[Work] Veiling glare

Veiling glare is a term used to indicate light which is scattered and reflected within the lens system. The net effect of veiling glare is to reduce imaging contrast.

The reduction in contrast of an optical image caused by superposition of scattered light.

Diffuse stray light at the image plane of an optical system that results in reduced contrast and resolution.

Veiling glare causes:

• Internal multiple reflections between the lens surfaces.
• Scatter from the surfaces of the lens elements due to scatches and the other imperfections in the polish, dirt and dust, fingerprints, grease, and poor antireflection coatings.
• Bulk scatter from the interior of the glass and from bubbles and striae.
• Scatter from optical cements.
• Scatter and reflections from the ground edges of the lens elements, from internal lens mounts and from the internal surface of the lens barrel.
• Reflections from the surfaces of diaphragms and shutter blades.
• Fluorescence of the glass or optical cements.

Flare comes in two flavors, veiling glare and visible flare. Veiling glare is the loss of contrast seen in darker shadows from light bouncing around off the lens elements and film. To evaluate veiling glare, you want a thin dark subject against a bright white background. The easiest real world subjects I have found are thin tree branches against the sky in winter. In summer, the leaves get in the way, so I may opt to use a powerline against a bright cloudy sky. Ideally, the branches or wires should be dark black and transition abruptly to the white sky. The less solid black the branches or wires look, the more you are seeing the effects of flare. If you have a downloaded lens test chart , you can also do this test easily with the black and white lines on the chart.

To evaluate visible flare, you can take a photo of a bright point light source such as the sun. You may see a hexagonal or other pattern from reflections off the surface of the lens diaphragm blades. You may also see bright blobs or streaks of light reflected from lens elements. Try a shot without your UV filter, if you use one, to see why some of us don't use UV filters (they add to flare). Some lenses such as my 20mm f/3.5 AI nikkor have surprisingly little flare, even in direct sun in the photos shots. Zoom lenses tend to have much higher flare than other lenses due partly to the larger number of elements having more reflective surfaces. Again, this is one major reason many of us prefer fixed or non-zoom lenses.

/The question of subject distance when testing a lens is important. Photographic lenses are normally optimized for middle distances, say, six to ten metres, the range in which they may mostly be used. The wider the aperture, the narrower the optimal range, a factor important from f/1.4. Source: Lens Test Charts by Geoffrey Crawley in British Journal of Photography, January 31, 2001, p. 26./

Links:
McCann Imaging
Veiling glare on Imatest