The term 'lens opening' refers to the mechanism that controls the amount of light entering a camera lens. APERTURE, which is an 8-letter word, is the size of the opening in the lens, and is measured in f-stops. IRIS, which is a 4-letter word, is the adjustable diaphragm that can be used to control the size of the APERTURE. Together, these two mechanisms are used to control the amount of light entering the lens.