This is part of a four-part series on exposure.
The opening to the camera lens is called the aperture. You are able control the size of this opening, in increments known as f/stops, thereby choosing the amount of light you want to let through the lens and into the camera in one go. The f/stop numbers go from small; when the aperture is at its widest, to large; when the aperture is small.
To demonstrate, here is a lens aperture at f/2.8:
And here is an aperture at f/16:
While the shutter speed lets you chose how long the light enters the camera, the aperture lets you choose how much light is able to enter in that time. Closing the aperture is known as “stopping down”, while opening it is “stopping up”. Even though modern digital cameras generally have the ability to be set to 1/2-stops, or 1/3-stops, the range of full f/stops is still useful to be aware of because it corresponds to the standard range of shutter speeds (which is also more varied on modern cameras).
Full f/stop range: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32
Both aperture and shutter speed halve the amount of light when stopped down one full stop or f/stop, and double the light when stopped up one full stop or f/stop. This means that if you stop down one full f/stop, say from f/4 down to f/5.6, and decrease the shutter speed one full stop, say from 1/500 down to 1/250, the same amount of light will be allowed into the camera and the same exposure can be achieved.
Depth of Field
Just as changing the shutter speed has an effect on the way motion is captured in your images, changing the aperture also has a visual effect on the image. When you open or close your aperture, it changes the Depth of Field (DoF) in the photo. When the aperture is small (e.g. f/16) the DoF will be high and much of your image will be in focus. When you open your aperture wide (e.g. f/2.8) on the other hand, the DoF will be lowered and some, or much, of your image will be blurred.
A large DoF is good for landscape photos, and times when you want everything to be in focus. Here I kept both the trees and St Paul’s Cathedral in focus by using an aperture of f/11:
Low DoF is great for being able to isolate a single subject from otherwise busy surroundings. Below are two shots of the same leaf. The first photo was taken with a small aperture, f/22, and as you can see the foreground is entirely in focus, as is a lot of the background. In the second image, I opened the aperture up to f/1.8 to lower the DoF. The background has become a soft blur, and the viewer’s eyes are drawn to the leaf in the centre.
When you open or close the aperture to achieve the desired DoF in your image, you are affecting the amount of light entering the lens, highlighting once more the relationship between aperture, shutter speed and ISO. As you make a creative decision with one of these three, you adjust the others to make the exposure work. Next week we’ll look at the effect the ISO has on your photos.