In addition to metering the quantity of light reaching the photo sensor, the aperture allows the photographer to control the amount of blur in the image foreground and background. Blurring parts of the image creates an aesthetic quality, known as bokeh, that brings out the subject drawing the viewer’s attention.
The range between the nearest and furthest objects that can be subjectively described as being in focus is the depth of field. Increasing the aperture size will decrease the depth of field creating an image with more bokeh. Of course reducing the aperture has the exact opposite effect.
The images below show the camera optics that create the bokeh. Both diagrams are the same except for the size of the aperture. The photographer has adjusted the camera to bring the subject into sharp focus as shown by the dashed lines. Light that reflects off a specific point that’s further from the camera than the subject will come to a focal point behind the photo sensor. Of course, the image is recorded at the photo sensor before these beams come into focus. The image of the point in the background will be spread across the portion of the photo sensor between the two peripheral beams of reflected light. The amount of blur is a factor of the area of the photo sensor stimulated by light from the same background point. Comparing the two diagrams, you can see that reducing the aperture size prevents some of the peripheral beams from reaching the photo sensor. Narrowing the incoming light reduces the area on the photo sensor that records the background object’s image. It is in better focus.
Because the aperture also controls the quantity of light entering the camera additional adjustments are needed to ensure that the image has the appropriate exposure. Most commonly this is done by pairing a large aperture size with a faster shutter speed. The photographer can also utilize filters and photo sensor light sensitivity (ISO setting) to influence exposure.
The three photos below were taken as an experiment to help visualize the foreground and background blurring at different aperture settings. They were taken in aperture priority mode allowing control of the aperture size while letting the camera automatically adjust the shutter speed to achieve an acceptable exposure.
In each photo compare the focus of the numbers along the yard stick. Remember that aperture size, as expressed by the F-number, is inversely proportional to the area of the aperture. Large apertures have small F-numbers.
The photo below provides a real world example of the use of bokeh. I took this photo of a lilac in Ohio. Behind the lilac bush is an ugly red fence. Opening up the aperture allowed me to minimize the distraction.
The bottom line is that controlling the depth of field of a photograph can add a nice aesthetic ambiance to the picture. This is accomplished by opening the aperture for more foreground and background blur (bokeh).