressed as an f- number, such as f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 or f/32. Changing the f-number changes the size of the whole. The higher the f-number, the smaller the aperture and less light that passes through the lens. The lower the f-number, the larger the aperture and more light that passes through the lens.
Aperture also controls depth of field. Small f-number ( f/1.4, f/2, f/2.8 etc.) or wide open aperture creates shallow depth of field. On the other hand big f-number ( f/11, f/16, f/22 etc.) or small aperture creates wide depth of field.
ISO:
ISO stands for the International Organization for Standardization — an organization that sets international standards for all different kinds of measurements. But, when in reference to your camera, the ISO is your camera’s sensitivity to light.
Digital cameras convert the light that falls on the image sensor into electrical signals for processing. Doubling the ISO sensitivity doubles the electrical signal. So, same amount of light creates double amount of electrical signal than before, creates one stop brighter image than previous.
For the film cameras ISO was fixed for a particular film. Film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The bigger the silver halide crystals are the more light sensitive the film becomes.
There has one down side of increasing the ISO and that is grain. Higher the ISO, higher the grain on a image. But now a days digital cameras working much better with higher ISO.
ISO normally measured by numbers like 50, 100, 200, 400, 800, 1600, 3200 etc.
From 100 to 200 it doubles the amount of light in the image. Same for 200 to 400 or 400 to 800 and so on.
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