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← Creative Hut LibraryPhotography basics · Aperture

Part 3 of 13 in the aperture guide

What is the difference between absolute and relative aperture?

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

Short answer. Absolute aperture is the real width of the opening in millimetres. Relative aperture compares that width with the focal length and is what we call the f-number.

  • Absolute aperture: the diameter, for example 50 mm.
  • Relative aperture: focal length divided by diameter, for example 100 mm ÷ 50 mm = f/2. It has no unit.
  • The same f-number gives the same brightness on lenses of any focal length, which is why exposure settings carry over when you change lenses.
  • Lens barrels sometimes print it as a ratio, such as 1:3.5 to 5.6, which means f/3.5 to f/5.6.
N = f / D
A 100 mm lens at f/2 has an opening 50 mm wide. Relative aperture is f/2 and absolute aperture is 50 mm.[1][3]
Dfsensorlight
Figure 3. D is the width of the opening. f is the focal length. The f-number is f divided by D.

One limit: the marked f-number is exact only when the lens is focused at infinity. At close focus, as in macro work, less light reaches the sensor than the marked number suggests, and photographers use a larger working f-number. The standard itself applies only to lenses focused at infinity.[2][4]

In photography
A 200 mm lens at f/2.8 has an opening about 71 mm wide, while a 24 mm lens at f/2.8 has one about 8.6 mm wide. Both give the same exposure.
In cinematography
A 25 mm and a 100 mm cine lens both set to T2.8 expose the same, so the DOP can change lenses between setups without re-lighting.
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Questions people ask

What is relative aperture?

It is the opening compared with the focal length. We write it as the f-number, the focal length divided by the width of the opening.

Why is the f-number a ratio and not a size in millimetres?

Dividing by the focal length means the same f-number gives the same picture brightness on any lens, so one exposure rule works for all lenses.

References

  1. Creative Hut Academy (2025) Aperture. Lecture slides by Abin Alex and Anu Joseph.
  2. International Organization for Standardization (2008) ISO 517:2008 Photography: Apertures and related properties pertaining to photographic lenses: Designations and measurements. 3rd edn. iso.org/standard/50089.html
  3. Encyclopaedia Britannica. Relative aperture. britannica.com/technology/relative-aperture
  4. Ray, S.F. (2002) Applied Photographic Optics, 3rd edn. Oxford: Focal Press.

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 2Next: part 4
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Guide by Abin Alex and Anu Joseph, Creative Hut Academy.