Is flare essentially a product of lens design? What part does coating, or lack thereof, play? For example, will the use of a multicoated filter on an uncoated lens decrease flare? Thanks for any enlightenment you may offer.

Dennis

-- Dennis Couvillion (couvilaw@aol.com), February 24, 2002

Answers

There are many kinds of flare.

But to answer your practical question - NO.

A multi-coated filter will not improve the quality of an uncoated lens. Flare is cumulative - an additional piece of glass may add more flare, or at best have a neutral effect - it cannot subtract flare.

A lens hood CAN prevent flare by cutting down the amount of extraneous light hitting the front elements or the inside of the lens/camera outside of the film areas.

Lens coatings are transparent atomic films about as thick as a half- wavelength of light. They actually create a SECOND reflecting surface - but the two reflections (from coating and glass) cancel each other out because of that 1/2 wavelength thickness - the light waves bounce off the coating and glass more or less exactly out of phase.

Since light actually comes in a spread of wavelengths a coating really only affects part of the light spectrum - which is why coatings give lenses those pretty blue/purple/green/yellow colors. Those are the colors that DIDN'T get cancelled out.

MULTI-coated lenses/filters have several coatings on top of each other - each a different thickness - so that combined they can eliminate reflections over most of the visible spectrum - leaving just a faint purple reflection.

But each surface has to have its own coating or multicoating - you can't 'cure' reflections on lens 'A" by coating the surface of filter 'B" and then combining them.

-- Andy Piper (apidens@denver.infi.net), February 24, 2002.

Flare is dependent on lens design. It is particularly sensitive to the number of air-glass surfaces in the lens, which is why zooms (with 10-20 glass elements) are much more prone to flare than single focal length lenses. Flare is due to internal reflections within the lens, while ghosting (a close relative to flare) is due to reflections off the diaphragm. The amount of flare also depends on the type of glass (index of refraction) and the quality of the coating (s). Certain types of glasses require little or no coating, while others may require multicoating.

Proper blackening of the inside of the barrel and recessing the front element are also design features that contribute to the protection against flare. You can NOT prevent flare by adding additional air glass surfaces. Filters can make the problem worse, but not better. The best you can do is use a high quality filter that does not add to the tendency of the lens to flare.

-- Eliot (erosen@lij.edu), February 24, 2002.

An example of what Eliot describes so well:

the 90mm f/2.8 'thin' Tele-Elmarit-M is notorious for flare. Most of that flare is created after the light has already passed through all the glass - due to reflections off the inside of the lens barrel at the very back by the focusing cam. No coating in the world can help a physical design problem like that.

The design parameters called for maximum compactness for this lens - but in the process the lens tube became so narrow that it acts like a mirror for extra light outside the image area - a design compromise.

-- Andy Piper (apidens@denver.infi.net), February 26, 2002.