Last week we spoke about controlling light through adjusting our aperture and shutter speed. In the photographic world we use the term “Stop” to measure our adjustments to our camera and lens as it applies to the amount of light we are allowing in the camera. For example, if we open up our aperture a full stop, then we have to speed up our shutter speed by a full stop to reduce the amount of light that the larger aperture will be allowing into our camera. (Note: A “stop” can also be referred to as an “f stop”).
First let us discuss f stop’s as they apply to our aperture setting. There are full stops, half stops, and third stops, we will discuss full stops to begin with. The full stop aperture settings would be f 2.8, f 4.0, f 5.6, f 8.0, f 11.0, f 16.0, and f 22.0. Each stop either doubles, (increases), or haves, (decreases), the amount of light entering the camera depending if you are opening your aperture or closing it down. As an example, if you changed your aperture setting from f 4.0 to f 2.8, you would double the amount of light entering the camera. When we shot with film camera’s these were our f stop’s, but with the advent of the DSLR, (Digital Single Lens Reflex), camera’s, we no have 1/3 stop measurements, giving us a greater range of f stops, (f 2.8, f 3.2, f 3.5, f 4.0, f 4.5, f 5.0, f 5.6, f 6.3, f 7.1, f 8.0 and so on). It still takes an increase of one full stop to reduce out light in half, but now we can adjust by 1/3 stops to make finer adjustments to our light.
Now, how do we apply this new found knowledge to our shutter speed? This is a little easier to understand. If we decrease our shutter speed in half, (one full stop), then we double the amount of light entering the camera. Conversely if we increase our shutter speed by doubling it, (one full stop), then we reduce the light by half. In the previous paragraph we used the example of opening our aperture a full stop by changing our f stop from f 4.0 to f 2.8 which doubled the amount of light entering our camera. If our shutter speed was 1/125 of a second, what would we have to do to our shutter speed to compensate for the increased amount of light created by opening our aperture a full stop? We would have to increase our aperture to reduce the light, and we would have to make a corresponding full stop change in our shutter speed. To achieve a full stop change from 1/125 of a second, we double our shutter speed to 1/250 of a second.
To summarize, if we took a properly exposed photo with an aperture of f4.0 and a shutter speed of 1/125 of a second, when we changed our aperture to f2.8 we would have to change our shutter speed to 1/250 of a second to maintain the same exposure.