Aperture Area Calculator — Lens & Optics
Compute the physical cross-sectional area of a circular aperture. Enter the diameter directly for a telescope mirror or antenna dish, or enter a camera lens's focal length and f-number to derive the entrance pupil diameter and area.
Input mode
mm
Physical cross-sectional area of the aperture opening
- 1
Radius (D ÷ 2)
100 ÷ 2 = 50 - 2
Aperture area (π × r²)
π × 50² = 7,853.98
How does this calculator work?
Aperture area = π × (D/2)², where D is the clear diameter in mm. For a camera lens, D = focal length ÷ f-number. Doubling the diameter quadruples the area and the light collected. A 100 mm diameter aperture has an area of ~7854 mm²; at f/2.8 with 50 mm focal length, the aperture is ~17.9 mm diameter (area ≈ 251 mm²).
Formula
How this is calculated
The aperture of an optical or imaging system is the clear circular opening through which light passes. Its area determines how much total light the system collects: doubling the diameter quadruples the area (and the light-gathering power), which is why telescope and camera engineers care so much about aperture size.
For a physical opening — a telescope primary mirror, a radio dish, or a pinhole — the aperture diameter D is measured directly and the area is simply A = π(D/2)². For a camera lens, the effective diameter of the entrance pupil is not labelled directly on the barrel; instead the lens is characterised by its f-number (focal ratio), defined as focal length divided by entrance pupil diameter. Rearranging: D = focal length / f-number. This is why a 50 mm f/1.4 lens has an entrance pupil of 50/1.4 ≈ 36 mm, while a 50 mm f/8 lens has only 6.25 mm — the aperture area differs by a factor of 33.
Note that this calculator computes the physical aperture area only. The effective light-gathering power also depends on optical transmission (T-stop vs f-stop), vignetting, and for telescopes the central obstruction. Antenna "effective aperture" (Ae = Gλ²/4π) is a separate concept that accounts for directional gain and is not what this calculator computes.
Frequently asked questions
A larger aperture collects more light, enabling faster shutter speeds, lower ISO settings, and shallower depth of field for a given exposure. Going from f/5.6 to f/2.8 (halving the f-number) quadruples the aperture area and therefore doubles the light collected per stop change — each full stop halves or doubles the exposure.
The f-number (f-stop) is a geometric ratio: focal length divided by entrance pupil diameter. The T-stop is the "transmission stop" — it adjusts the f-number to account for light lost to glass absorption and reflection inside the lens. A lens labelled f/2.8 might be T/3.1 if it transmits only 83% of light. Cinema lenses always specify T-stops; still photography lenses usually only state f-numbers.
Telescope light-gathering power is proportional to the mirror or lens area: a 200 mm aperture collects four times the light of a 100 mm aperture. This lets you see fainter objects. The focal ratio (f-number) determines field of view and magnification characteristics, but raw light collection is purely a function of aperture area.
Also known as
TG we-Calculate Editorial Team. (2026). Aperture Area Calculator — Lens & Optics [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/aperture-area-calculator
TG we-Calculate Editorial Team. "Aperture Area Calculator — Lens & Optics." TG we-Calculate. 2026. https://we-calculate.com/calculator/aperture-area-calculator.
TG we-Calculate Editorial Team, "Aperture Area Calculator — Lens & Optics," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/aperture-area-calculator
@misc{wecalculate_aperture_area_calculator, title = {Aperture Area Calculator — Lens & Optics}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/aperture-area-calculator}}, year = {2026}, note = {TG we-Calculate} }
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