F-Number Calculator

About the F-Number Calculator

The F-Number Calculator determines the f-stop from focal length and aperture diameter, or calculates any missing value when two are known. It also shows the relative light transmission, depth of field implications, diffraction limits, and equivalent f-numbers across sensor sizes, which is useful when comparing lenses or planning exposure. That gives you one place to connect aperture math with the photographic consequences of the setting.

The f-number (f-stop) is the ratio of focal length to the diameter of the entrance pupil: N = f/D. A lower f-number means a wider opening, more light, and shallower depth of field. Understanding f-numbers is essential for controlling exposure and creative focus effects in photography and cinematography.

Beyond the basic ratio, this calculator computes T-stops, shows the diffraction-limited resolution at each aperture, and provides an equivalence table for different sensor formats so you can compare depth of field between a full-frame and APS-C or Micro Four Thirds camera. It is also handy when you want to translate lens specs into a practical shooting decision.

When This Page Helps

Use this calculator when you need to convert between focal length, aperture diameter, and the resulting f-number. It is helpful for lens comparisons, depth-of-field planning, and checking whether a lens is operating near its diffraction limits or needs a different aperture for the shot. That makes it easier to choose a working aperture before you take the photo or compare lenses across formats.

How to Use the Inputs

1. Enter the focal length of your lens in millimeters.
2. Enter the aperture diameter or the desired f-number.
3. The calculator computes the missing value.
4. View depth of field, diffraction, and light characteristics.
5. Use presets for common lens configurations.
6. Check the sensor equivalence table for cross-format comparisons.

Example Calculation

Tips & Best Practices

• Peak sharpness is usually 2-3 stops from maximum aperture — f/5.6-f/8 on most lenses.
• For astrophotography, shoot wide open (lowest f-number) to gather maximum starlight.
• Larger aperture diameter (lower f-number) = more expensive, heavier lens.
• T-stop matters more than f-number when matching exposure between multiple cinema lenses.
• When comparing lenses across formats, multiply the f-number by the crop factor for DOF equivalence.

The Mathematics of F-Numbers

The f-number sequence 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 follows powers of √2 (approximately 1.414). Each step multiplies the f-number by √2, which halves the area of the aperture, which halves the light. This is why each "stop" represents a 2× change in light.

The area of a circular aperture is π(D/2)². Since f-number N = f/D, the diameter D = f/N, and area = π(f/(2N))². Area is inversely proportional to N², so doubling the f-number reduces light by 4× (2 stops).

Full-Frame Equivalence

When comparing cameras with different sensor sizes, the "equivalent f-number" for depth of field is the actual f-number multiplied by the crop factor. A Micro Four Thirds camera (2× crop) at f/2.8 gives the same DOF as a full-frame at f/5.6. However, the exposure brightness is the same — only the DOF characteristic changes.

Diffraction and the Airy Disk

The minimum resolvable detail is limited by diffraction: the Airy disk diameter ≈ 2.44 × λ × N, where λ is the wavelength of light (~550nm for green) and N is the f-number. At f/16, the Airy disk is about 21 microns — larger than most sensor pixels (3-6 microns). This is why sharpness drops at small apertures despite greater depth of field.

Frequently Asked Questions

• Why are f-numbers a confusing scale?

  Because they're ratios, not direct measurements. A lower number means MORE light, which is counterintuitive. The sequence (1, 1.4, 2, 2.8, 4, 5.6, 8...) follows powers of √2 because area scales with the square of diameter.

• What is a T-stop?

  T-stop accounts for light lost inside the lens (reflection, absorption). A lens rated f/2.8 might transmit only f/3.0 worth of light. Cinema lenses are rated in T-stops for consistent exposure across different lenses.

• What is diffraction?

  At very small apertures (high f-numbers), light bends around the aperture edges, softening the image. Most lenses hit the diffraction limit around f/11-f/16 on full-frame. Stopping down past this point reduces sharpness despite increasing depth of field.

• Is f/2.8 on a crop sensor the same as f/2.8 on full frame?

  For exposure, yes — the same amount of light reaches the sensor per unit area. For depth of field, no — the crop sensor gives deeper DOF. To match the full-frame DOF of f/2.8, a 1.5× crop sensor needs about f/1.8.

• What is the fastest lens available?

  The fastest commercial lenses are around f/0.95 (e.g., Nikon Z 58mm f/0.95 Noct). The fastest lens ever made for general use was the Canon 50mm f/0.95 for Leica mount. Theoretical minimum is around f/0.5.

• Why do zoom lenses have variable apertures?

  As focal length increases, the entrance pupil stays the same size, so the f-number (f/D) increases. Constant-aperture zooms (e.g., 24-70mm f/2.8) use larger, heavier glass elements to maintain the same f-number throughout the range.