Historically, the unit of exposure was a doubling or halving of the exposing energy. This is the origin of the f/stop. Initially, this adjustment was made by inserting a thin metal plate with a circular hole, into a slit in the lens barrel. The photographer had a series of these metal slides called Waterhouse Stops after John Waterhouse circa 1858. The Waterhouse stop was superseded by the mechanical iris diaphragm. (You can see early examples of both in this video from Roger Cicala at lensrentals.com) To achieve a 2X change the aperture diameter must be enlarged or contracted such that the surface area of the hole is doubled or halved. To accomplish a 2X change, the diameter of the hole must enlarged or contracted using a multiplier of divider of 1.414 (square root of 2).

As an example, say a 50mm lens is mounted and set to f/8. The diameter of the hole in the iris will be 6.25mm. To open up this lens to f/5.6 the revised diameter will be 8.82mm. To stop down to f/11, the revised diameter must be 4.42mm. What I am trying to say is, given that the leaves of the iris are adjusted by a gear train, precision is challenging.

To make a ½ f/stop change, the multiplier is the forth root 2 = 1.19. To make a 1/3 stop change, the diameter modification is the sixth root of 2 = 1.12. In other words, as we make smaller and smaller adjustments the needed precision adds cost.

Please note: With black & white films, the resulting negatives are useless until printed. The printing operation is comparable taking a picture of the negative substituting light sensitive paper for film. This second exposure (printing) allows for adjustments to be made to mitigate errors made during the initial film exposure. In all most every case, a camera precision of greater than 1 f/stop was not necessary.

With the advent of more complex materials like positive black & white and color slide film, the need to improve exposure accuracy is evident. This inspired the 1/2 and 1/3 iris adjustments.

Now long focal length lenses are the “norm” for large film cameras. When we are adjusting longer lens the amount of accuracy of the gear adjustment of the iris is not a problem because the hole size change to make a 1/3 stop change is hefty. If the focal length is short, then 1/3 f/stop changes become problematic. Example: A 28mm set to f/8 has a diameter of 3.5mm. To close down to f/11, the revised diameter works out to be 3.125mm, not an easy mechanical change.