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I want to know if smudges (finger prints etc) and grease marks in specific have any effect in increasing the lens flare. As those who wear glasses may already know about the problem of flares caused by grease or smudge marks on the lens.
I understand that there are are some questions such as how to control lens flares etc but none have discussed these factors. This is specific to smudges and grease.
Yes grease and smudges can cause flare, but instead of well defined circles or lines you are more likely to get an overall clouding effect with a visible glow around highlights and lightsources.
In fact it used to be a common technique with glamour and some portrait photographers to smear vaseline on a lens in order to get flattering (if cheesy) soft focus look. The same technique was used to simulate motion blur when shooting stop motion animation in films such as The Terminator.
Think of smudges, grease, and finger prints (usually caused by oil mixed with other things on your hands) as a semi-transparent mirror.
It causes light to refract and reflect at the points where the smudges and fingerprints are. As Matt pointed out, sometimes this is the desired effect for aesthetic appeal.
Smudges can't create "lens flare" per se. Smudges will definitely affect the result (the effect might even be interesting) but it is unlikely to cause what we traditionally call "lens flare". Niether can any similar problem such as dust, fog, or mold that can grow internally.
Lens flare will show up as a chromatic bright spot, often times several spots. It is caused by the source of light, the sun for instance, being reflected internally against the lens elements until those reflections are picked up by the film/sensor. The chances of them occurring go up and the focal length goes down. Wide angle lenses have a higher incidence of lens flare than telephoto lenses. They also go up the narrower the angle between your subject and the light source.
To reduce lens flare, shade the primary element (the outer piece of glass) of the lens. A lens hood is designed for this purpose, but your hand or anything that can block the light will do.
ghosting. The technical term flare refers to the rays of light that extend out from bright light sources, particularly when they are in the corner of the frame. The third related problem would be a loss of contrast or large areas of glare that reduce contrast...which is most likely what grease on the lens would introduce. Grease is also likely to introduce glare even with incident light (flare and ghosting tend to be created by non-incident light)...a hood might not fix it.
I would say no, well not by the definition of lens flare. Lens flare is more defined as coloured circles, grease etc will cause blurred images and "haze".
Unless of course its a huge chunk of clear grease (or indeed rain)- which could itself act as a small lens, in which case.. yes!
I might add - for god's sake keep your lenses clean! there is nothing worse for a lens than the act of cleaning it (well perhaps dropping it!) you will inevitably mildly (invisibly) scratch it and slowly erode the coating.
Anything irregular on the polished surface of the lens will generate non-imaging light; aka flare.
You can make a very long list of specifics, if you want, but there it is. It doesn't matter how thin, or thick it is. If it is non imaging illumination, it is flare.
Dust is individual particles, Moisture is individual droplets, grease is a thin non-planar layer of translucent oil, scratches are individual furrows, etc.
The coating is a controlled application of a thin (usually metal) surface treatment applied evenly to the polished surface of the lens for image enhancement or surface protection.
