A filter is usually a thin transparent sheet of glass or plastic that is attached to the front of the lens to alter the image as it is taken. There are filters with specific tasks to enhance the image, and there are filters that give photographers creative options, such as color or graduated filters. The former include infrared or UV filters, which ensure that neither UV nor infrared light passes through the optics and onto the sensor, adversely discoloring the image. Polarizing filters are also often useful, as they can prevent reflections and over-illumination. This prevents information loss in said areas.

An important field of application for filters is especially multispectral photography, since filters can be used to select a wide variety of light wavelengths and bring them specifically onto the sensor.