Motion picture lenses are fine tuned image making machines. The quality of the images they produce is directly related to the quality of the glass combined to create a lens. Cinema lenses must be maintained to the highest degree to ensure optimal performance. Obviously the most susceptible and delicate parts are the front and rear element of the lens (the exposed glass at the front and back of the lens). When accidents happen, it’s usually fairly simple to replace just the front element of a lens. A lot of folks ask if the glass can just be polished since most accidents are simply micro scratches that don’t go any deeper than the anti-reflective coating. Yes, this is possible, but it’s not usually a great idea. It’s sort of a last resort. Polishing and re-coating a lens element can be risky. If the polishing isn’t done perfectly, the curve of the element can change and alter the light path of the lens. Anti-reflective coatings are applied in a vacuum chamber with very special equipment. Every lens manufacturer has their own process and it can vary quite a bit between companies. So re-coating a lens is never going to yield the same results unless it’s re-coated by the manufacturer which is usually more expensive than buying a new element. Hence, replacing glass to maintain a perfect image is common in the motion picture industry. Think of it like car tires. You could save a few bucks and re-tread your old tires or you can buy new tires when you need them. Which one sounds safer?