Refraction of Light in Camera Filters
Filters not only absorb light, they also
refract or bend it. This has consequences that should be considered.
Camera lenses are designed to operate best with no other objects
between them and the subject. Filters placed in front of a lens can
disturb the image formation. The amount and type of disturbance depends
on the flatness of the filter, the parallelism of its surfaces, its
thickness, and the accuracy of its placement in the optical path.
To a degree, a filter acts like an
additional lens element. If the surfaces of a filter are not flat it
indeed acts as a lens, bending the light according to the curvatures
that are present, and definitely degrading the image. If the two
surfaces are not parallel, it will act somewhat as a prism, dispersing
light according to its wavelength. A glass filter also changes the
focal length of the lens according to its thickness, tending to defocus
the image slightly. Placing it in the optical path at an angle other
than 90 degrees to the lens axis causes further image disturbances.