These are the cover plates inside some optical lenses, which is used to press the shutter blades with high precision.
The material is generally stainless steel or phosphor bronze. However, because they’re accessories in the optical lens, the metal gloss causes interference, so matt black surface treatment is necessary for such parts.
The dimensional tolerance requirements of these cover plates are generally plus or minus 0.02mm, and the flatness requirement is 0.08mm.
It also has high demands on surface appearance, color difference, adhesion.