Case 1
- Focal length
- 50mm
- Aperture diameter
- 25mm
2.00
Open with these valuesF-Stop Calculator
2.00
Result: 2.00The f-number is the focal length divided by the diameter of the entrance pupil, so a 50 mm lens with a 25 mm opening is f/2. Only the ratio matters: 100 mm over 25 mm and 200 mm over 50 mm are both f/4. A smaller number means a wider opening and more light.
Worked examples
Case 2
- Focal length
- 50mm
- Aperture diameter
- 35.71mm
1.40
Open with these valuesCase 3
- Focal length
- 200mm
- Aperture diameter
- 50mm
4.00
Open with these valuesHow it's calculated
N = focal length ÷ aperture diameter
- StepEnter the focal length of the lens in millimetres.
- StepEnter the diameter of the entrance pupil, the opening you see through the front element.
- ResultRead the f-number; the lens is marked f/N with that value.
Reference table
| Focal length, aperture diameter in mm | Marked as | F-number |
|---|---|---|
| 50, 35.71 | f/1.4, a fast prime | 1.40 |
| 35, 19.44 | f/1.8, a common wide angle | 1.80 |
| 50, 25 | f/2 | 2.00 |
| 50, 12.5 | f/4 on a 50 mm lens | 4.00 |
| 100, 25 | f/4 on a 100 mm lens | 4.00 |
| 200, 50 | f/4 on a 200 mm lens | 4.00 |
Questions
How do I calculate the f-stop?
Divide the focal length by the diameter of the entrance pupil, using the same unit for both. A 50 mm lens with a 25 mm opening gives 50 ÷ 25 = f/2.
Does a smaller f-number mean more light?
Yes. A smaller f-number is a wider aperture, so more light reaches the sensor and the depth of field grows shallower. A larger f-number narrows the opening: less light, but more of the scene stays sharp.
Why do f-stops run 1.4, 2, 2.8, 4 instead of whole numbers?
The light that gets through depends on the area of the opening, not its diameter, so each full stop steps by the square root of two, about 1.41. Every step halves the light, which is why the scale reads f/1.4, f/2, f/2.8, f/4, f/5.6, f/8.
Which diameter should I enter?
The diameter of the entrance pupil — the opening as it appears through the front of the lens — not the raw width of the iris blades. Lens elements magnify the iris, and the f-number is defined against the entrance pupil.
What is the difference between an f-stop and a T-stop?
The f-stop is purely geometric, focal length over entrance pupil. The T-stop also counts the light lost inside the glass, so it is always a little larger. Cinema lenses are marked in T-stops because exposure has to match between lenses.
Sources and last check
Information, not professional advice.