Nikon’s 800mm F5.6 FL ED VR and 400mm F2.8 FL ED VR lenses earn the 'FL' in their names from the coating of fluorine applied to the lens elements. But what exactly is fluorine coating, and why is it important?  

In this two-minute long video from Nikon Japan, the coating's repellent properties are demonstrated as various elements are dripped, drawn and painted onto the surface. Essentially, the fluorine coating turns the front element glass into a large hydrophobic surface. This makes it much easier to remove offending liquids from the lens. 

It’s probably not that often permanent marker meets the front of the lens, but the coating prevents buildup of material on the glass, and makes it easier to clean without damaging the surface. 

According to Nikon, this coating is also more 'peel resistant' than the coating applied by other manufacturers.