Sand, dust and even intentional marks on a lens have little impact on the results. Grease and other smears are another story.

For a number of years I worked for a manufacturer of precision printers for the school finishing business. Our product was a high speed optical printer/enlarger. It exposed roll color paper , making a package of prints the student’s parents purchased. When the package came home, they used scissors to cut the pictures apart. The machines were sold all over the world. A typical package consisted of the following quantity and print sizes: 1 8x10 – 2 5x7’s – 4 3 ½ x5 – 9 wallet etc. (inches).

To accomplish this the machine featured multiple lens decks. Each deck was positioned at a different height paper-to-negative distance. Each deck was mechanically moved in and out of the optical path. The lenses on each deck were matched one to another as best practice allowed.

Nevertheless, when each machine was installed in the field and tested, many of the projected images yielded exposures that were a tad off. We would open up the aperture using a file. We would dot the lenses that were too bright with spots of paint on their surface.

Cluster cameras were also made. These exposed multiple images on a single frame of film.

What I want to tell you is: Defects such as dust or dirt on a lens only impart minuscule degrading. Think about it. We stop down the lens to reduce light transmission. The dust and dirt do exactly this; they stop some light. Now grease and oil diffuse but specks are just opaque matter.