Looking through the lens of many cameras, you can see a series of leaves, which can create a larger or smaller opening. This opening is the aperture, which is measured using a term called f/stop.
The f/stop of a camera is most commonly seen on your controls as a series of numbers that can start as low as 1.4 (a large opening) and range to 32 (a tiny opening.) Many get confused because a smaller opening is a larger number, and a larger number is a smaller opening. As expected, the larger opening allows more light to pass through the lens and reach the film or digital sensor.
This amount of light works with the ISO and Shutter speed to form the exposure triangle. A larger opening can allow a lower film speed and/or a faster shutter speed, while a smaller opening requires a higher ISO and/or slower shutter speed.
WHY CHANGE THE APERTURE
Besides controlling the light for correct exposure, the aperture setting can dramatically alter the appearance of an image by controlling the DEPTH OF FIELD, or the area in the image, from near to far, that is in focus.
DEPTH OF FIELD can help focus the eye on a subject by making the areas closer and further away appear out of focus. Frequently, an unattractive background can be minimized by using a shallow depth of field to make it blurry.
A large lens opening (smaller number) results in a shallow depth of field where only the item in focus is sharp. For instance, an aperture of 2.8 will result in a shallow depth of field, whereas an aperture of f/22 will allow objects closer and further away to be in focus.
Smaller apertures (larger numbers) can be used if you want the foreground and background to be clear, for instance, a car posed in front of a beautiful mountain scene. Greater depth of field can also allow a bit more latitude when focusing, as more of the photo is clear, and slight errors in focusing will not make the subject out of focus.
