What is aperture and what it does to exposure?
What is aperture?
The aperture is a mechanism that controls how much light hits the film or a sensor of a camera. The aperture is just like an iris of the eye, which controls light levels inside the eye. It's composed of interlocking metal plates. As you close the iris down to a smaller aperture, it stops more light from passing through to the image sensor. Therefore, as you are varying the aperture, you vary the size of the opening.
The size of the aperture is measured in f-stops and can be adjusted to different sizes. The aperture can be opened up to let in more light or closed to let in less. The higher the f-stop number, the more light your aperture is stopping and less light hits the image sensor. Conversely, the lower the f-stop, the more light hits the image sensor and the image is lighter. For example, a lens set on f8 has a smaller aperture than a lens set on f4.
If we want to decrease amount of light the lens transmits to the sensor, we can always decrease the size of the lens aperture. The lower the f-stop number, the larger the lens opening and the more light hits the sensor.
Opening up one full f-stop doubles the amount of light entering the camera. Closing down one full f-stop cuts the light in half.
The sequence of full f-stop numbers is as follows:
f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16
Figure 1: f-stop numbers
The speed of a lens is measured in terms of its widest aperture. Therefore, a lens that can be opened to f2.8 is a faster lens than one that can be opened only to f4.
In figure 2 you can see the same picture taken with different apertures. Everything else is the same.
Figure 2: Pictures taken with different apertures
The first picture shows an overexposed image. Too much light is gathered which results in image being too light or even washed out. The aperture is too big. The second picture shows properly exposed picture. The third and last picture shows an image that is underexposed. This image is to dark and muddy, because we didn't gather enough light. The aperture was too small (f-stop number was too big).
The aperture also has a huge impact on depth of field. You can read more about the depth of field in this article.
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