Those who have been playing with their digital SLR cameras or SLR cameras for a while now, probably know that the shutter speed and the aperture are two mechanisms that help you control the exposure or the amount of light entering your camera. Shutter speed controls the speed with which your focal plane shutter opens and the time duration for which the film or sensors are exposed to light. The aperture, which is measured in f-stops, is the opening of the lens that helps you control the amount of light entering your lens. Technically, Exposure is defined as a multiple of time and intensity:

Exposure = Time x Intensity

So if you want to keep exposure constant then you can achieve this by manipulating the aperture and shutter speed. You would have often seen that when you are shooting on aperture priority then with every stop of the f-stop that you decrease, the shutter speed increases by one stop. This is because the f-stop and the shutter speed are inversely related. If you see the table on your left, you will see that each set of f-stop values with the corresponding shutter speed will give the same exposure or the amount of light entering your camera, as the other. This inverse relationship between f-stop and shutter speed is called the reciprocity law.

Next time you want to shoot with a deep depth of field and the light is low, take a tripod and decrease the shutter speed as your f-stop will be high and you can’t change this. Similarly, if you are snapping a high-speed object then you cannot decrease the shutter speed - to achieve the same exposure you can open the aperture or decrease the f-stop.