Why did typical number of iris blades decrease from the 1950s to the 1970s and 80s?

Question

I have in my collection a number of 1950s "silver" lenses, and they all have a high number of iris blades: 10, some even 12.

On the other hand lenses from the end of manual focus era (late 1970s to early 1980s) tend to have much fewer iris blades; often just five or six.

This holds true even for a lens that kept its optical formula unchanged over the years, such as the long lived Carl Zeiss Jena Tessar 2.8/50mm. It went from 12 blades to 5.

Manufacturing cost was certainly a factor, but could not be the only one — the cost and complexity of having a lens with many blades was at first accepted, and then not.

What was the reason for going from many to few iris blades?

I realize this is a possible duplicate to Why do lens manufacturers produce lenses with few aperture blades? - but I am asking why did the many irised lenses go out of fashion, which I take as a different question.

To illustrate my point: these two lens are optically the same (but about 20 years apart in manufacture).

Answer 1

The standard type of iris diaphragms used in early lens designs featured numerous overlapping leaves. Each leave crescent shaped. The leaf is pivoted at one so that it can rotate about a fixed point. The other end of the crescent contains a pin. This pin is caused to slide forward and back in a slot attached to a control ring. As the control ring is rotated, the leaves change position such that the aperture diameter closed down or opened up. When the control ring is rotated to stop down the iris to a tiny diameter, the blades crowd together. Now the accuracy begins to fall off. Now tiny movements of the control ring are required.

The countermeasure is to change the shape of each individual leave so it resembled the shape of a “U”. This design delivered a linear change in the working diameter throughout the entire aperture range. additionally, fewer blades are required with this “U” shape design. Thus greater accuracy of adjustment is achieved. This improved accuracy is a must if the user is allowed to make f-numbers in 1/3 stop increments or finer.

Answer 2

The only way to know for sure would be to ask those who made such decisions for the manufacturers at the time. But we can make a few observations and educated guesses.

1. As another answer waxes forth in great detail, there was a need for greater accuracy while preserving linear changes in effective diameter with linear movements of the control mechanisms. The increasing market demand for 1/2 and 1/3 stop diaphragm adjustments in the 1960-70s required such a change.
2. Once a solution was created by modifying the shape of aperture blades, the increased accuracy could be achieved even with fewer blades than had previously been the case. Fewer blades mean simpler, more reliable designs as well as lower manufacturing costs.
3. Optical designs sometimes follow fads in creative practices. In the 1960s and 1970s there were a small number of high profile cinematographers who popularized the use of lens flare as a creative device. Easy Rider, Texas Chainsaw Massacre, Close Encounters of the Third Kind as well as other projects Douglas Trumbull worked on, and Terrence Malick’s Tree of Life are notable examples. Sci-fi and horror were genres where this was most often seen. This was at a time when most directors and DPs went to great lengths to eliminate lens flare from their shots. In more recent times, directors such as J.J. Abrams, David Boyd, and Michael Bay often replicate such looks, sometimes using CGI to create artificial flare. The auteurs who pioneered intentional flare often preferred the look of hexagonal (six sided) flare when using such techniques. The trend in movies crossed over to a number of popular still photographers. Among all of the other factors at play, lens designers may have been attempting to follow this trend as well. (Then, in the 1980s, the preference shifted to circular flare as the technique became even more widespread in cinematography.)