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Someone mentioned that "veiling glare" or "veiling flare" was to blame for reduced contrast in a photo. What does this mean exactly?
What causes veiling glare, and how can it be avoided? How does it relate to those floating polygonal lens flares one often sees in movies?
Veiling glare is light that's not intended to be part of the image, per se, but ends up on the recording medium (film or sensor) anyway. It's caused by reflections and scattering of light by optical elements and the lens barrel. This produces an overlay of general brightness, which raises what should be the darkest parts, reducing overall image contrast.
For example, imagine shooting on a sunny day, and framing a photograph which doesn't include the sun directly, but where direct sunlight still falls on the front glass of that lens. Some of that light still makes it to the sensor, even though it doesn't represent the form of anything in your image.
Smears or dust on the lens (or on an attached filter) can scatter light in unintended ways, compounding the problem.
Adding additional lens elements — like a glass filter, either for protection or for a special effect — can make this worse, for several reasons. First, it's another piece of glass, and usually a flat one at that. Second, many filters are of low quality and don't have good coatings. And, since they're usually right on the front of the lens, further away from protection from out-of-the-image light, they're prone to making the problem worse even when of high quality.
Your biggest defense is a lens hood — or otherwise keeping the front element of the lens shaded. All light that strikes the front element has the opportunity to scatter and bounce around in the lens, causing veiling glare — and bright sunlight can easily wash out the image.
Lens bodies are designed with matte black internal finishes and often have baffles and other features to control reflection. And, lens surfaces are given special coatings in part to minimize this reflection.
The visible lens flare — sometimes called "ghosting" seen in photographs or movies are related, but not quite the same. In those cases, the light is more focused and controlled, causing a bright highlight shaped like the aperture, or sometimes rays or lines. These are can also be caused by having bright lights hit the front element — like, say, having the sun in the frame! — but veiling glare can be there even if you don't see any of that.
This supplement to mattdm's answer contains a series of images to illustrate the effect that coating technology can have on veiling glare. They were all taken handheld with the camera set to the same settings with auto exposure enabled. There is a lamp shining toward the camera just outside the frame in the upper-left corner. All lenses were set to cover approximately the same angle of view with the aperture wide open. No hood was used with any lens.
Taken with a Canon FD 35-105/3.5 lens (1981), which features an FD bayonet mount ("New FD"). It is my understanding that all of the "New FD" lenses, except for the 50mm f/1.8, used super-spectra coating (SSC). Although SSC was Canon's most advanced coating at the time, it apparently could not handle this lens' 15 elements in 13 groups. Both veiling glare and flare ghosts may be seen.
Taken with a Pentax-A SMC 35-105/3.5 lens (1984), which also has 15 elements in 13 groups. Pentax's Super-Multi-Coating (SMC) was well regarded at the time. Veiling glare is significantly reduced compared with the Canon lens.
Taken with a FujiFilm XF 18-135/3.5-5.6 R LM OIS WR (2014), which has 16 elements in 12 groups. This lens sports FujiFilm's Super Electron Beam Coating (EBC). Glare is limited to the upper left corner of the image.
