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← Creative Hut LibraryPhotography basics · Aperture

Part 4 of 13 in the aperture guide

How is the f-number written, and why does a bigger number mean a smaller opening?

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

Short answer. Write it as f/2.8: a lowercase f, a slash, then the number. A bigger number means a smaller opening because the number is the divisor.

  • f/2 means focal length ÷ 2, so the opening is half the focal length wide. f/16 means focal length ÷ 16, a much smaller hole.
  • The slash shows the division. F2 hides it, which is why the f/ form is preferred.
  • The same quantity is also called f-stop, f-ratio, focal ratio or just f-number.
  • An easy memory: f/1.8 is like 1/1.8, a bigger fraction than f/8, which is like 1/8.

If italics are not available, f/2 is acceptable. Mathematically f/2 is the same as f ÷ 2 or 0.5f, but photographers, manufacturers and cameras do not write it that way.[1]

In photography
A kit zoom marked 18 to 55 mm 1:3.5 to 5.6 opens to f/3.5 at 18 mm and only f/5.6 at 55 mm.
In cinematography
Cinema lenses are marked with T numbers, such as T2.8, on the iris ring instead of f numbers.[2]
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A 100 mm lens is set to f/4. How wide is the opening?

Show the answer

25 mm

Questions people ask

How wide is the opening of a 100 mm lens at f/4?

25 mm. The width is the focal length divided by the f-number: 100 divided by 4.

Why does a bigger f-number give a smaller opening?

Because the f-number is the divisor. The same focal length divided by a larger number gives a smaller width.

References

  1. Creative Hut Academy (2025) Aperture. Lecture slides by Abin Alex and Anu Joseph.
  2. PetaPixel. F-Stops vs T-Stops: The Difference Explained in Plain English. petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english

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 3Next: part 5
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Guide by Abin Alex and Anu Joseph, Creative Hut Academy.