noun
: the diameter of the entrance pupil of an optical system ; specifically : the apparent diameter of the diaphragm opening in a camera lens as seen through the front of the lens
noun
: the diameter of the entrance pupil of an optical system ; specifically : the apparent diameter of the diaphragm opening in a camera lens as seen through the front of the lens
Webster's New International English Dictionary. Новый международный словарь английского языка Webster. 2012