F-Number Calculator
Compute the f-number (f-stop) of a lens by dividing its focal length by the aperture diameter. Use it when selecting lenses, comparing lens speed, or setting exposure in photography.
About this calculator
The f-number, also written as f-stop or N, describes the ratio of a lens's focal length to its entrance pupil (aperture) diameter. The formula is: f-number = focalLength / apertureDiameter. A lower f-number means a wider aperture and more light reaches the sensor, making it ideal for low-light shooting. A higher f-number means a narrower aperture, increasing depth of field and sharpness across a scene. For example, a 50 mm lens with a 25 mm aperture gives f/2, while the same lens stopped down to a 6.25 mm aperture gives f/8. Understanding f-numbers helps photographers control exposure, depth of field, and diffraction limits in a predictable, repeatable way.
How to use
Suppose you have a 85 mm portrait lens with an aperture diameter of 17 mm. Step 1 — Enter 85 in the Focal Length field. Step 2 — Enter 17 in the Aperture Diameter field. Step 3 — The calculator computes: f-number = 85 / 17 = 5. Your lens is operating at f/5. If you open the aperture to 21.25 mm, the new f-number = 85 / 21.25 = 4, one full stop brighter.
Frequently asked questions
What does a lower f-number mean for my photography?
A lower f-number indicates a wider aperture opening, which allows more light to reach the sensor in a given amount of time. This is beneficial in low-light conditions because you can use a faster shutter speed or lower ISO. A wide aperture also produces a shallower depth of field, creating that creamy background blur (bokeh) popular in portrait photography. Lenses with very low f-numbers like f/1.4 or f/1.8 are called 'fast lenses' and are prized for their light-gathering ability.
How does focal length affect the f-number of a lens?
The f-number is directly proportional to focal length — doubling the focal length while keeping the same physical aperture diameter doubles the f-number. This is why a 200 mm telephoto lens with a 50 mm aperture is f/4, whereas a 50 mm lens with the same 50 mm aperture would be f/1. Lens manufacturers therefore have to build physically larger aperture elements into long telephoto lenses to maintain wide maximum apertures, which is why fast telephoto lenses are large and expensive. When comparing lenses, the f-number provides a standardised, focal-length-independent measure of light transmission.
Why are f-stops expressed as fractions like f/2.8 rather than whole numbers?
The 'f/' notation is a reminder that the f-number is a fraction: focal length divided by aperture diameter. Writing f/2.8 makes it clear that 2.8 is the denominator — a larger denominator means a smaller aperture hole, which is counterintuitive if you see it as just a plain number. The standard f-stop sequence (f/1, f/1.4, f/2, f/2.8, f/4 …) is based on powers of √2, so each step halves or doubles the area of the aperture and therefore the amount of light. This geometric progression keeps exposure changes consistent and predictable across all lenses and camera systems.