depth-of-field preview button
/ˈdɛpθɒvfˈiːld ˈpɹiːvjuː ˈbʌtən/
n
中文：景深预览按钮
中文 · 景深预览按钮Español · botón de previsionado de profundidad de campoFrançais · vérification de la profondeur de champ
Meaning：Button that closes the diaphragm so that the relative clarity of various depths can be seen through the viewfinder.