Did you know that most picture taking situations have at least six possible combinations of f/sops and shutters speeds that will ALL result in the ‘same’ correct exposure? Furthermore, did you also know that despite having six possible correct exposures only one, maybe two, would be the ‘creatively’ correct exposure?
Every ‘correct’ exposure is nothing more then the quantitative value of an aperture and shutter speed working together within the ‘confines’ of a predetermined ‘ISO’.
For the sake of argument lets say we are both out shooting some flowers in a meadow and we are both using a film speed of 100 ISO and an aperture opening of f/5.6 and whether we are shooting in manual mode or aperture priority mode the light meter indicates a correct exposure at 1/250 second. What other combinations of aperture openings (f/stops) and shutter speeds can we use and still record a ‘correct’ exposure? If I suggest we use an aperture of f/8 what would the shutter speed now be? Since we have cut the lens opening in half (f/5.6 to f/8) I will now need to double my shutter speed time to a 1/125 second to record a correct exposure, (1/250 sec + 1/250 sec= 2/250 which equals a 1/125 second). On the other hand, If I suggest that we use an aperture of f/4 what would the shutter speed now be? Since we have just doubled the size of the lens opening (f/5.6 to f/4) I will now need to cut my shutter speed in half (1/500 second) to record the same ‘quantitive value exposure’.
Now let’s put this into practice and pretend that nine of us have all got together for an evening of shooting. We break into three groups. One third of the group will shoot this night scene at f/4 for at a ½ second, the other third will shoot the scene at a f/8 for two seconds while the remaining third of the group will shoot the scene at f/16 for 8 seconds. You know what? All of us just shot the exact same CORRECT EXPOSURE! Even though each group’s f/stops and shutter speeds were different, the end result was the same; all three groups recorded a correct exposure. However, it’s very clear in this case, that the group, which shot the longest exposure, ended up with the most exciting composition!
f4 1/2 second
f2 2 seconds
f16 8 seconds