Aperture Area Calculator
Enter a focal length and f-number to get the real aperture diameter and area. The f-number is the focal length divided by the aperture diameter.
How to use the aperture area calculator
- Enter the lens focal length in millimetres.
- Enter the aperture f-number.
- Read the aperture diameter in mm and the area in square mm.
Examples
50mm at f/2
50mm, f/2
25mm diameter, 490.87 mm2 area
50mm at f/4
50mm, f/4
12.5mm diameter, 122.72 mm2 (one quarter)
Frequently asked questions
How is the aperture diameter calculated?
The f-number is the focal length divided by the aperture diameter, so the diameter equals the focal length divided by the f-number. A 50mm lens at f/2 has a 25mm opening.
How do you find the aperture area?
The opening is treated as a circle, so the area is pi times the radius squared, where the radius is half the diameter you computed from the focal length and f-number.
Why does the area quarter from f/2 to f/4?
Going from f/2 to f/4 halves the diameter, and area scales with the square of the diameter, so the area drops to one quarter. That is the same as two stops less light.
Is this the same as the entrance pupil?
Yes, in practice. The f-number is defined by the entrance pupil diameter, which is the apparent size of the aperture seen through the front of the lens.
Why do two lenses at the same f-stop differ in size?
The f-number is a ratio, so a longer lens needs a wider opening for the same f-stop. A 200mm f/2.8 has a much larger front element than a 50mm f/2.8.
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