The third part of the exposure triangle is the part that lends itself most to creativity in photography and that is aperture. Aperture and shutter speed are the two settings that control the amount of light that reaches your sensor. We explored shutter speed in our last article so now we will explore aperture.
This setting tells the camera how large or small you want your lens opening to be when the shutter button is pressed. This is the confusing part because large apertures have a setting that is small, such as f2.8, and small apertures have settings that are large, such as f22. The f in this setting refers to what is known as the f-stop. Each change in the f-stop effectively lets half as much or twice as much light into the camera to reach the sensor so if you increase your aperture by one stop and decrease your shutter speed by one stop, you will have the same amount of light reaching the sensor. This happens because the amount of light reaching your sensor has just as much to do with aperture as it does with shutter speed so to keep the exposure correct, you would need to make an adjustment to shutter speed for every adjustment to aperture.
Aperture also controls how much of the scene you would like to be in focus. This is known as depth of field. A small f-stop, such as f2.8, means the opening in your lens is open as far as it can go (depending on the lens). This also means that you will have a very shallow depth of field making your subject the only thing that will be in focus. If you would like everything in the scene to be in focus, than a small aperture setting is needed, such as f22. One way to visualize this is to think of a cone…if the opening to the cone is very large, the tip of the cone does not reach very far (short focus distance), but if the opening is very small, the tip of the cone can reach much farther (longer focus distance).
This is how you can consider the aperture to be wide open at f2.8 (short cone) or closed down at f22 (tall cone). Now you may think, “Why would i not want everything in focus?” If you are trying to isolate your subject because the background is too busy, you would want the background to be out of focus. For this type of image, you would want a very large opening in your lens or a small aperture setting such as f2.8 so the focus does not reach that far. In the case of landscape pictures, you naturally would want all of the scene in focus so you can see every part of it, thus you would want the focus to reach as far as possible by using a large aperture setting such as f22. The images below give a clear example of how aperture is used and why it can help with creativity in photography.
Now I admit, this is a bad subject but it shows what I mean. If you went beyond f10, the background would become more clear.
This ends this series, but we will revisit this when we address the photography triangle (framing, light, and focus).