Fast and Slow While Sitting Still
Understanding the relationship between lens opening, shutter speed, and film or sensor sensitivity is one of the dividing lines between being a casual shooter and being a professional filmmaker. When you know how those values interact and the way they affect your image, you begin to control the technical quality of your images. The unique language that has grown up to describe those ideas is probably the barrier to easy understanding.
Perhaps the most confusing word is “fast.” We speak of fast lenses, fast shutter speed, and fast film/sensors. In each case, the word means a completely different thing.
The first lens characteristic to understand is focal length. It's very straightforward: it's just the distance from the physical center of the lens to a point in space where the light passing through the lens comes to as small a point as possible. That distance is usually written in millimeters, and it falls inside the lens—camera assembly between the lens and film or sensor. A normal focal length for a 35mm film camera lens is about 50 millimeters. A digital camera lens is usually not as long, since the sensors are smaller than the 35mm film area.
To arrive at the lens' focal ratio you need to divide that focal length, which is always part of the lens description, by another value—the width of the lens opening. (Actually, you don't have to do it, the optical designer does. The raw measurement of the opening isn't something the photographer uses.) That ratio, however, the focal length divided by the opening, is very important. So, if a lens has a built-in focal length of 80 millimeters and a maximum opening of 40 millimeters, it is called an f/2 lens. That's called a fairly fast lens. An 80mm lens with a max opening of 20mm is an f/4 lens, and so on. A 50mm lens An f/4 opening is not considered fast these days.
The reason that's important is that the ratio does affect your photograph. The wider the lens in relation to its focal length, the more light will get through the shutter while it's open and be spread over the sensor. Putting the right amount of light on the film or sensor is the essence of proper exposure. To control that light, you (or your automatic light meter) will pick the duration the shutter is open, and will want to match it with the right lens opening for the job. In theory, you could have a box of 50mm focal length lenses, each with a different opening, to give you the control you need. To avoid that expensive approach, the lens has an adjustment called an iris. It simply adjusts the size of the opening from maximum down to quite small, giving you or your meter a whole range of different values—the f-stops. These vary at the wide end from lens to lens, but fall into a normal pattern of max, 2.8, 3.5, 4, 5.6, and so on. Each of these f-stop numbers is the answer to the equation focal length divided by selected opening. Each of them normally lets twice as much light through as does the next larger number.
That's an f-stop. And that's a good place to stop.