Lens Flare
Lens flare usually refers to the stray light from the subject and the lens. An ideal lens would only transmit all the light from the subject to form a high-contrast image in the focal plane. In practice, a quantity of stray light also reaches the image or reflects off the lens to cause haze. You may be lucky in that this non image forming light may integrate to give a near uniform additional level of illumination at the image surface. But the overall effect is a loss of image contrast or reduced density range of a photographic image. S
For optical instruments in general, stray light is called veiling glare, but this is not a good term since it implies uniformity of the illumination level when in fact it is ‘structured’ so that stray images, ghost images, bright spots and diaphragm ghosts may appear as well. The more general term glare is preferable, but for photographic systems the term flare is more often used, the two being largely interchangeable.
Types and sources of flare
Flare is primarily due to a number of conditions, including the nature of the subject and its surroundings, inter-reflections between surfaces of a lens, reflections and scattering from various mechanical surfaces adjacent to or within the camera and lens, plus artefacts such as dirt or fingermarks on optical surfaces.
Reducing flare
To reduce the variety of flare effects a number of positive measures are possible. The lens designer may arrange that the flare image pattern is reduced and out of the primary image area.
The use of single and multi-layer coatings is the most useful technique.
The rims of lens elements can be blackened. The iris diaphragm blades, the inside of the lens barrel, and any mechanical features within can be treated to be of low reflectance to reduce the aperture value effect on flare. It is found that matte black paint is poor for this purpose, having a small but significant specular reflection. Machined ridges or striations are better but best of all is electrostatically precipitated black flocculent material. Interior baffles also help and are vital in catadioptric lenses and long focus lenses.
Externally, lens hoods, shadow boards and various shielding devices may be used to prevent sources outside the subject area inducing flare.
Lens flare usually refers to the stray light from the subject and the lens. An ideal lens would only transmit all the light from the subject to form a high-contrast image in the focal plane. In practice, a quantity of stray light also reaches the image or reflects off the lens to cause haze. You may be lucky in that this non image forming light may integrate to give a near uniform additional level of illumination at the image surface. But the overall effect is a loss of image contrast or reduced density range of a photographic image. S
For optical instruments in general, stray light is called veiling glare, but this is not a good term since it implies uniformity of the illumination level when in fact it is ‘structured’ so that stray images, ghost images, bright spots and diaphragm ghosts may appear as well. The more general term glare is preferable, but for photographic systems the term flare is more often used, the two being largely interchangeable.
Types and sources of flare
Flare is primarily due to a number of conditions, including the nature of the subject and its surroundings, inter-reflections between surfaces of a lens, reflections and scattering from various mechanical surfaces adjacent to or within the camera and lens, plus artefacts such as dirt or fingermarks on optical surfaces.
Reducing flare
To reduce the variety of flare effects a number of positive measures are possible. The lens designer may arrange that the flare image pattern is reduced and out of the primary image area.
The use of single and multi-layer coatings is the most useful technique.
The rims of lens elements can be blackened. The iris diaphragm blades, the inside of the lens barrel, and any mechanical features within can be treated to be of low reflectance to reduce the aperture value effect on flare. It is found that matte black paint is poor for this purpose, having a small but significant specular reflection. Machined ridges or striations are better but best of all is electrostatically precipitated black flocculent material. Interior baffles also help and are vital in catadioptric lenses and long focus lenses.
Externally, lens hoods, shadow boards and various shielding devices may be used to prevent sources outside the subject area inducing flare.
Lens hoods
The lens hood, sunshade or matte box, to give it its various names, has the prime and vital function of shielding the lens from glare sources outside the FOV of the lens. By restricting the acceptance angle to only image-forming rays from the subject, most oblique rays from elsewhere may be blocked.
The interior of the hood must be treated to reduce the possibility of reflections entering the lens in this way. Black velvet, flock paper, fine raised ridging and corrugated b
Lens hoods come in many designs and shapes and may even be built into the lens, though in such cases they are seldom of adequate length. The cylindrical form is cheapest to make and allows the lens to rotate during focusing. An extension sleeve may be adjustable for different focal lengths. A square or rectangular hood can give more exact shielding to the FOV, being the shape of the format in use. Some hoods have rear cut-outs so as not to obscure a viewfinder image or to permit insertion of a series-mounted filter. Some are of rubber and so can be folded back over the lens when not in use.