The Complete Handbook of Lens Filters

Chapter 1: Understanding Lens Filters

A lens filter is a flat or slightly curved optical element mounted in a frame, designed to be attached to the front of a camera lens. Its purpose is to modify, reduce, or selectively transmit light before that light reaches your camera's sensor. This simple definition hides a complex set of physical interactions between glass, coatings, light wavelengths, and your optical system.

Understanding what filters actually do requires stepping back from product marketing and looking at the physics. When light enters your lens, it travels through multiple glass elements, refracts according to the lens design, and eventually strikes your sensor. A filter is inserted into that path, introducing two new optical surfaces: the front surface where air meets the filter material, and the back surface where the filter meets the air again before reaching your lens. Each of these surfaces reflects and transmits light according to material properties and wavelength.

The Optical Reality of Filters

At any air-to-glass interface, some light is reflected and some is transmitted. The proportion depends on the refractive index of the glass (how much it bends light relative to air) and the wavelength of the light. A typical uncoated glass surface reflects about 4% of light at normal incidence—not dramatic, but measurable. When you stack two uncoated filter surfaces into the optical path, you lose roughly 8% of light before it even reaches your lens. Add an uncoated lens with 8-12 elements, and you're losing 20-25% of light to reflection