A UV filter is a clear screw-on filter originally designed to block ultraviolet light, which on color film could cause a bluish haze, particularly at high altitude or over water. UV blocking was genuinely useful in the film era because emulsions were sensitive to UV in ways that color rendition could not easily compensate for. Modern digital sensors include built-in UV/IR cut filters as part of the cover-glass stack, making the UV-blocking function of a screw-on filter essentially redundant on a contemporary camera.
As a result, today’s UV filters function primarily as protective glass for the front element of the lens. The argument for using them is straightforward: a 100-dollar filter that scratches or shatters is a cheaper casualty than a 2000-dollar front element. The argument against is equally direct: adding another air-glass interface in front of the lens introduces flare, ghosting, and very small losses in sharpness and contrast, which are most visible when shooting toward bright light sources.
Quality matters enormously when a filter is going to live on the lens. High-end multi-coated filters from B+W, Hoya, Nisi, and Breakthrough Photography use multiple layers of anti-reflective coatings, brass mounting rings, and high-grade Schott or comparable glass, which substantially reduces the optical penalty. Cheap uncoated filters can produce visible reflections, especially of bright off-axis sources, and they tend to bind on the lens threads when temperature changes. Spending less than ten percent of the lens cost on the filter is a common rule of thumb.
The protection debate is largely cultural. Landscape and outdoor photographers working in salt spray, dust, sand, and wind generally favor keeping a high-quality filter on, since cleaning and replacing a flat filter is far easier than dealing with a contaminated front element. Studio and clean-environment photographers often shoot without a UV filter, relying on the lens hood for mechanical protection and the deep recessed front element on most modern lenses for impact resistance. Either position is defensible.
A UV filter is no substitute for a proper lens hood. The hood blocks side light from striking the front element at oblique angles, which is the primary cause of flare and contrast loss. Many lens hoods also provide significantly better impact protection because they extend an inch or more in front of the actual glass. Using both a high-quality UV filter and the lens hood is the standard configuration for outdoor work.
Common pitfalls include stacking a UV filter with a polarizer or ND filter, which compounds reflection and vignetting risks, especially on ultra-wide lenses, and forgetting to clean the filter as routinely as the front element. A dirty filter undermines image quality far more than a clean lens with no filter at all, and the assumption that the filter is doing nothing visible can lull a photographer into ignoring its actual condition.