These are actually two filters, one UV and one circular polarizing. You can use them separately or stack them on top of each other. These are well made filters, the ring is very small so they don't cause vignetting even when stacking both filters on wide angle lenses. These screwed onto my lens smoothly and stayed secure. They only reduce the value a little and once screwed on there is an outer ring that allows you to rotate the polarizer to get the best effect. The UV filter helps reduce atmospheric haze and makes the air appear clearer. Many people also leave a UV filter on to protect the front element of their lens from dust and splashes. The circular polarizer reduces reflections from water, glasses and windows. Because it reduces reflections, you can see right into the water, making it look clearer and cleaner. It also helps provide a clearer sky image when photographing outside. This is the “K” series filter, they also have a “D” series. I'm not sure what the exact difference is between the two, as K&F's own website doesn't really explain it. As far as I can tell, the "K" series (this one) are slightly cheaper with fewer layers of coating, the K series doesn't seem to have a water and oil repellent coating. Definitely well made filters that do the job.