You can adjust the aperture setting and shutter speed to create several different correctly exposed images. The relationship between the aperture and shutter is known as reciprocity. Reciprocity gives the photographer control over the depth of field of the Image, which controls the area of the image that remains in focus. This is the easiest way to control what part of the image you want the viewer to pay attention to.
For example, opening the fens aperture by one stop and decreasing the shutter speed by one stop results in the same exposure. Closing the aperture by one stop and increasing the shutter speed by one stop achieves the same exposure as well. Therefor, f4 at 1/90 of second is equal to f5.6 at 1/45 of a second. The reason is that the camera's aperture setting and shutter speed combine to create the correct exposure of an image.