F Stop
An f-stop is a step along the aperture scale (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16), where each full step halves the light reaching the sensor compared to the step before it. Lower numbers mean a wider physical opening and more light; higher numbers mean a narrower opening and less light.
Because the f-number is a ratio of focal length to the aperture's effective diameter, the scale isn't evenly spaced the way shutter speed or ISO are — each full stop requires the diameter to change by the square root of 2 to double or halve the light-admitting area, which is why the numbers follow this specific, non-round sequence rather than simply doubling.
Frequently Asked Questions
Is a lower f-stop always better?
Not universally — a lower f-stop (wider aperture) lets in more light and produces shallower depth of field, both genuine advantages in low light or for background separation, but a higher f-stop is often the right choice for landscape or group shots needing more of the scene in focus.
Why do some lenses only show whole f-stops while others show thirds?
Most modern cameras let you adjust aperture in 1/3-stop increments for finer exposure control, while older or manual lenses often only marked whole stops on the aperture ring — both use the identical underlying doubling logic, just at different granularity.