(Aperture f/5.6, Shutter Speed 1/125 sec, ISO 200)
By now I have read quite a few books on photography. One of the first was Understanding Exposure by Bryan Peterson. It explains in very simple words the Photographic Triangle: Aperture, Shutter Speed, and ISO.
Before understanding this triangle I would leave my camera on Automatic. I stayed far away from Manual. My idea was that it was too complicated for me and the camera was smart enough to take good pictures. I never realized that the camera does not know what kind of picture I want to take. In Automatic mode the camera selects one correct exposure out of a whole set of correct exposures with completely different emotions.
The Aperture determines the size of opening of the lens, the Shutter Speed determines the duration of the opening, and ISO the sensitivity of the sensor. All three control the amount of light that is sensed by the sensor.
For Aperture each step in the sequence f/22 – f/11 – f/8 – f/5.6 – f/4 means doubling the amount of light. For Shutter Speed 1/500 sec – 1/250 sec – 1/125 sec – 1/60 sec – 1/30 sec also means doubling the amount of light per step. And, for ISO 200 – 400 – 800 – 1600 – 3200 each step means doubling the sensitivity of the sensor.
So, a triple (Aperture, Shutter Speed, ISO) represents the lighting of an exposure. If, for example, (f/5.6, 1/250, 100) is a correct exposure then (f/8, 1/125, 100) is a correct exposure as well: the Aperture is halved and the Shutter Speed is doubled, giving the same exposure; the same is true for (f/8, 1/250, 200): the Aperture is halved and the ISO is doubled.
For every triple there are an arbitrary large number of triples with the same exposure. Of course, there are limitation, for example, the widest Aperture of a lens or the lowest ISO of a camera.
So, take control of your camera and start using the Manual mode, and decide yourself what type of picture you want to take: highlight the subject by a wide Aperture or visible movements by a slow shutter speed. The same exposure, different emotions. In upcoming posts I will elaborate on these choices.
Update: I am a great fan of Sean Tucker. Here a YouTube video of him explaining the exposure triangle.