How this calculator works
The f-number is focal length divided by entrance pupil diameter. Area follows from the circle formula. The entrance pupil is the apparent aperture seen from the object side and can differ from a physical diaphragm opening. The calculation assumes a circular unobstructed pupil; it does not subtract secondary mirrors or account for transmission losses.
Formula
N = f/D; D = f/N; A = πD²/4. Area changes with the square of diameter.
Worked example
A 100 mm focal length lens at f/2 has a 50 mm entrance pupil and an area of 19.635 cm2. At f/4, its area falls to 4.909 cm2.
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Frequently asked questions
Does f/2 collect twice as much as f/4?
For the same focal length, f/2 has twice the diameter and four times the pupil area.
Is this the glass area?
It is the clear entrance pupil area. The glass can be larger than the usable pupil.
Can I use this for an obstructed telescope?
It gives the outer circular area. Subtract the central obstruction area separately.