Exposure: Getting it Right Everytime, Part IV
Take the same scenario as above. The meter is telling you that you do not have enough light for the current setting of f2.8 at 1/60 sec with 100speed film. What do you do, wait and come back later when the light is better? No not really. Maybe you can't use a slower shutter speed because you don't want to risk a blurry picture or you don't have a tripod or you may be using a long lens. There are still at least two other quick ways to get the right exposure.
The last of the big three is the film speed. We talked about the difference in f-stops between each speed of film. There is one f-stop difference between each film speed. In our above scenario, we need to make up two whole f-stops of exposure. If we are using 100 ISO speed film then we could switch to a 400 ISO speed film therefore gaining the needed two f-stops to maintain the correct ratio.
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
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