Glossary
Aperture blades
Aperture blades are the overlapping leaves that form the opening inside a lens, and their number and condition determine the shape of out-of-focus highlights and whether the aperture works at all.
Also called diaphragm blades, blade count, oily blades
How it works
Blade count sets the shape of a stopped-down highlight. Six blades give a hexagon, eight an octagon, and the fifteen or more blades on some Soviet portrait lenses keep highlights essentially round at every aperture. Wide open, the shape is a circle regardless of count.
An odd number of blades produces twice as many sunstar points as blades, because opposite edges are not parallel. Even counts produce the same number of points as blades.
Oil on the blades is the most common vintage fault. Lubricant migrates from the focus helicoid onto the diaphragm, making it sluggish or stuck. It is a routine and inexpensive service item, but a stuck aperture means you cannot stop down at all until it is fixed.
Why it matters when buying
Work the aperture ring through its full range before buying. The blades should snap crisply at every stop. Visible oil, sticky movement, or blades that stay open are all the same repair, so price accordingly.