stop up

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Verb

stop up

1. To fill a hole or cavity, or block an opening or passage, as with a plug.
   • 1885, Mark Twain, Huckleberry Finn, ch. 37:

     So then we […] scratched around and found an old tin washpan, and stopped up the holes as well as we could.

2. (photography) To increase the aperture of a photographic lens, moving from an f/stop represented by a higher number to an f/stop represented by a lower number and causing more light to pass into the camera.
   • 2002, Kathleen Tracy, The Complete Idiot's Guide to Portrait Photography, ISBN 9780028643687, p. 18 ↗:

     To stop down means to narrow the aperture; to stop up or open up means to expand it.

Antonyms

• (increase the aperture of a photographic lens) stop down

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