noun
numerical aperture
Technical term in optics and photography; measures the light-gathering ability of a lens.
このレンズは開口数が大きいので、暗い場所でもよく写る。
This lens has a large numerical aperture, so it performs well even in dark places.
Compound of 開口 (opening) and 数 (number), a calque of English 'numerical aperture'.