The school photo finishing industry uses cluster lenses to project multiple identical images on photo paper. Try as you might to make all the lenses on a board identical, some pass more light than their neighbors. We resorted to dotting the lenses with opaque spots of paint to equitize. These act to reduce the amount of light that transverse the lens. As rule, no appreciable degrading of image quality resulted. The amount of light reduction is simply the percentage of the surface area opaqued.
On the other hand, once upon a time portrait lenses were favored it they yielded a soft focus. One scheme was to opaque a large circular portion in the center of the lens. This caused the lens to image using only rays from the peripheral which inherently are poorly corrected for aberrations.