What the f-stop

One of the most important elements of photography, and one of the more difficult to grasp, is the f-stop and its effect on the light gathering ability of the camera and the focal distance captured in the image.

Let’s start by defining f-stop. The f-stop number is the focal length of the lens divided by the aperture width formed by the blades in the lens. So, for example, f8 translates into an aperture that is 1/8^th of the focal length. This would translate to a 10 mm aperture on an 80 mm lens, or 12.5 mm on a 100 mm lens. This leads us to the understanding that the f-stop is not a fixed size, but a relative size based on the length of the lens.

The first important aspect of f-stop is the effect it has on the amount of light the sensor, and the film in older cameras, could gather. Intuitively, but incorrectly, one might assume that widening the aperture might increase the field of view for the sensor. Changing the f-stop does not alter the width of the scene for the camera, but only changes the amount of light from that scene that hits the sensor. So, a camera set for f8 will see the same scene as it would at f16; at f8, the camera will gather four times as much light as it would at f16. Each standard f-stop number effective doubles the amount of light going through the lens compared to the number larger than it. So, f2.8 doubles the amount of light hitting the sensor compared to f4, and f4 again doubles that light compared to f5.6. The standard f-stops are 2.8, 4, 5.6, 8, 11, 16, & 22. Some lenses will go as low as f1.4, and others will go as high as f32.

A 100 mm f1.4 lens will have a maximum aperture of 71.43 mm, compared to a 100 mm f4 lens with a maximum aperture of 25 mm. With this greater light gathering capability, the f1.4 lens will be able to work at faster shutter speeds, and is hence called a “fast lens.”

The other aspect of f-stops is their effect on depth of field. The greater the aperture, and smaller f-stop number, the shallower the depth of field for an image. Simply put, depth of field is the amount of an image from front to back that will appear in focus. At f1.4 or f2.8, very little of the image will be focus in front of and behind the subject. At f16 and f22, most of the foreground and background will be in focus. For group shots, a photographer might pick a higher f-stop number to get all of the faces in the image in focus, while a macro or wildlife photographer might pick a lower f-stop in order to blur all but the primary subject of the image.