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← Creative Hut LibraryPhotography basics · Aperture

Part 2 of 13 in the aperture guide

What is a diaphragm, and how does it work?

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

Short answer. The diaphragm is a ring of thin, overlapping metal blades inside the lens. It opens and closes to make the aperture. It has changed from loose metal plates to a motor-driven iris.

  • Waterhouse stop: a metal plate with a hole, slid into a slot in the lens barrel. Each plate gave one fixed opening. It was invented by John Waterhouse in 1858.
  • Rotating diaphragm: a disc with holes of different sizes, turned to bring the chosen hole in front of the lens.
  • Iris: tissue-thin leaves that overlap like flower petals and sit between two lens elements.[1]
  • Blade count: most current lenses have about 5 to 10 blades. Straight blades make a polygon-shaped opening and curved blades make a rounder one.
Wide openStopped downNearly closed
Figure 2. The opening shrinks as the blades close. A six-blade iris makes a six-sided opening.

Today the aperture ring, or the control wheel on a digital camera, still drives a multi-bladed iris, but through motors. When you press the shutter button, the camera reads the exposure settings and signals a small motor in the lens, which turns the ring that moves the blades to an exact angle.[1] Automatic diaphragms came first on SLR cameras: the lens stays wide open while you compose, so the viewfinder is bright, and closes down only during the exposure.

In photography
Look at a street light that is out of focus. Its blur takes the shape of the opening, so a lens with rounded blades gives round highlights and one with fewer straight blades gives polygons. This soft background look is called bokeh.[1]
In cinematography
The look of out-of-focus lights is part of a lens’s character. Faculty to add a lecture example from a cine lens test.
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[Lecture example slot: photographs of the Waterhouse stop, rotating diaphragm and iris, and a bokeh comparison at 5 and 9 blades]

Questions people ask

How many blades does a lens diaphragm have?

Most current lenses have about 5 to 10 blades. Straight blades make a polygon-shaped opening and curved blades make a rounder one.

Where is the diaphragm in a lens?

Between two lens elements, inside the lens barrel.

References

  1. Creative Hut Academy (2025) Aperture. Lecture slides by Abin Alex and Anu Joseph.

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 1Next: part 3
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Guide by Abin Alex and Anu Joseph, Creative Hut Academy.