- non-flat or non-parallel surfaces, reflections (minimised by optical coating), scratches, dirt - affect the image.

In digital photography the majority of filters used with film cameras have been rendered redundant by digital filters applied either in-camera or during post processing. Exceptions include the ultraviolet (UV) filter typically used to protect the front surface of the lens, the neutral density (ND) filter, the polarising filter, color-enhancing filters, and the infrared (IR)