Refraction of Light in Camera Filters

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.

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