The size of the aperture determines the amount or volume of light reaching the film from a given subject and lighting. The shutter speed determines the length of time this light acts upon the film. Mirror flip-up, Lens diaphragm stopped down, light reaching the film exposure is formed Shutter curtain closes, reflex mirror stays down, back to TTL viewing Total exposure on the film is determined by the combination of aperture and speed. The specific combination you choose under given lighting conditions will depend upon the degree to which you want the greater depth of field associated with smaller apertures or greater movement-blur preventing ability of faster speeds . Thus, allowing more light reaching the film to compensate for the duration of the shutter curtain traveling time.
Exposure is determined by the combination of shutter speed and aperture .
As the numbers on either the aperture ring or shutter speed dial increase by one increment, the amount of light striking the film is reduced by approximately one half. As apertures and shutter speeds will have respective influences in the final effect of the photograph. The camera's metering is just a guide for a theoretical good exposure, you can alter the combinations to suit the effect that youdesired. Basically, apertures varies the depth of field . While selection of shutter speed affects expression of movement, freeze of action or just solely avoiding camera shake.