F-Number (f-stop)

f/\# = \frac{f}{D}

The f-number (or f-stop) is the ratio of focal length to aperture diameter. Lower f-numbers mean larger apertures and more light gathering.

Variables

f/#

F-number

Allowed values: greater than 0

unitless

Focal length

Aperture diameter

Allowed values: greater than 0

Your notes

0/5000

Check value

Round inputs run through this page's calculator. Use it to test your own working or a rearranged form. It is a test case, not a real-world example.

f = 2 m (Focal length)
D = 3 m (Aperture diameter)

Result: f/# = 0.66666667

Other optics formulas with the same quantities

Mirror Equation (Reflection) shares Focal length
Focal Length and Radius (Reflection) shares Focal length
Thin Lens Equation (Lenses) shares Focal length
Lensmaker's Equation (Lenses) shares Focal length
Lens Power (Lenses) shares Focal length
Spherical Mirror Focal Length (Reflection) shares Focal length

Where this formula is listed

Optics formulas, under Optical Instruments, alongside Magnifying Glass Magnification, Microscope Magnification, Telescope Magnification and Numerical Aperture.
In the optics list it comes after Numerical Aperture and before Depth of Field.
Optics formula sheet, one of 34 formulas printed with symbol keys and SI units.
The full formula index, grouped by category and topic.

Common questions

What is the F-Number (f-stop) formula?

F-Number (f-stop) is written f/# = f/D. The f-number (or f-stop) is the ratio of focal length to aperture diameter. Lower f-numbers mean larger apertures and more light gathering. It belongs to Optics, under Optical Instruments.

What are the units in f/# = f/D?

In SI units, f/# is f-number (dimensionless); f is focal length, measured in m; D is aperture diameter, measured in m.