Aperture is nothing more than a fancy word for a hole or opening through which light is admitted! In Photography, the size of this "hole" is measured in f/stops, or stops and controls the amount and angle of light rays that fall on the image plane or digital sensor.
The Aperture or f/stop can be set on most cameras while in the Creative
Zone. Again, I use Canon cameras, so check the User's Manual for your
particular camera. Select(M) or (Av) on the Mode Dial, look thru the
viewfinder and rotate the Quick Control Dial in (M) Mode or the Main
Dial in (Av) Mode to select or change the f/stop setting.
A large Aperture (f/2.8) would admit a large amount of light at very steep angles to the image plane, which would result in an image that was very sharp around the point of focus with everything else blurred. This f/stop would be said to have a very shallow Depth of Field.
A small Aperture (f/22) would let in considerably less light, but at a much shallower angle to the image plane resulting in a image with a much larger area of acceptable focus in front of and behind the point of focus. This f/stop would be considered to have a very long Depth of Field.
Depth of Field is defined as "the area of acceptable focus in front of and behind the point of focus". There is about twice the Depth of Field behind the point of focus as there is in front at any given Aperture.
In modern cameras, Aperture is controlled by a diaphragm. In cameras with a fixed lens, one you can not remove from the camera, the diaphragm is mounted in the camera. With a DSLR camera, the diaphragm is located in the lens. Both systems work the same way. You turn a dial to the required f/stop and the diaphragm adjusts to that f/stop.
Question: If f/11 lets in less light than f/2.8, how come the brightness of the image in the viewfinder stays the same?
Answer: The diaphragm stays at it's widest setting for the brightest image possible in the viewfinder until the shutter is pressed. Then instantly stops down to the correct f/stop to record the image, then returns to full open, ready for the next shot.
If you would like to see what the image would look like if viewed at the aperture you have set, press the Depth of Field preview button, (check your owner's manual to see where it is located on your camera) the aperture will close down to that setting. The image will become dark and hard to see. Imagine trying to focus or compose an image in those conditions!
You're on your vacation in Yellowstone and you stop at the Norris Geyser Basin. You can't believe the vastness of the scene and want to capture it just like it appears for all of your friends to see! You select the smallest f/stop available to you for the greatest Depth of Field, f/22 or maybe f/32. Don't forget to use your tripod for the sharpest possible image.
You focus on something about one third of the way into the scene. Still holding the shutter release half way down to lock focus you re-compose and take the shot. Everything from the closest bush to the farthest peak should be in focus.
At your next stop you come upon one of the big three in the park. A Grizzly Bear. But he/she is standing in front of a very distracting background. Select a smaller f/stop, f/4 or f2.8. Both of these f/stops are considered to have a shallow Depth of Field. Focus on the bear's eyes and "get the shot"! The shallow Depth of Field will throw the background out of focus, separating the bear from it! This is a technique used by Professional Portrait, Child and Pet Photographers.
Learning how to control aperture to create interesting and unusual effects will not only make your photographs more professional looking, you will become a better photographer. And you'll have a lot of fun doing it!