Why did many lenses go from 10–12 aperture blades in the 1950s to 5–6 blades by the 1970s–80s?
Asked 6/12/2017
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Older 1950s manual-focus lenses often have 10–12 aperture blades, while many later lenses from the late 1970s and early 1980s use only 5–6 blades, even when the optical formula stayed essentially the same. For example, some versions of the Carl Zeiss Jena Tessar 50mm f/2.8 changed from a 12-blade diaphragm to a 5-blade one.
Beyond simple cost cutting, what changed in lens design or camera use that made manufacturers accept fewer blades? Why did high-blade-count diaphragms fall out of favor?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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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.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
0
Generated from our catalog & community — verify before relying on it.
Cost was part of it, but not the whole story. Older iris designs often used many overlapping crescent-shaped blades to keep the aperture opening fairly round. As lenses and cameras evolved, manufacturers wanted more precise and more linear aperture control, including finer 1/2-stop and later 1/3-stop click settings.
Improved diaphragm blade shapes and mechanisms made that possible with fewer blades. In other words, better blade design reduced the need for lots of blades to achieve accurate aperture changes. Once that happened, fewer blades became attractive because they simplified construction, improved reliability, and lowered manufacturing complexity and cost.
There was also a change in priorities: a very round stopped-down opening became less important than precise, repeatable operation and simpler mass production. So many-blade diaphragms did not disappear because they were useless; they became less necessary once newer mechanical designs could deliver acceptable aperture shape and better control with fewer parts.
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