Definition: Non image-forming light in optical system that degrades image quality. * Light may (a) enter optics obliquely and reflect around inside of camera or lens (b) leak into system e.g. through hole in bellows (c) be deviated from forming image by marks or particles on lens surfaces or by defects in glass. with net effect of reducing contrast of image. * In bad cases it may form one or more spots of light, often of shape derived from that of diaphragm or aperture stop. * Flare is reduced by (a) coating or multi-coating all air-glass surfaces of optics (b) providing shade or hooding to prevent entry of stray light into optical system (c) providing stops within optical system to block oblique rays (d) blacking or flocking all reflective surfaces e.g. rims of individual lens components and interior of lens and camera to absorb stray light.
When no flare is bad flair
The funny thing is that lens flare sometimes authenticates -- almost validates -- an image as a photograph so its presence in a digitally manipulated image helps make it look more 'realistic'. Most image manipulation software offer some kind of flare filter effect. There is even a plug-in for PhotoShop that offers nothing but lens flares.

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