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← Creative Hut LibraryPhotography basics · Aperture

Part 5 of 13 in the aperture guide

Why does each f-stop multiply by 1.4 and not by 2?

Written by Abin Alex, Founder and Director, and Anu Joseph, Faculty, Creative Hut Academy. Reviewed 29 October 2026.

Short answer. Light follows the area of the opening, and area depends on the square of the diameter. To double the light, the diameter only has to grow by the square root of 2, about 1.4.

  • One stop means double or half the light.
  • Area = π × (D ÷ 2)², so double the area needs D × 1.414.
  • That is why the full-stop series is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.
  • The diameter never doubles or halves from one stop to the next.
StepWorking (from the lecture)
Circle 1D1 = 20 mm. Area = 3.14 × 10² = 314
Circle 2One stop brighter, so double the light: 2 × 314 = 628
Compare(D1 ÷ D2)² = 314 ÷ 628 = 1/2, so D2 = D1 × √2
ResultD2 = 20 × 1.414 = 28.3 mm, about 28 mm
D = 20 mm, area 314D = 28 mm, area 628→
Figure 4. Doubling the area needs only about 1.4 times the width. Drawn to scale.
In photography
Going from f/4 to f/5.6 halves the light, so to keep the same brightness the shutter changes from 1/250 to 1/125.
In cinematography
Closing from T2.8 to T4 halves the light on the sensor. The DOP must double the light, or raise the sensor’s sensitivity by one stop, to hold the same exposure.
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Check yourself

You close from f/5.6 to f/11 and keep the same brightness. By what factor must the exposure time grow?

Show the answer

four times, as the light falls by two stops

Questions people ask

Why do f-numbers go 1.4, 2, 2.8, 4?

Each step halves the area of the opening, which needs the diameter divided by the square root of 2, about 1.4.

How much does one f-stop change the light?

One stop doubles or halves the light reaching the sensor.

References

    All 13 questions in this guide

    1. What is aperture, and what does it do?
    2. What is a diaphragm, and how does it work?
    3. What is the difference between absolute and relative aperture?
    4. How is the f-number written, and why does a bigger number mean a smaller opening?
    5. Why does each f-stop multiply by 1.4 and not by 2?
    6. What are full, half and third stops of aperture?
    7. What is effective aperture?
    8. What is lens speed, and what does maximum aperture mean?
    9. How do I set the aperture on my camera?
    10. What is the difference between an f-stop and a T-stop?
    11. Does a smaller aperture always make a sharper picture?
    12. How does aperture affect depth of field?
    13. How do cinematographers use aperture?
    Previous: part 4Next: part 6
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    Guide by Abin Alex and Anu Joseph, Creative Hut Academy.