Perfect exposure settings: Reciprocity is the law of the relationship between shutter and aperture. It stipulates that one stop increases in aperture is equivalent to the shutter duration doubling. Both increase light by one stop.
In addition to this the photographer may wish to over expose or under expose. Understanding the law of reciprocity allows them to do this in a controlled and intuitive manner, understanding the exposure differences on the image PLUS the different changes to the depth of field or any motion blur increase or decrease.
Sometimes however, the aperture you want and the shutter speed required do not give an adequate exposure with the available light (either natural or including flash). Fortunately there is another variable which effects exposure with the same degree of effect and working in the same measurements of stops of light.This is ISO and refers merely to the cameras sensor’s sensitivity to light.
You therefore have THREE variables at your disposal. Each however alter the image beyond just the brightness of the final image.