Lens f-number by Focal and Diameter

Lens f-number from focal length and aperture

The f-number is just N = f / D, with f as the focal length and D as the effective aperture diameter, also called the entrance pupil. Put a 25 mm pupil on a 50 mm lens and you get f/2. Light intensity follows pupil area, so stepping from f/2 to f/1.4 doubles the light, which is one stop. On a movie set, the T-stop takes N and corrects it for the light that actually gets lost passing through the glass elements.

Applications

Portrait shooters lean on a shallow depth of field at f/1.4–f/2.8. In low-light situations a fast aperture like f/1.4 keeps you off high ISO. When you want maximum sharpness, the sweet spot sits around f/5.6–f/8. And the formula lets you match aperture across lenses with different focal lengths.

FAQ

Why is a smaller f-number "brighter"? N is a ratio, so a smaller N means D is larger relative to f, and more light gets through. f/1.4 has about 2× the area of f/2.

What is the difference between f-stop and T-stop? The f-stop is a geometric figure (f/D). The T-stop measures the light that's really transmitted, since it accounts for absorption and reflection inside the lens.

Does a lower f-number always mean better image quality? No. Wide open, most lenses give up some sharpness and start to vignette. The sweet spot usually lands 2–3 stops down from maximum.