Enter Two Values, Get the Rest
Sample Aperture Values
| Focal (mm) | F-number | Diameter D (mm) | Relative light (1/N²) | Stops vs f/2.8 |
|---|---|---|---|---|
| 50 | f/1.4 | 35.71 | 0.5102 | 2.00 |
| 50 | f/2.8 | 17.86 | 0.1276 | 0.00 |
| 85 | f/1.8 | 47.22 | 0.3086 | 1.27 |
| 200 | f/4.0 | 50.00 | 0.0625 | -1.03 |
| 24 | f/8.0 | 3.00 | 0.0156 | -3.03 |
Formulas Used in This Tool
- F-number: N = f / D where f is focal length, D is aperture diameter.
- Diameter: D = f / N.
- Aperture area: A = π(D/2)².
- Relative light: proportional to 1/N² (ignoring losses).
- Stop difference: stops = 2·log₂(N₂/N).
- T-stop estimate: T ≈ N/√τ using transmission τ.
- Airy disk: d = 2.44·λ·N (λ in µm gives d in µm).
How to Use This Camera Aperture Calculator
- Enter any two of focal length, f-number, and aperture diameter.
- Optionally enter crop factor to see DoF-equivalent f-number.
- Add a comparison f-number to compute stop differences.
- Use wavelength to estimate diffraction blur size on sensor.
- Click Calculate to show results above the form.
- Use the export buttons to download CSV or PDF.
F-number ratio explained
An f-number is a ratio, not a fixed hole size. Using N = f/D, the same f/2.8 yields different openings at different focal lengths. Example: 24 mm at f/2.8 gives D ≈ 8.57 mm, while 200 mm at f/2.8 gives D ≈ 71.43 mm. This is why long, bright telephotos are large today.
Solve for the missing value
Enter any two of focal length, f-number, and diameter to compute the third: D = f/N, N = f/D, or f = N·D. The tool also computes aperture area A = π(D/2)², so you can compare openings even when focal lengths differ. Area doubles when diameter grows by √2.
Stops and light scaling
Relative light is proportional to 1/N². So f/2 to f/2.8 is one stop, about half the light, and f/1.4 to f/2.8 is two stops, about 4× less light. The comparison field shows stops = 2·log₂(N₂/N) and ratio (N₂/N)². Use this to estimate ISO or shutter changes.
Crop factor equivalence
Crop factor affects depth-of-field equivalence, not exposure. With crop factor c, DoF-equivalent f-number is N_eq = N·c. Example: f/2 on 1.5× behaves like about f/3 for blur compared with full-frame, while exposure stays f/2. Pair this with equivalent framing when comparing images.
Transmission and T-stops
Transmission losses explain T-stops. With transmission τ, estimate T ≈ N/√τ. If τ = 0.90, an f/2 lens becomes about T2.11, and if τ = 0.80 it becomes about T2.24. Use this when matching exposure between lenses for video or multi-camera work.
Diffraction quick check
Diffraction grows as you stop down. The Airy disk estimate is d = 2.44·λ·N. At λ = 550 nm and f/11, d ≈ 14.7 µm, which may soften detail on small-pixel sensors. If your pixel pitch is near 4 µm, consider f/8 for critical sharpness.
Workflow and exports
Choose an f-number for your look, confirm diameter and area, then compare against a baseline aperture for stop changes. Export CSV for planning and PDF for sharing with a crew or students. Repeat at other focal lengths to see how aperture scales for portraits, landscapes, and product shots.
FAQs
What should I enter to get a result?
Provide any two of focal length, f-number, and aperture diameter. The calculator solves the third and then adds area, relative light, and optional comparisons.
Does crop factor change exposure?
No. Crop factor only changes depth-of-field equivalence when framing is matched. Exposure is still governed by f-number, shutter speed, and ISO.
How accurate is the T-stop estimate?
It is an approximation using T ≈ N/√τ. Real transmission varies with wavelength, focus distance, and lens design, so measured T-stops may differ slightly.
Why does the light ratio use (N₂/N)²?
Because image irradiance at the sensor is proportional to 1/N². Comparing two apertures cancels constants, leaving the squared ratio of their f-numbers.
What wavelength should I use for diffraction?
550 nm is a common green-light reference for daylight. For red light you might use ~650 nm, and for blue ~450 nm. The tool converts nm to micrometers internally.
Can I use this for smartphones?
Yes, if you know the real aperture diameter or f-number. Note that very small sensors hit diffraction and depth-of-field limits quickly, so equivalence and Airy estimates are especially useful.