Exposure relates to the amount  of light that is used to form the image and is controlled in three ways.
The Shutter Speed, the Aperture and Sensitivity to light (ISO).

Shutter Speed is the length of time that the sensor or film is exposed to light. The shutter has a time value control running from whole seconds through to fractions of a second. The normal range on most cameras is 30 sec (very slow) to 1/8000 sec (very fast). If light conditions allow, a fast shutter can be used to freeze the subject; if you wish to convey motion in an image, then a slow shutter can be used, where any movement is rendered as a blur. Both fast and slow shutter speeds can be used creatively and is a matter of choice and what the photographer wishes to convey.
Slow shutter speeds can introduce unwanted elements, such as camera shake, so if this is not what is intended then it is advisable to use a camera support such as a tripod, monopod or beanbag, etc.
The length of shutter speed that can be used is dependent upon two things, the available light (largely beyond the photographer’s control) and the Aperture set (within the control of the photographer).

Aperture is the quantity of light allowed to pass through the lens and on to the sensor or film. If you think of the aperture as an adjustable hole in the lens that can be set to large (allowing a lot of light through) or small (allowing little light to pass through) you get the idea.  Apertures are given in f numbers. Eg f 2.8, f 5.6, f 8, etc. The smaller the number the more light passes through and the larger the number the less light passes through. It does sound confusing but to attempt to understand the relationship between the f numbers imagine putting a 1 above the number, hence  1/2.8 is larger than 1/16.
The Relationship between Shutter and Aperture The control of the aperture is directly related to the time that the shutter is open. For the same amount of light to reach the sensor or film, if the aperture is made smaller, then the shutter speed has to be slower. — 1/125 sec @ f 8 will give the same exposure as 1/60 sec @ f 11

Sensitivity to light (ISO) this is  the ‘speed’ in terms of a film’s sensitivity; slow films might be 50 ISO, whereas fast films might be 400 ISO. In digital photography, the sensitivity of the sensor can be adjusted from the equivalent of 100 or 200 ISO up to and beyond 3200 ISO. Increasing the sensitivity is a little like turning up an amplifier; a side effect is that in addition to increasing the speed of the sensor to allow more light to be registered, is that also noise can be increased and this shows up as tiny specks of grain appearing in the image and can detract from the quality of the photograph.

So why are there so many things to control exposure? An explanation of this is partly given here.
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