What Does Opacity Mean When Talking About Photography?

Opacity is the measure of an objects impenetrability to electromagnetic radiation. When talking about photography the term is used to describes exactly how much an object blocks light from passing through it. An object that completely blocks light is called opaque, no light is able to pass through it. The opposite of opaque is transparent. Many opaque objects will reflect light which can be problematic when taking photos. Knowing the opacity of objects is also very important when designing lenses; lens elements despite very low opacity values will still block small amounts of light, this has to be compensated for within a camera when taking photographs.