Introduction What is photography? The history of photography What this site will do for you Light: The most important element Why light is important to a photograph The many types of light Controlling light in your pictures The camera The basic function Types of cameras Choosing the right camera Putting the image together: the Lens How the lens bends light: a tutorial A brief introduction to apertures The variety of lenses Choosing a lens Exposure: a film tutorial How film records an image Understanding film speed Print vs. Slide film Film recommendations Taking Pictures Depth-of-field Apertures and shutter speeds Composition and experimentation: the basics Metering: when you can't guess The many types of picture-taking Photography with a point-and-shoot Accessories Tripods: for when you can't stay still Lens filters Post-processing: after development Scanning photos The digital darkroom Photo and equipment storage
How apertures work
Apertures, called f-stops or stops by most professional photographers, are numbers which determine the amount of light the aperture diaphragm in a lens is permitting. As I said before, the aperture diaphragm controls the amount of light that will reach the film. F-stops are numerical values that tell you about the size the diaphragm is set to. All f-stop numbers are always displayed with an "f/" before them. For example, "f/11" would be an example of an f-stop number. The f/, obviously, stands for f-stop, while the number tells about the size of the diaphragm. The thing that is most confusing about the f-stop value is that the larger the number is, the smaller the diaphragm is, so the darker the image will appear. However, if your f-stop value is "f/2", the diaphragm will be nearly or completely open, so the image will appear bright.
Faster means more light
We can identify the brightness of a lens by finding out its speed. Lenses are identified by two speeds: fast lenses and slow lenses. The speed is not how fast light reaches the film, but rather how much light reaches the film at the lens’ maximum aperture. For example, if the maximum aperture you can set your lens to is f/1.7, your lens would be considered fast. However, if your lens’ maximum aperture is f/8, your lens would be considered slow. The fastest lens is at f/1.0, while the slowest lens is anything above f/18.
To learn more
F-stops will be covered in detail in the Taking Pictures section.