That is how we use the three major ways of controlling exposure with an slr camera. But what if you have struck out all three possibilities and still need more light? That is when your flash comes into play and it too is based on the f-stop system of halving and doubling. More about how to use your flash in another article. For now, just mount the flash and set the shutter speed to 1/60 second, this is the speed at which most cameras will synchronize with the flash. Set your flash for the right exposure.
When all else fails and you get confused, with 100speed film just set the camera to f/8 with the shutter speed at 1/60. If you leave your flash on it will synchronize at 1/60 if you need more light. What if you have 400-speed film in the camera? Remember the rule about film speed, each film speed is one f-stop difference from the lower one. So 400-speed film is two stops more sensitive than 100-speed film so you have gained two f-stops. Now what? This gain of two f-stops means that you can use two f-stops smaller, because the film is more sensitive you need less light or less time to get the correct exposure. How much less? Two f-stops less. Therefore, if you have the old rule of 100-speed film with f/8 at 1/60sec, then you adjust one part of the equation. You have already adjusted the film speed by two stops so you will either adjust the aperture by two f-stops or the shutter speed by two settings: one or the other but not both. If you want you can split the two gained f-stops and give one on the aperture and one on the shutter speed. But not two stops on both, it is two stops combined.
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