Ok let's go all the way back to basic with a definition of what the aperture ring is. The definition of aperture is "an opening, hole or gap." It means the same thing in photography, and the size of the hole can be adjusted by a ring of blades that contract or expand the opening. Each lens has a slightly different aperture, and some have different shapes that make for cool bokeh affects.
Bokeh is what happens when you are photographing with a very small aperture value, and your subject has small lights out of focus in the background. The small lights turn into blurry shapes or balls of light floating in the distance. If your aperture is hexagon shaped, the bokeh turns into hexagons and if you have rounded blades, the bokeh is more circular.
Having the right aperture setting for the situation can make or break an image. The changing of an aperture will dictate the depth of field in the image. Depth of field can be explained simply as how far the focus plane will extend in front and beyond the point to focus. The focus plane is essentially the area that is in focus. So with a higher aperture value, you'll have less depth of field, or more of the image will be in focus. Here is a diagram to explain how the change of aperture will change how much of an image is in focus.
Changing depth of field is probably the most common reason for changing the aperture in a photo. I was talking to the owner of a small watch company located in the Bay Area one day about how he photographs his watches for his website. He said that he uses a computer software that takes a bunch of photos at different focal lengths and compiles them all together to make one crystal clear image. In order to do this, he uses a macro lens with an extremely small aperture value (probably around f/1). Since the computer moves the focus across each part of the watch, it is able to make sure that there is no focus fall off where he doesn't want it. That's an example of shallow depth of field being used practically in a real world setting.
I was talking to a corporate photographer and videographer about determining depth of field when taking portraits. This professional told me that he has a personal rule that he learned in college and still follows. He said that when taking portraits, use an aperture of f/2.8-f/5.6 and nothing else. Anything less than 2.8 will make the subjects ears and sides of face out of focus, and anything more will make the subject blend into the background. This image is taken on a telephoto lens at f/2.8. Notice how his ears are just beginning to fall out of focus ?
Finding sweet spots and manipulating each setting to work together is crucial to capturing compelling images. In situations when you want to photograph something extending into the distance, or something far away, you'll want to use a higher aperture value to lengthen the depth of field. For example, if you are taking a picture of San Francisco, but you are standing on the Marin side of the Golden Gate Bridge. Say you want to be able to see the bridge in focus and the skyline in focus. Because of the distance between the bridge and the city, you are going to need to set the aperture to something high, like f/16-f/22, in order to have a long enough depth of field to keep everything sharp and in focus.
Another reason would be to control the brightness of an image. When you lower the aperture value, you are in turn opening a hole in the front of the camera. The larger the hole, the smaller the aperture value and vice versa. Here's a visual example for what I'm talking about.
Since we are working with such small openings and values, each minor change makes a great impact on the overall outcome of an image. Aperture is used to manipulate the brightness of an image more when shooting film than digital. This is because when shooting film, you are constrained to a fixed ISO value, and when using a digital camera, you are able to change your ISO freely with some limitations that is covered in my blog on ISO.
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