spitfire

<p>The biggest issues with filters:</p> <ol> <li>For filters that are not modifying the image with other than a flat surface (e.g. halo and many soft focus filters), it can introduce aberrations due to not having dead flat surfaces, or if its two surfaces are not perfectly parallel.</li> <li>Flare from light reflecting off of the two air-glass interfaces that have just been introduced into the optical train. Light reflects from all boundaries between materials with different refractive indices, and that includes glass and air boundaries. It is the reason lenses have optical coatings which reduce the amount of reflected light. Flare is most pronounced when a specular or point light source is in front of the lens, casting light onto its surface even if it's not within the field of view.</li> </ol> <p>The best filters, such as B+W or Heliopan, have the flattest and most parallel high precision surfaces. While multi-coated filters cannot completely eliminate flare, they can reduce it. Ultimately flare control also requires light and composition control, including the use of lens hoods and other means to keep stray light out of the optical train. If the optical train is a low quality lens with aberrations, low contrast, poor flare control and low resolving power (i.e. poor MTF characteristics), filter quality and optical coatings won't make much difference. However, if the lens optics are world class, made by a firm such as Carl Zeiss or Schneider-Kreuznach, a poor quality filter can immediately plunge it into utterly mediocre optical performance.</p> <p>A filter is another element and group added to the front of the lens. Its quality should be commensurate with the quality of the lens behind it. Keep that in mind when selecting and using one.</p>