The F-Stop

The aperture of a photo lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with the of shutter speed, the aperture size will determine the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. Each "stop" refers to a factor of √2 (approx. 1.41) change in f-number, and it represents a halving of the light intensity from the previous stop.

Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one.

Shutter speeds are arranged in a similar scale, so that one step in the shutter speed scale corresponds to one stop in the aperture scale. Opening up a lens by one stop allows twice as much light to fall on the film in a given period of time, therefore to have the same exposure at this larger aperture as at the previous aperture, the shutter speed is set twice as fast (i.e., the shutter is open half as long).

Although this may be a little confusing, it is good to know that f/8 is f/8, f/11 is f/11, no matter what lens or film you use. Through all of the different focal lengths, aperture areas and even film formats, the brightness of the illumination reaching the film plane or sensor is exactly the same for a given f/stop. That's why handheld light meters work. They don't know if you're shooting digital, film or movies, if it's a DX or FX format sensor or 35mm, 120 or 4X5 inch film.

The important thing to know in all of this, is the doubling and halving relationship of shutter speeds and how it works with exposure. This will be the key to how your final photography turns out. You need to know that a stop down will give you more depth of field in your photo (click here for the article on aperture control). You don’t need to carry a calculator though to figure out your aperture areas for your lens and f-stops; once you have played with it a time or two you will get the hang of it.

Settings used on this photo were: F-stop: f/5, Shutter Speed: 1/25 sec, with Aperture 4.

Join me next month for a lesson for everyone, from the point and shoot user to the users of SLR cameras.