UV filters are most commonly used to protect the front of your lens. A first line of defense against scratches, rain, dirt and dust. Hoya's NXT Plus UV is a powerful upgrade to the best-selling NXT series. The new filter applies a waterproof top coat to Schott B270 glass polished smooth for easy cleaning. Dirt, smudges and water wipe away easily. Using 10 layers of Hoya's patented anti-reflection (AR) multi-coatings your images show improved contrast, color and sharpness with a 98% light transmission. So, leaving it on all the time will not hurt your images. The NXT Plus filter frames are precision milled from lightweight aluminum for a slim profile that makes it perfect for all lenses including super wide-angles. The frame sports full front filter threads that can accept a normal lens cap or allows you to stack additional filters like a circular polarizer or neutral density. What else does a UV filter do Most photographers leave a UV filter attached to the lens all the time as a form of protection. But the filter also plays an important role in filtering through UV Haze. Not a huge problem when you are taking pictures at sea-level but as you hike into higher altitudes the filter definitely helps cut through the haze to increase contrast and improve color. The NXT plus has a UV Cut Point of 360nm or L36 which is superior to most filters at this price point.